IFATCA The Controller - January/March 1970

D 20418 F

selected

aiiimMX iiMB Off for Malmo and six other sites The ATCR-2T is an all weather high power, L band, surveillance radar with a revolu

tionary digital Mil - video integrator. It is normally supplied with 2 channels which can operate simultaneously in frequency diversity. A dual beam arrangement reduces

'angels' phenomena and enhances MTI performance without degrading low coverage.

SOUTH AFRICA HAS ORDERED THREE AT C R - 2 T R A D A R S

- i SELENIA INDUSTRIE ELETTRONICHE ASSOCIATE SpA RADAR DIVISION ROME ITALY

Corporation Members of the International Federation

of Air Traffic Controllers' Associations AEG-Telefunken, Ulm/Donau, Germany The Air Transport Association, Washington D. C., U.S.A.

Wolfgang Assmann GmbH., Bad Homburg v.d.H. Compagnie Generale de Telegraphie sans Fil Maiakoff, Paris, France

Cossor Radar and Electronics Limited, Harlow, England The Decca Navigator Company Limited, London ELLIOTT Brothers (London) Limited Borehamwood, Herts., England FERRANTI Limited

Bracknell, Berks., England Glen A. Gilbert & Associates, Washington D. C., U.S.A. IBM World Trade Europe Corporation, Paris, France

International Aeradio Limited, Southall, Middlesex, England ITT Europe Corporation, Brussels, Belgium Jeppesen & Co. GmbH, Frankfurt, Germany The Marconi Company Limited Radar Division Chelmsford, Essex, England N.V. Hollandse Signaalapparaten Hengelo, Netherlands N.V. Philips Telecommunicatie Industrie Hilversum, Holland

The Plessey Company Limited Chessington, Surrey, England Selenia - Industrie Elettroniche Associate S.p.A. Rome, Italy The Solartron Electronic Group, Ltd. Farnborough, Hants., England Texas Instruments Inc., Dallas 22, Texas, USA

Whittaker Corporation, North Hollywood, California, USA The International Federation of Air Traffic Controllers' Associations would like to invite all corpora tions, organizations, and institutions interested in and concerned with the maintenance and promo

tion of safety in air traffic to join their organization as Corporation Members.

Corporation Members support the aims of the Federation by supplying the Federation with technical information and by means of an annual subscription. The Federation's international journal "The Con

troller" is offered as a platform for the discussion of technical and procedural developments in the fi e l d o f a i r t r a f fi c c o n t r o l .

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THE CONTROLLER Frankfurt am Main, January/March 1970

Volume 9 • No. 1

P u b l i s h e r : I n t e r n a t i o n a l F e d e r a t i o n o f A i r T r a f fi c C o n

trollers' Associations, S. C. 11; 6 Frankfurt am Main N.O. 14, Bornheimer Landwehr 57a.

Officers of IFATCA: M. Cerf, President; J. R. Campbell, First Vice President; G. Atterholm, Second Vice Presi

dent; G. W. Monk, Executive Secretary; H. Guddat, Honorary Secretary; B. Ruthy, Treasurer; W. H, Endlich. Editor.

Editor: Walter H. Endlich, 3, rue Roosendael, Bruxelles-Forest, Belgique Telephone: 456248 Publishing Company, Production and Advertising Sales Office: Verlag W. Kramer & Co., 6 Frankfurt am Main N014, Bornheimer Landwehr 57a, Phone 434325,492169, Frankfurter Bank, No. 3-03333-9. Rate Card Nr. 2.

Printed by: W.Kromer&Co., 6 Frankfurt am Main NO 14, Bornheimer Landwehr 57a.

Subscription Rate: DM 8,— per annum (in Germany).

Contributors are expressing their personal points of view

and opinions, which must not necessarily coincide with t h o s e o f t h e I n t e r n a t i o n a l F e d e r a t i o n o f A i r T r a f fi c

Controllers' Associations (IFATCA).

IFATCA does not assume responsibility for statements made and opinions expressed, it does only accept re sponsibility for publishing these contributions.

Contributions ore welcome as are comments and criti

C O N T E N T S

cism. No payment con be made for manuscripts submitted for publication in "The Controller". The Editor reserves

the right to make any editorial changes in manuscripts, which he believes will improve the material without altering the intended meaning.

Written permission by the Editor is necessary for re printing any part of this Journal.

A u t o m a t i o n i n A i r Tr a f fi c C o n t r o l

S. Ratcliffe, B. Sc.

Passive Horn May Clean Up Radar Displays Tirey K. Vickers I FAT C A 7 0

Lasers to Determine Visibility at Airports R. L. Burr and E. T. Hill Mathematical Models for the Prediction o f A i r Tr a f fi c C o n t r o l l e r W o r k l o a d

S. Ratcliffe, B. Sc. Advertisers in this Issue: Decca/HARCO (Back cover); ELLIOTT Space and Weapon Automation Limited (Inside back cover); IFATCA 70 (14, 15); Ferranti Limited (13); Marconi Ltd. (2, 3); Standard Elektrik Lorenz (30); Selenia S.p.A. (Inside Cover).

Picture Credit: NASA (24, 25, 26); Plessey Radar Ltd. (16); Ratcliffe (20); Vickers (11, 12, 23, 25, 27).

Book Review

The Improvement of Wet-Runway Operations Tirey K. Vickers Terminal Area Approach Control Sequencing E. Crewe and T. E. Foster I FAT C A A d d r e s s e s

A u t o m a t i o n i n A i r T r a f fi c C o n t r o i Paper to the United Kingdom Symposium "Electronics for Civil Aviation", 1969* S. Ratcliffe, B. Sc. Royal Radar Establishment, M a l v e r n , Wo r c e s t e r s h i r e

Introduction

It is easy for aviation and electronic engineers to look on air traffic planning as a tradition-bound organisation that will adapt itself to exploit technological advances only with reluctance and under extreme pressure from growing traffic. Members of this school of thought point out that most features of the present day air traffic system are dictated by the weaknesses of various electronic aids which were developed without clear guidance from the

the much greater flexibility of the device. The advent of Autoland, for example, now faces the planners with only one major decision, whether to fit. If a decision is taken to introduce a digital computer into an air traffic process, there are an immense range of roles that the device could play. In practice, the choice of the role for the computer is

inextricably bound up with arguments about reliability. It is proposed, therefore, to discuss this topic before proceed ing further.

ATC authority as to the precise operational need that was to be met, and which were often intended primarily for some quite different application.

Reliability

Basically, the difficulty stems from the standards of reliability which are expected of civil aviation in general

A computer is only worth installing if it is to serve a useful purpose. The more useful the purpose, the greater

and air traffic control in particular. These are very high,

and it has been argued (1) that with the growth of traffic they should become even higher. Electronics designers must face the serious implications of this demand. It is easy

proudly to point to some brave new device which avoids

the well-known drawbacks of an established technique, but the hard-bitten air traffic planner may be justified in his refusal to exchange the "devil that he knows" for the lesswell-tested and therefore dangerous novelty.

Aviation and electronics techniques have grown up

together and have persistently exercised a significant in

fluence on each other. Despite everything said above, elec

tronic developments tend to match the operational require

the penalty when the machine fails. In most ATC applica tions, a significant hazard will then arise. If it is proposed to overcome this difficult by "reversion to manual", three conditions must be fulfilled: —

(i) There must be available at all times an adequate num

ber of control staff with the necessary training and adequate recent experience in the appropriate arts. (ii) These staff must always be familiar with the current situation.

(iii) The computer must not be allowed to use its powers to generate a traffic situation which the human control lers cannot safety unscramble.

to air traffic control, the situation is very different. The

Since such a system contains the staff who are capable of dispensing with the computer, the simplest way of ensuring that they preserve their skills and stay in touch with the traffic situation is not to install a computer at all.

If this solution is not adopted, it Is necessary to call for a computer system with a probability of failure so low that

explosive growth of computer technology has produced a

one can tolerate a relatively hazardous situation during

situation where adequate reliability statistics are only

the down time of the system, or, at least, until the operating

available for machines using obsolete techniques, and where air traffic control represents only a small corner of

rules can be changed to ease the load on the defective system. The flight plan processor now being installed at LATCC West Drayton, for example, has to meet a speci

ments of civil aviation. The willigness of the military to

adopt new techniques and to use them on a large scale has made an invaluable contribution to the building-up of con

fidence In many innovations.

When we come to discuss the applications of computers

the potential market for digital devices, and cannot hope significantly to influence the broad lines of either hardware or software development.

There are advantages as well as disadvantages in this situation. The existence of a broad-based market for com

puters helps to keep down the cost of developing both

fication that only once in five years allows a loss of service for more than 30 sees, or an undetected error in a single c h a r a c t e r.

Such a stringent requirement poses serious problems —

hardware and some of the more basic software. System

not only for the equipment designer. Consider, for example, the problem facing the authority conducting the accept

weaknesses which lead to subtle and not readily detected

ance tests on the equipment of the previous paragraph.

hazards in ATC may prove much more conspicuous in other

Even if it were possible to build a single non-redundant

applications of the same machinery.

computer having an adequate reliability, a successful ac

Another striking difference between the digital com

ceptance test could hardly take less than ten years or so.

puter and nearly all other aids to air traffic control is in

In practice, the proposed system is not based on a single computer, there are several operating channels, enough

' Reprinted with kind permission of the author and the U.K. Ministry

comparison mechanisms to detect a failure, and enough redundancy to enable the system to survive one or more

of Technology

6

equipment failure. This approach may be much quicker and less expensive than one that requires the development of

Given the inevitable residual uncertainty about the

long-term reliability of such systems as are within the pre

components to reliability standards far higher than "good commercial practice". If the customer is prepared to accept arguments based on the independence of failure me chanism, it is possible now to calculate the probability of a system failure from the much higher, and therefore measur able, figures for components failures.

The failure-survival redundancy is commonly applied at computer level (2). Whereas in automatic landing, say, it is necessary to triplicate virtually the entire control system,

sent state of the art, the case for introducing computers into the system may be more subtle. Certainly there is no

advantage is often token of the flexibility of the stored-

puters into ATC. These include the need to build-up opera tional experience of automated systems before rising traffic

programme digital computer to enable a spare machine to sand-in for more than one main-line computer. It is possible, to some extent, to re-apportion roles between the surviving apparatus to ensure that if the number of faults builds up to the point where operational efficiency must suffer, the blow will fall first on the least important func tions of the system.

The weakness of this approach lies in the complexity of the data highways, the switching system, and the rest of the hardware and software involved in transferring data and programmes between computers. In these areas, amongst

others, there are possibilities of systematic failures which can invalidate the assumptions on which the probability of a system failure were calculated.

An experimental "reliable" computer exists in Vv'hich

the redundancy is applied at a lower level. Error-correct ing codes are used to check that words are stored and

transmitted correctly. Any failure should result in a fairly precise indication of the location of the fault, since both

the logical element and the faulty digit can be specified.

Since there is no known way of performing arithmetic operations on an error-correcting coded number whilst pre serving the error-correcting property, this technique cannot

be used throughout the computer, and other techniques, such as triplication, are used in limited areas. This scheme

offers the possibility of building a reliable computer with less than twice the number of components that would be

required in its "unreliable" counterpart. As always, the danger lies in the risk of several non-independant and near-simultaneous failure, such as might be triggered by a fault which caused a small fire.

A separate problem is the risk of a flaw in the pro gramme logic. In a complex on-line system it is nearly im possible to test all combinations of circumstances, and several years is needed to establish confidence in the soft

ware. It is improbable that, in practice, such a period could elapse before changing circum.stances made it necessary to modify the programme, each modification bringing a new risk of error. In the present state of the art it may be that this problem sets a bound to the confidence one can

have in an automated system.

Choice of area to be aufomafed If computers and their associated programmes were foolproof, possible reasons for introducing computers into air traffic control might be: — (i) to save manpower by taking over some of the tasks presently performed by human controllers or their assistants, (ii) to eliminate human errors,

(iii) to perform tasks which are beyond the capability of a team of human controllers working under time press ure.

evidence to show that any saving in m.anpower has, so far resulted from the introduction of computers into ATC, and so long as decisions are taken by human controllers and communication with the aircraft is by voice, there is still plenty of scope for controller errors, together with contribu tions from the machinery and the programmers.

There are, however, other grounds for introducing com

makes them essential.

For one reason or another, many ATC authorities are now using or experimenting with computer systems. The tendency has been to begin by introducing a machine to print flight progress strips. This is a relatively easy task, there is no obvious difficulty in reverting to manual opera tion, and input of flight plans to the computer is a necessary

step towards most more advanced proposals. One possible line of further develepment is to delay print-out of flight progress strips until a departure message is received, or even until nearer the ETA at the appropriate reporting

point, to give on indication of procedural conflict with

other aircraft, or even to suggest modifications to resolve the confliction (3).

Experience has taught that the superficial simplicity of this task is deceptive. Particularly troublesome mechanical problems ore involved in delivering a legible strip, loaded in its holder, silently and rapidly to the working position.

The requirement for delayed printing of the strip leads to a demand for very high integrity in the flight plan storage system. In the Flight Plan Processing System at

West Drayton the specification lays down that a failure involving damage to a single character or a break in ser vice lasting more than 30 seconds must not occur more

often than once in 5 years. To meet this specification it is proposed to duplicate most features of the installation and to triplicate the central facilities such as the computer

system. There are also programming complexities which

arise from the need to detect faults, to reconfigure the faulty system, and to retrieve the damage data.

At least one attempt has been made (4) to extend the automation process to provide a travelling printing head which can update flight progress strips after they have been placed in position in front of the controller. To pre

serve the controllers' freedom to rearrange his strips, it is necessary to code the strip or its holder to enable the computer to sense the location of a given strip on the dis play.

An alternative approach to the problem, is to replace

the printed paper strips by a multiplicity of electromecha nical indicators. This lends itself more easily to numerous

revision of the flight plan data, but in practice, controllers found it difficult to read the strips (5) or to input revisions,

and rearrangement of flight progress strips is a rather cumbersomie process.

Both these schemes are open to the criticism that in ATC systems which are, in practice if not in theory, radar based, it is not worth introducing this degree of complexity into

mechanising the procedural data processing alone. The Apollo scheme (3) escapes this criticism because the U. K. authorities chose to begin their experiments in the Shanwick centre controlling N. Atlantic traffic, not because this pro-

blem was most acute, but because the low data rate and the absence of radar minimised the complications.

Systems based on printed strips or electromechanical indicators have the m.erit that even a major electronic

catastrophe is unlikely to damage more than a very small amount of displayed data. The advent of the touch-wire display technique (6) and advances in CRT character-writ ing (9) have led to a weakening of the objections to the use of CRT's for the display of procedural data. CRT displays have the added advantage of being compatible with radar

data display systems, and the two sets of data can even be merged on a single display.

Just as the automation of a procedural system starts

placed by a digital automatic link. The use of such a link for routine messages is technically feasible but may be some way off for other reasons.

We have already discussed the processes for getting basic ATC data into the computer. Detection of procedural conflicts presents no particular problem. Simple pro grammes for the automatic detection of radar conflicts are not very helpful, even when height information is available to the computer. Unless adequate "intention" data can also be provided, the conflicts search must be based on worstcase assumptions, which lead to a prohibitive false-alarm rate. This is precisely the problem that has for many years held back the provision of an airborne collision warning

with computer storage of flight plans, so the automation of

device.

a radar-based system starts with computer storage of air

In the absence of automation, most of the operations involved so far, including elementary conflict checks are, or

craft tracks. Although manual tracking techniques are used in certain limited applications (7) the manpower required to track large numbers of aircraft usually drives designers to the conclusion that it is necessary to provide automatic

tracking. The problems involved in this process are dealt

with in another contribution to this symposium (8) and are only indirectly relevant to the present paper.

The starting point for the exploitation of the radar track data is its "association" with flight plan data. The computer

system can be used to drive a "labelled radar display" in which one or more computer-driven symbols are super

imposed on the raw radar display, or, alternatively, the computer may drive a "synthetic display" based entirely on computer-driven symbols (9). Because of doubts about the integrity of the data and the reliability of the apparatus, most designers prefer the "labelled radar display", on

could be, performed by an ATC assistant. Since the con troller needs to be familiar with the situation, however, there is a case for involving him in at least some of the

clerical operations, as a means of impressing the data on his memory. Automation has, so far, done little to ease the controllers* lot, therefore.

The problems of data transfer between controllers are

interesting. In a system of only moderate complexity, the layout of the control centre mimics the route structure, so that controllers of adjacent sectors sit shoulder to shoulder.

This enables them easiliy to converse or to consult, or

amend even, each others' data. In a large TMA, for ex ample, where traffic from several airways may be merging

avoid both automatic association of tracks with flight plan

at a common holding point from which each aircraft is released to one of a number of destinations, such a con venient layout is no longer possible, and recourse must be had to telephones, closed circuit television, or other data transmission techniques. Similar problems occur at the

data and the use of flight plan data to aid the automatic

interfaces between different control centres and between

tracking process.

different control agencies.

which the controller can personally check the credibility of

the computer tracks. For similar reasons, most designers

Up to this point, the case for introducing computers has

Telephone communication between controllers presents

rested on the intuitive argument that automation is bound

certain problems. Each controller must maintain a listening

to be needed in the end and that we must begin to get

watch on his R/T channel. Such channels are quite frequently loaded up to 50% of theoretical capacity. If an attempt is

experience of the operational problems. The processes of getting the data into the computer are inescapeable, but at some stage In the process a large number of alternative

then made to phone the controller, there is an even chance that he cannot answer at once because of the R/T. For a

paths open up before the system designer, and it is ne cessary to choose which of the controllers many tasks can

pair of controllers working under these conditions, there is only a 25% probability that both will be free at a given

profitably be automated.

In an ideal world, there might be a formula which made

possible the prediction of the workload which a given ATC system imposed on each controller. It might then be pos sible to compare the costs of various control system con

figurations having different degrees of mechanisation, and

hence to perform a convincing cost/benefit exercises. In reality, quantitative prediction of controller workload is not possible (10), and such modelling and operational gaming techniques as are available (11) are commonly used mainly as props to support an intuitive decision, previously arrived at, rather than to make a more objective choice of the approach to the problem.

The main tasks facing a controller can be broken down

instant. Controllers can, to some extent, evade this diffi culty by passing messages through their assistants, who

act as buffers to hold the message till they can capture the controllers' attention. In a mechanised system, the more

routine messages, such as estimates or offer or acceptance of a handover, can be puffered in a computer.

Inter-controller liaison problems are a crucial factor in

decisions to automate. With the growth of air traffic, all the problems concerned with aircraft one at a time can be

handled either by increasing the control staff or, possibly, by automation. The problems of testing a given aircraft for conflict are proportional to the traffic density in the vicinity, and the problems of resolving any conflict rise also with the number of controllers involved. The controllers on

into the following headings; —

watch at a given time already tend to outnumber the air

(i) voice transfer of data between pilot and controller, (ii) data transformation, extrapolation, conflict detection,

craft under their control, and there is clearly an upper limit

(iii) data transfer controller-controller, (iv) planning, including conflict resolution.

given sector.

In the present state of the art, there is no immediate possibility of mechanising (i) unless the voice link is re 8

to the number of controllers who can share the tasks in a

It is possible to organise the control system so as to minimise the need for inter-controller liaison, by dividing the airspace and runways into reasonably watertight

compartments. For example, all traffic inbound to Heath

row from the North might use runway 28R, and all traffic from the South might use 28L. Similarly, at an intersection such as Dunsfold, traffic inbound to Gatwick might be con fined to 2,000 ft., Gatwick outbounds to 3,000 ft., Heathrow outbound traffic to 4,000 ft., and traffic inbound to Heath row via Epsom to 5,000 ft., or above. By constraints of this type, one can minimise the need for inter-controller liaison, but only at the expense of some loss in capacity.

Delays in an ATC system, assuming it is not hopelessly overloaded, arise because of the irregularities in the

demand for certain facilities. If the traffic is split up into a number of separate compartments, these irregularities, and hence the delays, are aggravated. For the Heathrow in bound traffic quoted above, a more regular demand on the landing runways would ensue if arriving aircraft could be assigned to either runway on a flexible basis. Simulation studies at the Royal Radar Establishment and by the Avia tion Operational Research Branch, Board of Trade, suggest that a suitable strategy could increase the landing capacity

at Heathrow by at least 10%, with a smaller but significant reduction in departure delays.

It does not seem likely that controller/controller liaison con be much improved by further automation of most

existing systems. It is sometimes agreed that the role of automation is to relieve the controller of his more humdrum

tasks, thus freeing him to spend his time on the decision-

(13, 14) has been used to solve both this runway problem and a more generalised version where each aircraft has a choice of two runways (15).

In the digital computer simulation of large ATC systems (11), it becomes necessary to provide automatic solutions to all, or nearly all, of the conflict problems that arise. It is too much to claim that these programmes produce perfect solutions, but there is evidence that some of these program.mes are, at least, com.parable with those produced by human controllers working under time pressure.

Techniques for solving the planning problems con be split into two categories. The first, here termed the algorith mic method, conducts on exhaustive search of all the pos sibilities with a view to finding the optimum solution. The second, termed the heuristic method, uses some rules of thumb, based on human experience, to find on adequate, even if not optimum, solution.

The difficulty with the algorithmic approach is the numiber of possible olternotivs. Exhaustive enumeration is out

of the question. Consider, for example, the runway problem mentioned above. If the programme looks only 12 moves

ahead, there are 2^^ 12! or about 2 X 10^^ possible solutions.

By the "branch and bound" technique, it is possible to find the optimum solution without exploring more than a very small fraction of the possibilities in detail. By suitable pro

gramme organisation and by storing intermediate results it is usually also possible to ovoid calculating the behaviour

taking processes. Since the computer cannot remove the need for the controller to familiarise himself with the data, the saving in time may be quite small. In any event, it is

of each sequence a b initio. The experimental airport

d i f fi c u l t t o b e l i e v e t h a t w h a t a m o u n t s t o a c o m m i t t e e o f

situation to a depth of 8 moves ahead. Some increase in

about 50 controllers could plan movements through a large terminal area at a rate of, say, 160 movements/hr. except by virtue of on extensive set of constraints on the possible solutions.

An alternative approach is to attempt to use a computer to devise the brood overall plan for the traffic movements

in the next ten minutes or so, leaving the controllers responsible for the safe implementation of the computer's plan. The possibility of using computer planning has been demonstrated in the course of work on large scale fasttime computer models of air traffic systems. The problems involved in this approach will form the main topic of the rest of the present paper.

Automated planning Much of the fundamental work on planning by computer

takes the form of attempts to programme a machine to play games whose rules and objective are explicitly for mulated. Some of the more highly developed computer programmes can play games quite well. There exists a

draughts playing programme (12) which is nearly up to the standard of a minor champion.

Another problem — not a game — which has received attention in recent years is the "Travelling Salesman". It is

required to find the round-trip path through a sucession of points which will minimise the total cost, the cost of travel beween each pair of points being provided as input data.

This problems bears a very close resemblance to that of deciding on the best arriving and departing given runway, "best" order which minimises technique known as

order in which a waiting queue of aircraft should be allowed to use a being defined, for example, as that the total delay to all the aircraft. A "backtrack" or "branch and bound"

programme, using these techniques, can run at about real

time on a medium speed computer whilst exploring the speed is no doubt possible, but It seems unlikely that a pro

gramme of this type will ever be capable of handling the planning problems of a whole terminal area.

The "heuristic"approach to this problem, commonly used in large-scale computer simulation of ATC systems,

uses a set of rules based on controller experience. It is usually necessary to go through a protracted process of programme testing and modification as the controller is

confronted with the consequences of what he formerly be lieved to be the rules on which he acted. Eventually, such programmes can give quite satisfactory results and are fast in operation. There ore two drawbacks to this techni

que. Firstly, there is no yardstick by which the efficiency of the process con easily be judged, and, secondly, there is a danger that the laborious process of programme deve lopment will have to be repeated whenever there is a

change in route structure or mode of operations. Such changes are relatively frequent in ATC, and it will probably prove essential to modify the programme in the field.

In an algorithmetic programme such as the runway

programme quoted above, the rules of operation are all incorporated into a data array. Since the actual problem solving method mokes no use of "common sense", it is

improbable that a change in the rules will invalidate the problem solving mechanism, though it may change the speed with which the programme can run.

The discussion in this section of the paper has, so far been confined to the solution of idealised problems, where the "rules of the game" are precisely defined, where the situation is deterministic, i. e. the future can be precisely

predicted from on adequate knowledge of the present, and where there is some convenient yardstick which indicates the relative merit of two alternative solutions. In the real

world, it is difficult to do more than approximate to these 9

conditions. The runway programme, for example, whilst producing a very whortwhile reduction in the total delay, had an irritating tendency to impose a quite preposterous

interactions take place mainly, if not exclusively, through the computer system. This is an area where considerable experimental work is needed.

delay on some isolated aircraft, a light aircraft in a sequence of jets, for example. If automated planning is to play a part in live ATC, it seems clear that provision must be mode for human intervention, if only to cope with emergencies and other situations where the account must be taken of factors beyond the comprehension of the

Acknowledgement Contributed by permission of the Director R.R.E. Copy right Controller H.M.S.O.

planning programme. Consideration of the problems of introducing automated planning into ATC suggests that, even if the above argument did not apply, it would be necessary, until the necessary experience had been gained

References

and confidence built up, to start with a system in which the

1. Lundberg, B. K. O. "The Allotment of Probability Shares

decision-taking task was shared in some flexible manner, between the computer and the controller.

Method, a Guidance for Safety Measures in Aviation".

Proceedings of Symposium on Civil Aviation Safety. Swedish Society of

Controllers and computers A certain amount of nonsense is talked about ''man

Aeronautics.

Stockholm April 1966.

2. McLachlan, W. L. "Redundancy in Ground Data [Hand

versus machine". As ref. 17 points out, the principle involv ed in "automatic" control is that part of the control task is

ling Equipment for Air Traffic Con

performed by a human being planning in advance, by a

Symposium on "The Use of Redund

computer programmer for example. The remainder of the control task is performed by the controller working "on line". Viewed in this light, the advantage of introducing computers into the planning process, for example, is that we can bring human effort to bear on a co-ordination problem on a scale and at a speed that would be otherwise impossible. The main drawback, as Majendie (18) points out, is that the system may have been intended for a task other than the one with which it is faced.

The real difficulty is to formulate a sufficiently com prehensive operational requirement and to arrive at an economical division of effort between the computer pro grammer and the on-watch controller.

Reference 16 describes some experiments on the Travelling Salesman and other problems in which a compa rison was made between unaided human operators, operator-computer systems, and a fully automatic, heu

ristic, system. The heuristic programme nearly always produced the best solution, eventually, but a dramatic in crease in speed is possible if use is mode of a man's ability to produce at least a rough solution nearly instantaneously.

The authors point out that the Travelling Salesman problem is one which has been the subject of considerable research, and that for most problems the man-machine system is likely to prove far more satisfactory.

For our real life problems, this argument applies with irresistable force. It may be helpful to think of the control ler as playing a game on the computer. The game is based on a possibly rather idealised version of the real-life situation, so that the result of the game is regarded as a guide to action rather than a mandatory decision. Work is in progress on a Computer Aided Approach Sequencing system which is Intended to incorporate some of these ideas. In particular, it is planned to provide computer guidance as to the best sequence of runway events.

In the experiments of ref. 16 only one man was convers

ing with the computer. In the CAAS system, about four controllers will eventually be involved, although the inter

actions between them need be extensive. If an attempt is made to extend the technique to more general planning problems, we must face a situation in which the controllers' 10

trol"

ancy in System Design".

Society of Instrument Technology, London February 1964.

3. Cherry, fH. "Evaluation of Computer Facilities for Oceanic Control"

3rd International Aviation R&D Symposium,

Federal Aviation Agency.

Atlantic City, November 1965.

4. Moraski, J. J. "ATC Data Processing Central" J o u r n a l o f A i r Tr a f fi c C o n t r o l

April 1959.

5. Smit, J.S. "Survey-of Experiences with the SATCO-System"

3rd Aviation R&D Symposium, Federal Aviation Agency.

Atlantic City, November 1965.

6. Johnson, E. A. "Touch Display — A Programmed Man-Machine Interface"

Ergonomics 10 2 pp. 271—277, March 1967.

7. Martin, D. A. "Application of a CAAS System" 17th lATA Technical Conference.

Lucerne October 1967.

8. Ord, G. "Automatic Tracking in ATC" This Symposium.

9. Evans, D. R. "Displays for ATC" This Symposium.

10. Ratcliffe, S. "Mathematical Models for the Pre d i c t i o n o f A i r Tr a f fi c C o n t r o l l e r Workload"

This Symposium.

11. Laite, P. J. "Computer Simulation in ATC Sy stems"

This Symposium.

12. Samuel, A. L. "Some Studies in Machine Learning, Using the Game of Checkers"

I.B.M.J. Res. Dev. 3, 210—229, July 1959 Continued in:

I.B.M.J. Res. Dev. 11, 601—617, November 1967.

13. Little, J. D. C. et al. "An Algorithm for the Travelling

Michie, D.

Salesman Problem"

Operations Research 11, 972—989, November 1963

14. Golomb, S. W. "Backtrack Programming" X Ass. Computer Machinery 12,

Edinburgh University Press 1968.

"Human Error and Accidents"

516—524, October 1965.

1 5 . Ratcliffe, 15.

R a t c l i ff eS. , S . " A u t o m a t i c S o l u t i o n o f I M A AT C Assignment Problems" 17th lATA

"A Comparison of Heuristic, Inter active, and Unaided Methods of Solving a Shortest-Route Problem" Machine Intelligence 3.

Design No. 116 August 1958.

(Council of Industrial Design).

Majendie, A. M. A.

Te c h n i c a l C o n f e r e n c e

"The Human versus the Automatic

Navigator" J. Inst. Navgn. 13, 1, January 1960.

Lucerne October 1967.

Passive Horn May Clean Up Radar Displays

by TIrey K. Vickers Senior Consultant

James C. Buckley, Inc.

On June 20, 1969 the US FAA signed a contract with Raytheon Corporation for the modification of an ASR

antenna, utilizing the passive horn principle. The main ob jective of this development is to eliminate most ground

FEED

HORN

clutter from the radar returns which are fed to the receiver.

Thus it should greatly assist the MTI circuitry in providing a clean, clutter-free radar picture.

With a conventional radar antenna, as shown in Fig. 1, the radar energy from the transmitter proceeds through a waveguide to a feed horn, which bounces it off a reflector screen, to focus it into the desired radar beam (Pattern A in Fig. 2). Part of this energy bounces off aircraft and ground targets, and returns to the reflector screen, which focuses it into the feed horn. From there it travels back

down the waveguide and into the receiver.

A characteristic problem of conventional radar installa tions is to get the beam low enough to provide good cover age of the lower altitudes at maximum range, without re flecting too much energy off the terrain and obstructions close to the antenna. Such reflections can produce very strong returns at short ranges, as shown in Fig. 2. Some of these returns may be too strong to be cancelled out by the MTI circuitry. The resulting ground clutter may obscure air craft targets at short ranges.

If the tilt of the radar beam is raised to the point where the ground clutter is eliminated, then the nose of the beam is well above the altitudes used by air traffic. This has the effect of reducing the maximum range of the radar for air

RECEIVER M-fCIRC.H—fT/R

TO

D I S P L AY S

SYNCHRON IZER

4 TRANSMITTER

traffic control purposes. Consequently, the beam tilt is usu ally adjusted to some compromise angle (usually 3 to 5 degrees) where the low altitude coverage is considerably less than ideal, but the ground clutter can still be tolerated, or cancelled out by the MTI circuitry.

Figure 1 Simplified block diagram of a conventional radar.

Reprinted from ATCA Journal with kind

of the Editor

11

How then can better coverage be obtained, without greatly increasing the ground clutter? This is where the passive horn comes in. It will be mounted on the antenna assem.bly below the feed horn, and looking upward at a VEFfY STRONG GROUND RETURNS FROM THIS AREA

slightly higher angle, as shown in Fig. 3. This will provide a receiving pattern similar to Pattern B in Fig. 4. The posi tion of the horn in relation to the reflector screen will be

Figure 2 Typical transmitting/receiving paftern of airport surveillance radar.

adjusted so that Pattern B barely grazes the ground with

out strongly illuminating any nearby objects which could cause objectionable clutter.

The passive horn will not be connected to the transmit ter. Instead, as shown in Fig. 3, it will be connected through a diode switch, to the radar receiver. The diode switch will connect the normal feed horn o r the passive horn alter

nately to the receiver. The switch will be adjustable, to flip automatically at desired points in explained below.

As soon as the transmitting pulse horn is switched on to receive returns targets within Pattern B, but blocking

the duty cycle, as is fired, the passive coming from nearby returns from Pattern

A. As Pattern B will be relatively free from close-in ground returns, this portion of the radar sweep will be relatively free from ground clutter.

At a time equivalent to 10 to 15 miles range (depend

ing on the switch adjustment) the diode switch flips auto matically to the alternate position. This blocks the returns from Pattern B but allows the returns from Pattern A to

feed through the normal (feed) horn to the receiver. As the

ground retuns contained in this portion of Pattern A, from the switchover point out to maximum range, are much weaker than those close to the antenna, the outer portion of the sweep will be relatively free of ground clutter. Fig. 5 shows the parts of the patterns actually used.

Using this combination of receiving patterns (Pattern B from zero range out to the switchover point, and Pattern A from the switchover point out to maximum range) it is ex

pected that the number and strength of ground returns reaching the receiver will be drastically reduced. This should make the job of the MTI circuitry much easier, in eliminating objectionable clutter, particularly in instances where the clutter is complicated by precipitation.

A by-product of the passive horn may be an improve ment in radar coverage at higher angles or altitudes close to the station. It is also expected that, since the close-in returns of Pattern A will not be used anyway, it will be practical in many cases to lower the elevation angle (tilt) of the antenna to obtain better coverage of the lower alti tudes at long ranges. This in turn should provide better targets from small aircraft at greater distances from the a n t e n n a .

The passive horn project is part of a continuing inservice improvement program for FAA radars. The project is under the direction of the Radar Systems Section of FAA's Systems Research Development Service, and will require about a year for completion.

If the passive horn development is successful, its retrofit to ASR antennas in the field will be complicated somewhat by the fact that each installation will be a hand-tailored

I SWITCHOVER POINT

job. Radar waves behave like light waves, so improving a radar antenna pattern is basically a matter of optics. Each fitting is a unique job tailored to that specific installation, like fitting a pair of spectacles. Optically, this one is equi valent to bifocals!

Figure 5 Portions of receiving patterns utilized.

12

TKV

Pity the Air Traffic Controller when they come in like this An exaggerated picture perhaps but not for long. Air Traffic is increasing so fast that the controller's j oh needs an entirely new appraisal. And one of the things we've got to look at is the method of training controllers. Is it adequate to meet the demands of the Seventies?

T h e fl e x i b i l i t y o f t h e F e r r a n t i R a d a r Simulator provides the answer窶馬ow and for the future. It gives the trainee con t r o l l e r p r a c t i c e i n A i r Tr a f fi c C o n t r o l under conditions so realistic that when he takes over control of real aircraft he'll

n o t o n l y b e f u l l y t r a i n e d h u t c o n fi dent too.

Digital techniques readily permit mod ifications to accommodate changes in a wide range of parameters, including aircraft type and speed, radar and geo graphical data. Raw radar or fully synthetic output can be provided to drive any type of display. The system can therefore

simulate

new

aircraft

and

procedural techniques not even envis aged at this stage.

Ferranti have the capability and ex perience to design and develop a system to suit any individual requirements. If you have an ATC training or evaluation problem talk to Ferranti.

F E R E W ^ I ATC training systems Ferranti Limited, Digital Systems Department, Bracknell, Berkshire, England. RG12 IRA

DS21/2

i t s M § c a To MONTREAL,

M AY 1 9 7 0

T h e I n t e r n a t i o n a l F e d e r a t i o n o f A i r T r a f fi c C o n t r o l l e r s

Association, will hold its 1970 annual conference in the Q u e e n E l i z a b e t h H o t e l a t M o n t r e a l o n M a y 11 - 1 4 .

It promises to be the best conference yet. The programme will have an e x c e l l e n t b l e n d i n g o f s e m i n a r s o n a l l AT C a d v a n c e m e n t s a n d p r o b l e m s , w h i l e on display will be the latest electronic equipment available, manufactured by the worlds' leaders. All this, coupled with the off hours of enjoyment that o n l y M o n t r e a l c a n o ff e r.

I f y o u a r e i n t e r e s t e d i n J o i n i n g t h e c o n f e r e n c e a s a d e l e g a t e o r o b s e r v e r, t h e n fi l l o u t a n d m a i l i n t h e a t t a c h e d f o r m .

IFATCA 70 is actively being supported by: A.

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15

Lasers to Determine Visibility at Airports

by R. L. Burr and E. T. Hill Radar Equipment Division, Plessey Radar Limited.

Aircraft and airports are extremy costly to rund. Clo

sure of major airports and aircraft diversions cost millions of pounds every year and are mainly attributable to fog.

To minimise this loss, aircraft tend to operate in the worst

possible visibility conditions commensurate with the stipu

lated safety regulations. These safety regulations are very

ments at all airports where Category II conditions are now

operative. However, monitoring RVR alongside the run way is no precise guide to the visibility conditions existing along the glide path itself. Comparatively clear conditions on or near the ground can be accompanied by poor visibi lity tens of feet above the ground and the converse can

also occur. Visibility can decrease as the pilot loses alti

stringent but even so accidents con still occur.

The recognised solution to this problem is Instrument

tude resulting in complete loss of visual contact with the

Landing System (ILS), and with the ultimate instrument, safe, fully automatic landings in zero visibility will be pos

ground below his decision height — typically 200 feet up.

The need to monitor Slant Visual Range (SVR) from the

sible. However the ultimate ILS instrumentation is not yet

ground is equally, if not more important than RVR. Unfor tunately the latter is easier to achieve than the former.

available and for the next ten years or so, the majority of ILS landings will be semi-automatic with ILS guidance down to 100 ft followed by manual control to touch-down. The essential features of a runway equipped for semi-automatic

Current Category II ILS systems are only reliable down to 100 ft, where the pilot is committed to the landing and must be able to maintain visual contact with the ground

landings are shown in Fig. 1.

for azimuth guidance. The pilot must know before commit

The International Civil Aviation Organisation has evolv ed three categories of ILS equipments, these being outlined

ting himself to the final landing that he will be able to see a sufficient landing light pattern during the final approach stage to land safely. In particular, during the final 300 feet of descent he must know the heights at which his descent will take him into fog and impair his vision and at which

in Fig. 2.

For categories other than III (C), accurate measurements of Runway Visual Range (RVR) and cloud base are essen tial for category classification.

he will see sufficient ground or guidance lights. A pre-

Prior to complete blind landing systems being available, there is increasing pressure on airport authorities where traffic densities are high, to progress rapidly to Category II operations. It is mandatory to have automatic RVR equip-

knowledge of adequate RVR is also essential to ensure satisfactory roll-out and final taxiing to the disembarka tion point. RVR is also of vital concern during the take off phase.

Aircroft decision height

^t. I ceiling RVR 800

^prox.limit of reliable ILS guidance. Pilot takes cantral of azimuth painting in autaflare

Pilot takes over from

" -^^wCat. II ceiling 400 m RVR Autoland ,after touchdown ^Floreout starts

Threshold / 3°

To u c h d o w n

800m RVR Cotl

300 m (1000 ft.)

I 400m RVR Cot.I ▶

600m

Approach lights F

loreout

RVR

Monitors

ISOm^ J

To u c h d o w n

Cloudbase monitor

Runway lights

Figure 1 A typical instrumented runway.

16

To date, RVR has been estimated by trained observers at the threshold counting the number of runway lights vis

computations are required to convert the measured ex tinction coefficient to the RVR values required by the pilot.

ible to them. As a method it is subject to human error and does not lend itself to being automated as is required by ICAO for Category II operations.

Several automated methods of measuring RVR have been proposed including the use of closed circuit TV came

Although the assessment of RVR is perhaps fairly in volved, the optical instrumentation is relatively simple. To

ras, but it is now generally accepted that the transmissiometer is the preferred instrument. The transmissiometer uses

a light source at one end of a baseline and a detector at the other end. The loss of light flux per unit length between the transmitter and the receiver is known as the ''atmo

spheric extinction coefficient". This loss is primarily due to scattering at visible wavelengths.

The ability of a pilot to see the lights Is a function of the eye's sensitivity and the minimum contrast it can discern

between the apparent brightness of the light source and the brightness of the background. The problem is made more difficult due to the considerable variations in background

assess SVR, the optical instrumentation now becomes diffi cult. An RVR type system using a mast on which are mount ed a number of light sources at various heights could be used. Receivers on the ground then measure the vertical profile of the extinction coefficient. For obvious reasons, the most cannot be placed on the glide path nor con its

height be greater than about 100 feet. For stable and hori zontally homogenous fog conditions, vertical profiles up to 100 feet can be obtained and extrapolated to 300 feet.

This data in conjunction with cloud base data etc. can give good results in terms of contact height sequence. However, during patchy fog or the formation stages of fog, forecasting becomes Other systems such as flares, burning on the extendable masts, lights on balloons etc. have

and visual lifting and unreliable. glide path, been con

brightness and hence visibility which occurs between day and night even for identical fog conditions. In practice RVR is defined as the maximum range at which the runway lights

sidered but hardly seem to offer safe and dependable

can be just discerned by the pilot.

RVR is a function of the atmospheric extinction coeffi

scatter from optical beams are now being investigated. It is known that the passage of light through most unpolluted

cient, the sensitivity of the human eye under the prevailing conditions, the level of the background illumination and the power of the runway lights. These four parameters are linked together by Allard's Law. In an instrumented system, the extinction coefficient is the only parameter measured, the other three being pre-set. Thus RVR can be determined

fogs is attenuated almost entirely by multiple scattering.

and passed to the pilot.

Two basic types of transmissiometer for RVR measure ment exists. The first uses a light source and a light receiver

separated by a straight baseline. The second uses a com parison technique, the light source and detector being mounted close together in a common unit and the base line folded back by an optical reflector. This arrangement allows a reference signal to pass directly between the

characteristics.

Indirect methods involving the measurement of bock-

The main problem is to establish a relationship between

this scattered light and the atmospheric extinction coeffi cient. The light bock-scattered can be monitored by a single

ended instrument combining transmitter and receiver. In struments of this type exist using pulsed light sources, but in general the light sources ore not powerful enough.

Effort is now being directed to the improvement of these systems and in particular towards the use of laser radars (lidars). Lidars can be used in a variety of ways, the most promising enabling the lidar to scan a volume of fog and determine its spatial distribution. Precautions need to

be taken however to ensure that these high power laser beams do not illuminate any aircraft on final approach be

transmitter and detector to provide a continuous monitor

cause of the possible eye hazard to the crew. This protec

of light output and detector sensitivity. This obviates the problem of maintaining highly stable light outputs and detector sensitivities necessary in the first system.

tion can be achieved using a simple radar which inhibits

If accurate measurements of RVR are to be obtained

to obtain RVR, SVR and cloud base data makes the laser

over the operational range from 50 to 1500 metres a double

an extremely attractive proposition. A significant saving in

baseline transmissiometer is necessary, utilising baselines

data handling equipment should also result from such a

of approximately 15 and 150 metres. Fairly complex data

concentration of functions within the one instrument.

Ceiling

Break off

height

A

220

metres

The possibility of using a single scanning lidar system

Remarks

Equivalent to non instrumented operations

800 metres

400

the lidar when aircraft are in the hazard zone.

30

m

metres

50 metres

Important, requires RVS, SVR and

Type of landing ISL assisted to 60 m similar to non

assisted landing ISL assisted to 30 m

cloud base monitoring

then manual control to touchdown

Fully automatic landing

Fully Automatic

Fully automatic landing sufficient to taxi

Fully automatic landing too poor to taxi

Fully Automatic

Fully Automatic

Figure 2 Categories of ILS Installations — ICAO. 7

Mathematical Models for the Prediction of Air Traffic Controller Workload Paper to the United Kingdom Symposium "Electronics for Civil Aviation", 1969*

Royal Radar Establishment,

which are deemed to characterise the problem, and which

Summary

Much effort and expenditure has gone into the auto mation of ATC, but we lack quantitative data about the controller workload which automation is meant to relieve.

It is unreasonable to expect more than a rough measure

of workload, however defined. A "mathematical model"

yield a predicted value for the ensuing workload. The algebra should be of the minimum complexity and the parameters of the minimum number necessary to give an adequate fit to the observed data.

Published papers (1, 2, 3, 4) treat the number of air

tnkes the form of one or more algebraic expressions which

purport to predict the workload. One way to construct such

craft under control as the main parameter and express the workload by an expression of the general form: — L = a + bN + cN^ 0)

a model is to break down the controllers' task into a num

ber of rudimentary components, to measure the time spent

where a, b and c vary with the nature of the control task and the organisation. Equation 1 can undoubtedly be

on each sub-task, to multiply these times by suitable weight

ing factors and add to obtain the total load. Alternatively, a workload equation may be arrived at intuitively, and the coefficients adjusted to fit the observed facts.

The paper is a critical review of models so for suggest ed, and of techniques for measuring controller workload.

The author has little confidence in any of the published

results. When faced with a serious overload situation, the controller normally preserves air safety and his own sanity

by slowing down the traffic demand, by one means or

another. In such a situation, tests on controller loading may

be an insensitive method of measuring the traffic delays which are of primary importance.

arrived at by arguing that it is convenient to use a power

series expansion, that a and b are certainly non-zero, that a linear model is inadequate, and that if more than one

additional term is added the experimental evidence (if any)

will be inadequate to determine values for the coefficients.

Techniques, other than pure intuition (2, 4), for the con struction of workload models will be classified, in the

present paper as "synthetic" or "analytic". A synthetic

model is arrived at by assembling, by one method or an other, a list of the various tasks which a controller must

perform, e. g. "conflict search", breaking each task down

into manageable components, and determining the amount

of work involved in each component, and adding the

results, with suitable weighting, to determine the total work.

Basically, this is the technique xysed in motion and time

Introduction

Given the scale of effort and expenditure that has gone

into the mechanisation of air traffic control, it is perhaps

surprising that there is relatively little quantitative data

study (6). The "analytic" approach takes the actual ATC

situation as a whole, and attempts to determine the contribution to the total workload due to each factor of

interest by analysis of variance or other market research

about the nature of the nature of the workload which automation is meant to relieve. If there existed on adequate

techniques. An alternative terminology would describe the

controller workload, it might be a much easier matter to

of the behaviour of the system; the "synthetic" approach being described as "prescriptive" — giving an account of

tool for the prediction of the various components of this

compare the economics of various possible control con figurations which might be adopted to deal with a given task.

.

.

It is a formidable task to get even an approximate solu

tion to this problem. Controllers differ markedly in the

"analytic" approach as "descriptive" — giving an account how the system should behave.

The next two sections of this paper will consider these techniques in greater detail.

amount of work they manufacture for themselves in a

Synthesis

actions between different members of the control team

of sub-tasks, the first need is for a breakdown of the job into its components, and for the construction of a flow

given situation; and in their ability to deal wit it Inter

may considerably confuse any simple arithmetical ap proach. "Workload" is not defined with any rigour and

subjective estimates of work difficulty are confused by the

If the controllers' task is to be studied as an assembly

diagram showing the sequence of events required to deal with a known task. This is a coarser scale version of the

variability of controller ability. For the purposes of the

problem facing a computer programmer who attempts to mechanise some aspect of ATC. It is not enough to cate

nition of "workload" that is not in conflict with common

gorise the various problems, it is necessary to know the strategy employed to solve them. Basically, the method is

present paper, the author is prepared to accept any defi English usage and which lends itself to measurement.

The scientific approach to the prediction problem is to

set up a mathematical expression or expressions into which

are substituted the appropriate values of the parameters

to interrogate one or more controllers. An extension of this technique, termed "instigated introspection" by some

psychologists, requires the subject to solve a series of problems whilst simultaneously giving a running commen

tary on his mental processes. The difficulty here is that the Reprinted with Irind permission of the outhor and the U.K. Ivlinistry of Technology

18

commentary introduces unreality into the situation, the controller may describe, not what he usually does, but what

he thinks he does or even what he thinks he should do.

cient of routine load" which varies with the nature of the

Further, there is the temptation to spend more time in de scribing the easy processes and less on those, common in ATC, which ore extremely difficult to explain to an outsider.

Leplat and Bisseret (5) used the technique of the previous paragraph, followed by a series of trials under laboratory conditions. Their study was confined to a particular con troller task, the test for procedural conflicts. In the labora tory trials, the controller was faced with a flight progress

task and the traffic mix.

board depicting a traffic situation at a fixed time. He was then faced with new aircraft wishing to join the system, or

with requests for a level change. Measurements were made of the time needed to solve each problem. The trial was repeated for on adequate sample of controllers and for a

set of problems designed to stimulate each branch of the flow diagrams.

If this technique is to provide an overall measure of controller workload, it is necessary to have a measure of

the relative frequency with which each sub-category of task will face the controller. It may be possible to measure these frequencies by analysis of the results of normal ope ration, but, except where there is already extensive me chanisation, the labour involved in data collection and analysis may be intolerable. In any event, this technique

may not be applicable to a hypothetical new organisation.

An alternative is to use fast-time simulation (10) to deter

mine the magnitude of the various control tasks.

By confining themselves to a procedural system, and by discussing the conflict detection task only, Leplat and Bisseret were able to avoid the more subtle situations that

arise when information is arriving by more than one channel at a time, e. g. by ear and eye, and where the controller may be uttering ritual words over the R/T and simultaneously be thinking of something different. The classical techniques of time and motion study have been the subject of heavy criticism (7) even when they were applied to almost purely manual tasks. The application of this technique to a largely cerebral activity will need con siderable justification.

Traffic was, in fact, broken down into "standard" and "non-standard" aircraft, and additional allowances made

for traffic in the following four categories: — — — — —

Vertical handoff, TMA handoff, Climbing and descending, "Pop-up" (a/c demanding impromptu admission to

the IFR system).

A "specific weight", empirically derived, is then as

signed to each class of traffic and the weighted mean of these weights is K^.

The term in Lj is assumed by ref. 1 to be of the form: —

^ 2K,aVN^ gS where a is a parameter fixed by the rules of separation (nm/ac).

V is the average traffic speed (kts) N is the number of aircraft under control (ac) S is the sector size (nm)2 Kj is the coefficient of airspace load. g is the "equivalent volumetric flow organisation fac tor" (see ref. 8 para. 1.6.11.2(g)).

Arad defines a unit of work as the load generated by "one standard aircraft overflying the Sector in straight and level flight when no interaction with other aircraft is con sidered". This is the "Dynamic Element of Work" or DEW.

The unit of load is termed the "Dynamic Element of Load" or DEL. One DEL equals one DEW per hour. The units of L, and L2, are therefore, DEL. The units of and K2 are DEW/a.c. There is a dimensional error in equation (3), since,

as shown in ref. 9, Appendix II, the right hand side of the equation does not have dimensions DEL. The reader should

see ref. 9 for details of a tidied-up and dimensionally correct version of equation (3) which was used in the computer programme for the evaluation of the Arad model.

It should be noted that the definition of a DEW is ap parently local to the Sector under consideration. it is unfortunate that none of the published papers on

Analysis

the Arad model discuss the axioms on which it is based.

The work by Bar-Atid Arad (1) appears to be one of the earliest attacks on a significant ATC system, and is almost certainly the most ambitious so far attempted. Arad chose to study the entire contemporary U.S. en-route ATC environ ment, with traffic loading and sector configuration as the

These imply that the L^ term represents work generated when aircraft enters or leaves a Sector, and that this work,

main variables.

The study breaks down into three components: —

or another, is directly proportional to the number of air craft entering or leaving and independent of the time which the aircraft spends in the Sector. Similarly, the L2 term is implicitly taken to be proportional to the number of conflicts arising between aircraft in a given Sector.

(i) formulation of a workload model,

(il) experimental determination of coefficients, (iii) effect on workload of changes in sectorisation.

The present section will be concerned with (i) and section 4 with (ii).

Arad states as an axiom that the control workload con sists of three terms: —

(i) a "background" load, Lq independent of N, the num ber of aircraft under control,

(il) a "routine" load, L^, directly proportional to N, (iii) an airspace load, Lj, proportional to N^.

In practice, the Lq term is ignored. The term in is as sumed to be of the form: —

L,

presumably communications and data entry of one type

(2)

N is the number of aircraft under control, T is the average time that aircraft are in the sector, and is the "coeffi

Since the difficulty with which a conflict can be resolved is itself a function of traffic density, it can easily be argued that higher order terms are necessary in the workload equation.

It is a relatively simple matter to measure the time spent on R/T or on inter-controller conversation, and although this does not measure workload, in DEW units or

otherwise, the results throw an interesting light on the It is necessary to spend only a little time listening to

R/T to realise that the nature of the messages changes with loading. In a busy period, a burst of noise as the R/T key is flicked replaces the message "good day to you, sir" which might well be passed in a slack period. This phenomenon may well complicate the relationship between the routine workload and the number of aircraft.

19

NUMBER OF AIRCRAFT Figure 1 Total speech versus traffic level.

J 14

I 16

I 18

I 20

I 22

I 24

L 26

28

30

32

N-UMBER of aircraft Figure 2 Curve fitting.

A recent study (11) at LATCC, West Drayton, has yield ed, amongst other things, figures for the total speech load (expressed as a percentage of the study period) on control lers on Sectors 5 and 11, as a function of the number of

aircraft passing through the Sector during the half-hour period over which each loading was measured. To quote

from ref. 11: — "Sector 5's sphere of operations is a bi directional airway bounded at its western extremity by a IMA and at the eastern by the convergence of two busy airways. The incumbent is frequently more heavily engaged in liaison than in actual controlling. On the one hand, he is engaged in almost continuous liaison with the Garston stack controller — on the other, he is liaising with the

Sector 11 controller, initial descents on inbound aircraft, a n d cl i mb s o n o u tb o u n d . Th e Se cto r 11 co n tro l l e r, i n co n

trast to his neighbour is primarily engaged in active con trolling, and since his area of operations is almost entirelyover water, his duties in so far as inbound releases to, and clearances from airports, are concerned ore limited with a consequent reduction in telephone workload".

20

The present author has fitted to this data quadratic

curves giving the least-squares best-fit prediction of speech loading as a function of the number of aircraft in the Sector

(m a half-hour period). The results are shown in fig. 1. The

curve for Sector 5, in particular, suggests that there must be a maior part of the total speech load which does not depend on the number of aircraft in the Sector. This conciubiun IS, at least partly, supported by the quotation from ref. 11 given in the paragraph above.

The difficulties of curve-fitting are illustrated in fig. 2,

based on the data from Sector 5. The best-fit curve is here

shown together with the dots which mark the experimental results and elongated "I's" which mark the ±1 sigma

limits about the mean loading for each traffic level. (Sigma

was calculated on the assumption that the scatter about

the mean for each level of loading constituted a sample

from the same population). It will be seen that whilst the

main features of the curve are almost certainly correct,

sampling errors may be playing a significant role. It would be interesting to have a larger sample of data (say,

10 times the present size, or about 600 half-hour periods) on which to work. Ref. 11 gave a breakdown of the speech load into various components: — R/T, intercom., GPO lines, and direct liaison. Unfortunately, the sampling errors for most of these components are even greater than those in fig. 2. because the samples are smaller, and the present writer has achieved no meaningful breakdown of the re

tative estimate of the relative capacity of two systems, but it is often impossible to define a precise point where

sults.

tually bear only a very rough relationship to that normally

What does seem clear, at any rate, is that the Arad assumption that the workload curve passes near zero for zero traffic is seriously inaccurate for the Sector 5 results

used. The technique has the added drawback that the pro

shown in fig. 1.

Using methods which are discussed later in this paper, the F.A.A. mounted a study (9) to check the validity of the Arad model. This compared the accuracy with which three

different models could predict the average order in which controllers would rank the difficulty of handling given numbers of aircraft in various sectors.

The three models were: —

(i) the Arad model (termed the "M" model in ref. 9) (ii) a variant of (i) in which only the "routine load" com ponent was used (the model) (iii) a model which assumed the load to be proportional to the "equivalent traffic count" i. e. the average num ber of aircraft under simultaneous control in the sector

(the "E" model).

Tests of the predictions of the three models against con troller judgment showed that the differences of proximity

were statistically significant, and that the order of closeness was:

—

(i) the E model (ii) the M model (iii) the L, model.

The difference between the E and L, models is parti

cularly interesting. As was pointed out above, the L, term is calculated on the assumption that the routine load depends on the number of aircraft entering or leaving a Sector in a given period. The E model assumes that the load depends on the number of aircraft within a Sector simultaneously.

It seems clear that the poor showing of the present Arad model could be improved by taking the E model as the routine load component instead of as presently defined.

Measurement of Workload

It was pointed out at the start of this paper that "con troller workload" has not been rigourously defined. If we are prepared to adjust the definition of workload to suit the method of measurement, possible techniques include: — (i) External observation of control activities (basically,

the method used by Leplat and Bisseret).

the control system "breaks down". A controller who is approaching saturation will progressively adopt more and more tricks to reduce his workload and postpone his pro

blems, possibly with a significant loss of expedition to the traffic concerned, until his mode of operation may even

cess of building-up a saturation situation is necessarily a fairly slow one. Since it is clearly necessary in any measure ment of workload to take a big enough sample of control lers and of traffic to keep sampling errors down to a reasonable level, the simulation process can become very expensive in time and effort.

Method (iv) suffers to a somewhat reduced extent from the objections to method (iii), but has the added drawback that it is not easy to measure controller errors. At best, the process is laborious, and the results may be ambiguous.

For example, if a busy controller had decided that updating of a particular piece of data on his display was really irrelevant, and omitted to make the revision, would this constitute a mistake? If not, how can one be sure that this is not the explanation of an "error"? The base-load task avoids this difficulty, but there is now on additional un wanted variable, for controllers will differ considerably in the priority they accord to this task when the work-load begins to build up.

The workers at the FAA (8, 9) apparently decided, in the face of the above difficulties, to adopt method (v). It con be argued that the consensus of controller opinion is based on a much larger sample of traffic situations than can be covered in any simulation, and, because the opinion is based on the real world, it avoids the systematic errors

that are always possible in simulation. It remains to obtain a large enough sample of controllers to reduce effects due to personal bias to a reasonable level, to devise a suitable technique for extracting a quantitative judgment of control work-load, and to show that there is a meaningful con sensus of opinion.

In the evaluation of the Arad model, Jolitz (9) selected

five air traffic control centres, and studied a total of 16 sectors, each of which hod been worked in common by at least two experienced controllers. The sectors were selected to have different functions, but each had "complete radar capability" and was bounded by other sectors that nor mally used radar handovers to the sector under study.

Further, the selected sectors hod average or above average activity.

Subject controllers, having experience on two sectors,

A and B say, were faced with questions of the form: — "How many aircraft under simultaneous control in sector A would you judge, on average, create the same

(ii) Physiological tests for strain in the controller.

load as N aircraft in sector B?"

(iii) Simulator trials in which traffic levels are pushed up to the point where the controller is saturated. (iv) Methods of detecting controller overload by measure ment of his error rate, either in performing his normal

6, 8, 10 and 12 and by reversing A and B. These were ar

task or in some artificial base-load task involving, say, elementary arithmetic.

(v) Controller judgment as assessed by questionnaire and interview (the method used by Arad and Jolitz).

Method (i) has been discussed in section 2. Method (ii) does not seem to have been applied with any success. Me thod (iii) is perhaps capable of being used to form a quali

Twenty-four questions were generated by putting N = ranged in pseudo-random order and put to each subject.

After a lapse of about one week, the questions were re

arranged and again administered to each subject. At the time of the first interview, the subjects were not told to expect the second.

The FAA project team decided to eliminate from the data any set of answers which contained "reversals in the judgmental response". (The example quoted in ref. 9 is a subject who stated that the load due to 6 aircraft in Sector 21

A was equivalent to 4 aircraft in Sector B, but who, in

4. Rosenshine, M. "The Application of Automation to

reply to another question, stated that 8 aircraft in Sector A were equivalent to 10 in Sector B.

t h e S o l u t i o n o f A i r T r a f fi c C o n t r o l Problems".

FA A T h i r d I n t e r n a t i o n a l Av i a t i o n

No detail is given of the train of reasoning that led to this decision. It seems intuitively obvious that the workload in any Sector will increase monotonically, but it is far from

R. & D Symposium — Automation in

obvious that the curves for two sectors will never cross.

5. Leplat, J. "Analyse des Processus du Troite-

Consider, for example, the speech load curves of fig. 1. A

Bisseret, A. ment de L'information chez le Con-

controller whose answer to questions comparing Sectors 5 and 11 reflected the situation depicted in fig. 1. would have had his responses deleted as inconsistent.

chologiques XIV no. 1—2 pp. 51—67,

A i r Tr a f fi c C o n t r o l . N o v e m b e r 1 9 6 5 .

troleur de la Navigation Aerienne" Bulletin d'Etudes et Recherches Psy1965.

Ref. 9 does not reveal the percentage of the replies that were censored out of the data collected, though it is possible to deduce from local irregularities in the sample size (in Table III, for example) that at least 5% of the results were rejected for one reason or another.

Available in poor English transla

tion in Controller 5 no. 1 pp. 13—22, January 1966.

6. Barnes, R. M. (Ed) "Motion and Time Study" Chapman & Hall 1949.

7. Gillespie, J. J. "Dynamic Motion and Time Study" Paul EIek 1947.

Conclusion The objects of air traffic control are stated to be "the safe and expeditious movement of air traffic". There are

8. Arad, Bar-Atid "Notes on the Measurement of Control Load and Sector Design in the En-route Environment"

dangers in accepting without question the assumption that reduction of controller workload is a useful intermediate

step in the search for more efficient ATC. When faced with a serious overload situation, the controller normally pre serves air safety and his own sanity by slowing down the traffic demand, by one means or another. In such a situa tion, tests on the controller loading may be an insensitive method of measuring the traffic delays which are of primary importance. It is unreasonable to expect that " c o n t r o l l e r w o r k l o a d " c a n b e d e fi n e d o r m e a s u r e d w i t h t h e

FAA SRDS June 1964.

9. Jolitz, 9.

Johtz,G.G. D D. "Evaluation of a Mathematical Mo del for use in Computing Control Load at ATC Facilities"

FAA SRDS Report No. RD-65-69 June 1965.

10. General Precision "Contract No. C/38/0/65 for the comSystems

Time) Simulation Study".

precision customary in the physical sciences, but attempts

Final Report Vols. I—III.

to date at defining, predicting or measuring controller

January 1968.

workload can be termed succesful only if judged by extremely relaxed criteria.

Acknowledgement The author is indebted to Messrs. C. Dowling and W. Feison of the FAA who spent some time discussing the Arad model and its evaluation, to Mr. M. Rosenshine of Cornell Aeronautical Laboratory, and to ATCEU Hum for permission to quote from their study of workload at LATCC West Drayton.

Contributed by permission of the Director, R.R.E. Copy right Controller H.M.S.O.

pletion of an Arithmetical (Fast-

Book Review StromungsmeStechnik

By W. Wuest, German Languoge, Friedr. Vieweg & Sohn Braun

schweig, ,9,9, 93, pp. clothbhlg DM zo,JU.

^

This book intends to give an introduction to the problems of aerodynam.c testing and the appropriate use of instruments and measure ment techniques. The author, a well-known authority in the Beld, gives first a description of the various types of wind tunnels from the low

speed range up to hypersonic velocities. The fundamental techniques to so ve problems in oeroayncmic testing - the measurements of forces, velocities, pressures and temperatures - are then treated in a logical

References 1. Arad, Bar-Atid, et al.

"Control Capacity and Optimal Sector Design" FAA SRDS Interim Project Report No. 102-llR, December, 1963.

sequence. Emphasis is on the instruments ond techniques which are used more frequently. The more specialized methods are mentioned and the original literature is cited. Two chapters ore devoted to measurements m VISCOUS flows (boundary layers) and turbulence measurements. The

2. Ratcliffe, S.

"Congestion in Terminal Areas" J. Inst. Navgn. 17, 183 (1964)

book IS completed by the description of the visualization techniques of water and air flows and the electrical and hydraulic analysis. The optical methods are stressed because of their importance in practical use.

This book IS the result of the experience of the author in the field in which he has been working for some twenty-five years and a series of

3. Chandler, G. A.

"ATC Capacities at Sydney Kingsford Smith (Mascot) Airport and

and new-comers primarily. This nevertheless is very useful to the specia list smce It allows him to find further details easily. The list of olmosf

Controller Saturation Levels".

600 reference papers represents another source of informotion.

J. Inst. Navgn. 18, 42 (1965) 22

lectures which he has given on the subiect. It is addressed to students

H. Uebelhack

The Improvement of Wet-Runway Operation

by Tirey K. Vickers Senior Consultant

One factor which con limit the acceptance rate of an airport is the time required for a landing aircraft to de celerate after touchdown to a speed at which it can safely make a turn and exit from the active runway. This factor,

which is sensitive to runway surface conditions, becomes particularly important at locations where takeoffs and

landings must share the same runway.

When runways are wet, rollout distances and runway occupancy times tend to increase. One reason is that the

presence of water on the runway can lead to a condition known as hydroplaning, which can greatly degrade the braking capability and the directional control of the air craft during the landing rollout. There are three different types of hydroplaning, as described below: Viscous Hydroplaning (thin-film lubrication) can occur

if the runway surface has been polished smooth by repeat ed landings and the coincidental buildup of repeated layers of burned rubber, as well as other contaminants such as soot and oil. In this case a very thin film of water,

less than .001 inch deep, can keep the tire from contacting the runway. Instead, the tire skims along the top of this film, where it cannot contribute either to the braking action or the directional control or stability of the aircraff. This type of hydroplaning can occur down to relatively low taxi

with a locked wheel reduces its cornering or steering capa bility to zero.

Dynamic Hydroplaning can occur when there Is a layer of standing water on the runway surface. If the water can not get out of the way of the speeding tire fast enough, it forms a liquid wedge which lifts the tire off the surface, as shown in Figure 1. Figure 2 shows how this wedge of water reduces the footprint (runway contact) area of the tire, thereby degrading the braking action and increasing the stopping distance of the aircraft.

Test data incidates that the minimum dynamic hydro planing speed of conventional aircraft tires, in knots, is about 8.6 times the square root of the tire pressure, in pounds per square inch. For example, a typical executive jet aircraft uses 135 pounds pressure in the main tires and 45 pounds pressure in the nose wheel tire. The calculated hydroplaning speed for the main tires is approximately 100 knots. However, if a high-speed turnoff is anticipated, it should be noted that the calculated hydroplaning speed of the nose wheel tire is slightly less than 60 knots.

If the runway is wet, a tire can hydroplane at any speed above the value calculated above, which simply represents the lowest speed at which dynamic hydroplaning can start.

Once it has started, however, the condition can be sustain-

speeds.

Reverted Rubber Hydroplaning can occur during a prolonged skid with a locked wheel. The resulting friction generates heat at the point where the tire contacts the runway. When the rubber reaches a temperature between 400 and 600 degrees F, it reverts back to its uncured (sticky) state.

Until recently it was believed that if there was water on the runway, the reverted rubber could form a seal which delayed exit of the water from the tire footprint area — and that the high temperature within this pocket changed the water instantly to high-pressure steam which lifted the tire a microscopic distance off the runway surface.

Recent research at the University of Michigan provides a completely different explanation for reverted rubber hydroplaning: If the runway is wet, the combination of the wet film on the runway with the reverted rubber forms a highly flexible bearing surface which changes shape to "flow" over and around irregularities in the runway surface, with very little friction. Even if the reverted rubber subsequently is cooled down to the ambient temperature, the tire may continue to skid smoothly, down to a speed of five to ten knots.

A tire can produce a cornering or side force for lateral stability or steering only when it is rotating; thus a skid

D - D I R E C T I O N O F A I R C R A F T; W - W AT E R L AY E R O N R U N W AY ; L - L I F T F O R C E F R O M W E D G E O F W AT E R ( S H A D E D A R E A ) ; H - H E I G H T O F T I R E O F F S U R FA C E ( N O T E : H E I G H T E X A G G E R AT E D FOR CLARITY)

Figure 1 Dynamic hydroplaning

23

ed on down to a speed somewhat lower than the minimum starting speed.

Hydroplaning has been the direct cause of a number

hydroplaning incidents occur off the side of the runway. It would appear more rewarding to develop a means of pre

of accidents in which aircraft have run off the end of the

of surface water from the pavement itself.

runway. Although there hove been relatively from this cause, aircraft damage has been instrument weather conditions on overrun often wipes out the localizer antenna, which close the airport for o long period.

Most present runways are designed with a transverse grade (slope) up to IVj per cent, to drain surface water to the sides, as shown in Figure 3. To obtain faster drain age, FAA engineers are now considering the possibility of

few fatalities extensive. In of this type in turn may

During hydroplaning, the reduced traction reduces the pilot's directional control of the aircraft. As a result, a hydroplaning aircraft tends to weathercock into any ap preciable crosswind, skidding sidewise down the runway

venting hydroplaning, by obtaining more positive drainage

increasing the maximum transverse grade to two per cent.

One of the most effective means of reducing hydro planing problems is the use of runway grooving. The groov ing consists of small slots cut across the runway to facilitate water drainage and to improve tire traction. First tried in

— or drifting off the downwind side into the mud, a situa tion which may require closing of the runway to other

several major U.S. airports with excellent results.

traffic, until the unfortunate aircraft can be retrieved.

Erecting barriers or lengthening runways is not neces sarily the answer to the wet-runway problem, as most

In the United States, most of the research on this subject has been conducted or sponsored by NASA. Figure 4 shows NASA's experimental grooved runway at Wallops Island,

Figure 2 Photos looking up through a glass runway, showing the foot print area of a 20 X 4.4 aircraft tire, under partial and total hydroplaning conditions. Vertical load = 500 pounds, tire pressure 30 pounds per

square inch, water depth = Vr inch. Tire motion is from left to right; tufts show direction of water displacement. Speeds in knots: A = 28,

24

England in 1956, the concept has been implemented at

B

=

56,

C

=

71,

D

=

88.

—

NASA

Photos

Virginia. Here a number of different groove patterns were

tested under damp, flooded, and slushy conductions, and over a speed range up to slightly over 100 knots. Figure 5 shows a typical test.

It has been found that transverse grooving tends to reduce the amount of time in which any type of hydroplan ing can occur, as the grooving expedites drainage of the surface water off the runway surface. This effect is quite noticeable in comparing the amount of standing water

Figure 3 Cross-section of typical runway showing transverse grade (slope) G for drainage.

(Drawing not to scale)

remaining on grooved and ungrooved portions of a run way after a shower; the grooved portions tend to dry off

immediately. This greatly reduces the amount of spray

thrown up by aircraft during takeoff or landing.

Runway grooving con prevent viscous hydroplaning by providing a continuous series of edges on which the tire can grip, in order to secure positive traction, even though the runway surface may be covered by a thin film of water as well as other contaminants. Grooving can also reduce

reverted rubber hydroplaning, as a locked wheel condition

is less likely to occur on a runway which has a uniformly high coefficient of friction. The provision of additional

gripping surfaces also tends to start the tire rolling again.

Runway grooving con eliminate dynamic hydroplaning by providing multiple escape paths for the water beneath the tire, thus preventing buildup of the type of high-pres sure wedge shown in Figure 1.

25

-

-'•-*^ai'

1

f S f ^ — NASA Photo

Figure 5 Surf's up!

NASA tests made with a Convoir 990 iet transport on grooved portions of o flooded runway, with 'A inch of

coefficient) than ungrooved surfaces. This is particularly

water above the grooves, and using smooth tires, indicate that braking is identical to dry-runway conditions at run way speeds below 106 knots. These results have been con

them utilize onti-skid systems based on the principle of reducing the brake pressure (to prevent wheel lock) when

important in the operation of heavy aircraft, since most of

ever the system senses an incipient skid condition. The more

firmed by aircraft users at Washington National Airport, Kansas City Municipal Airport, J. F. Kennedy Airport, and

allows the braking system to maintain a constant braking

uniform friction characteristic of the grooved surface

Chicago Midway Airport.

torque, as opposed to the intermittent torque required for

Comporotive tests made on grooved and ungrooved runway surfaces hove shown that the grooving tends to

a smooth stop on on ungrooved wet runway. The ability to

use constant torque tends further to reduce the stopping

produce more uniform friction (less variation in the friction

distance.

Figure 6 Maximum-traction grooving, spacing 1" (25 mm), width Vi" (6 mm), depth 'A" (6 mm). — NASA Photo 26

The greater the cross-sectional area of the groove, the more wa\er it con hold before overflowing. If the groove is too wide, however, it will trap stones and small debris, creating a housekeeping problem. The narrower the spac ing between adjacent grooves, the faster the runway sur face will be dried off. If the spacing between grooves is too narrow, however, its top surface is more subject to shear damage.

Of the various groove configurations tested so far, the V4 inch wide and V4 inch deep, spaced one inch between centers, as shown in Figure 6. Figure 7 compares various operational grooving installations.

The cost of grooving existing runways varies over a wide range. One variable is the total volume of material which has to be removed. Another variable relates to the

restrictions imposed by aircraft operations. For example, if the work has to be done during night or early morning

•

•

.

•

4

B

^

-

•

♦

^

D

C

Groove

Groove

Angle

W i d t h

Depth

of Cut

Washington Nctiooai

1/8"

1/8"

9 0 *

Kansas City Municipal

1/8"

1/4"

9 0 -

Chcrlsston W.Va. Municipal

!/4"

1/4"

9 0 *

Chicogo Midway

1/4"

1/4"

90*

JFKtnnsdy Inttrnational

3/8"

1/8" •

45*

AIRPORT

and BmIs AFB

hours, in order to avoid traffic peaks, the labor cost is likely to be considerably high than if the work could be done during regular daytime hours. Similarly, if the grooving equipment has to be pulled off the runway periodically, in order to permit resumption of aircraft operations, the additional labor and waiting time will increase the cost.

h—3/8"

w

JUS us ai r p o r t s .

Runway 18—36 at Washington National Airport was the first operational runway to be grooved by the FAA. The job required thirty-five days of work during the off-peak hours between 2300 and 0700 local time. Diamond saw machines were used; each machine cut thirteen grooves simultane

ously, into the asphalt surface, at a cost of about S 0.09 per square foot.

Although the number of aircraft operations which hove been mode on this grooved runways is now approaching one million, so far there has been surprisingly little de terioration of the groove pattern. In the touchdown zones,

some of the patterns have been distorted slightly, but this has been due to shifting between the upper and lower layers of the pavement, rather than to movement of the

grooved portion relative to the upper layer.

One problem which is involved in cutting grooves into an existing runway is the need to flish out and dispose of

the large amount of abrasive waste which is produced by this process. Otherwise, this material can create a dust (or slurry) problem which can be damaging to jet engines and aircraft wheel-well components. One of the newest types of grooving machines has an automatic clean-up feature, to take care of this problem.

grooving provides against a catastrophic hydroplaning accident.

Because an aircraft tire may be recapped ten or more times during its useful life, there was some initial concern

as to whether the vibrations produced in riding over the grooves would create a fatigue problem with the tire cords.

The preliminary data from recent tests indicate that there is no appreciable difference between the stresses produced in the cords of tires running on grooved and ungrooved runways.

Initially, some of the aircraft manufacturers were con cerned about another aspect of runway grooving — whether the touchdown conctact of a non-rotating wheel

with, a grooved runway during the landing would produce a much greater spin-up drag load on the landing gear, than if the aircraft were landing on an ungrooved runway

(Spin-up drag is the rearward force transmitted to the axle when the wheel touches down and accelerates to the

landing speed). The preliminary data from a recent test program indicates that the difference in the runway sur

Ultimately, it may be desirable to develop a means of casting or rolling the grooves into new runway surfaces

face (grooved or ungrooved) makes no appreciable dif

instead of having to grind them out at some later date.

The overall effect of runway grooving is to permit a wet runway to approach the braking capability of a dry run way. Up until the present time, the FAA has made no

High-speed photographs of tires encountering grooves indicate that under heavy loads the tire tread pushes down into the grooves. This allows the tire to get a bite on the runway, for traction purposes; however, it also gives the runway a chance to bite back. As a result, some tires which

have been subjected to prolonged operation on grooved

ference in the spin-up drag.

allowance for the presence of grooved runways, as far as

runway length is concerned. The present regulations require a runway length adequate to allow a full-stop landing (based on the aircraft type certification tests) within

runways have shown a pattern of tiny transverse or chevron-shaped cuts on the tread.

It is conceivable that certain grooved configurations

60 percent of the effective length of the runway.

will result in less tire wear than others. Additional research

operation into wet or slippery runways. There is now con siderable evidence that the original 15 percent allowance was not realistic, and should be increased for turbojet

may be desirable, particularly for those airports where runway length is not a problem, to determine the optimum compromise between tire traction and tire wear. Mean while, it appears to be the general concensus of the U.S. airline operators that a slight amount of additional tire wear is more than justified by the protection which the

Beginning in 1966, Federal Air Regulation 121.195 (d) has required an additional 15 percent runway length for

transport aircraft landing on smooth wet runways. If this increase is made, it is expected that it will apply to un

grooved runways, but not to adequately grooved runways under wet conditions.

27

Because the elimination of hydroplaning restores the ability of aircraft tires to provide side forces for lateral stability and steering, it is hoped that the installation of grooving will permit the authorized maximum crosswind component for wet runways to be increased to the limit allowed for the same runways under dry conditions.

Runway grooving offers definite advantages from the standpoint of airport capacity. With ungrooved runways, b r a k i n g d i s t a n c e s a r e i n c r e a s e d s i g n i fi c a n t l y i n w e t weather. High speed runway exits which are ideally placed for dry weather operation may be 1,000 feet or more too close to the touchdown point to be used by the same air craft in wet weather, because of reduced braking action.

In addition, if pilots anticipate hydroplaning conditions, they will not start any turnoff until well below their mini mum hydroplaning speed.

All this naturally increases runway occupancy time, so air traffic controllers have to increase approach intervals

accordingly, when such problems are anticipated. This reduces airport capacity and tends to increase traffic

In surveys made at Washington, Kansas City and Ken nedy Airports, the majority of the controllers who were

polled, definitely felt that runway grooving aided most pilots in controlling their landing run, and that the turnoff point from a wet grooved runway was identical in most instances to that for dry operations on the same runway.

Controllers reported that this definitely improved runway traffic management, and increased the acceptance rate over that possible with the original ungrooved runway in wet conditions.

Any accident in which a large aircraft slides off the side of a runway, or overshoots the end, can cause a critical

tie-up of the runway, and sometimes the entire airport, for

long periods. The disruption caused by a single accident

can be very costly in terms of airport traffic capacity and r e v e n u e .

The elimination of hydroplaning should practically eliminate accidents caused by wet runways. Besides this obvious advantage for safety, runway grooving can help controllers to maintain airport traffic capacity in wet weather conditions.

delays.

Terminal Area Approach Control Sequencing Paper to the United Kingdom Symposium "Electronics for Civil Aviation", 1969*

by F. J. Crewe & T. E. Foster Airspace Control Division Elliott Automation Limited

A major airport could be defined as a place where

several airways each ten miles wide and about 18,000' high are compressed into a piece of airspace 300' wide, zero feet high with a lower surface made of concrete. Anyone who has spent hours carving out a racetrack in the sky over Southern England, New York, Chicago or any other busy terminal control area will agree that there are times when the quart will not fit into the pint pot. There are two ways of eliminating this problem, but each has severe practical limitations. The first is to build more airports —

controversial, expensive, often impracticable — and the second is to reduce or practically eliminate the separation minima — possible only by the application of area navi gation methods to VTOL aircraft.

So the problem will be with us for many years, and we must think in terms of reducing it rather than removing it.

With modern jet aircraft, the practical limit of separa tion on final approach is about 3 miles or 90 seconds. If we

Ten seconds too much and one landing in nine will be lost.

Thus the separation standard and the tolerance on it could

be written as 90 seconds, —0 -f 5. If it were possible to maintain this separation to these limits, then in a fairly short time, great savings in holding time, airline money and controller wear and tear could be effected. Unaided,

however, the controller cannot work to this accuracy, for reasons both subjective and objective. Subjectively, the need to avoid an overshoot or potentially dangerous situation will introduce an element of caution. Objectively, the controller cannot observe deviations from planned

flight path or speeds quickly enough or accurately enough to guarantee making a five or ten second time slot.

Let us look at the basic problem and the way in which it must be solved. From some point in the terminal control area, an aeroplane has to be directed to touch down to

arrive there at a specific, closely defined time. To do this

them becomes of paramount importance. Ten seconds too

the aircraft must fly a flexible stretching flight path at a known speed, and its progress constantly assessed and corrections made. This flight path could easily be 40 miles

little and an overshoot may mean the loss of a landing slot.

long in a fairly small TMA and will involve a descent

wish to work on these limits, then accuracy in achieving

through at least 5,000 ft., so that many changes in wind velocity — including shear conditions — could be en Reprinted with of Technology

28

of the authors end the U.K. Ministry

countered. Intermediate approach may start from an ill defined point in a holding pattern commencing with a

large turn. At any one time in a small TMA there maybe up to twelve aircraft on intermediate approach all des

cending towards ground level and all requiring separation

— possibly in IMC.

How could a computer help to solve these problems?

The first requirement must be that it has a complete record of post and present position and some means of identifying the aircraft. The most straightforward method of achieving this will be to use secondary radar, since the carriage of transponders in this environment will become mandatory very shortly. A system of allocating a unique identity code will be required and all confusion could be avoided if an

unallocated mode were utilised for this purpose. After being decoded in a conventional decoder, the aircraft replies will be processed in a plot validation unit which establishes the validity of a train of replies and the "centre of gravity" of the plot, correlates it with the mode and passes the data to the computer for processing into the system.

The computer will hold in store performance data on all the most common aircraft using the TMA and the meteoro logical data, particularly the wind velocity distribution over the area including variations in the vertical plane.

From any point in a terminal control area, and within the limits imposed by the dimensions of the area, a flight path of any length can be drawn to the runway touch down point. This will consist of two straight legs — inter mediate approach and final approach. Thus the time to touchdown of an aircraft of given performance in given wind conditions is theoretically dependant only on the heading given for the intermediate leg. This determines the length of both ports of the flight path. The progress of the aircraft can be monitored constantly during the inter mediate phase and the heading changed as required by such circumstances as change of wind velocity and pilot inaccuracy.

Before discussing in any more detail the system require

For many reasons, the aircraft will deviate from the ideal flight path. The computer will be comparing continu ally the actual and ideal positions and recalculating new flight paths to ensure that the allocated time slot is met.

The changes in heading required will be displayed to the controller and will flash to attract attention. The computer will however, be working in terms of accuracy, at limits to which no pilot could fly. The controller will interpret the computer demands and pass reasonable instructions to the pilot. Normally, the aircraft will fly at the speed close to

their performance data held by the computer, which will be divided into two categories — intermediate approach and final approach. It will be possible, however, for the com puter to demand speed changes within very narrow limits,

if these ore required to maintain the timetable.

It should be noted that although time at touchdown is the critical factor in the system, the problem is eased some what by the fact that the computer need consider the timing only as far as a "gate" point on final approach probably about six miles from touchdown. After this point, the air craft will fly the ILS localizer course at constant speed.

Successive aircraft will be affected by very nearly the same wind effects, so that providing their arrival time at the gate is correct, their landing time will be correct.

No mention has been made so for of aircraft separation or descent. In fact, the computer will arrange the sequenc

ing working in only two dimensions. Since all aircraft will commence intermediate approach with height separation, descent timing is left to the controller who can ensure that vertical separation is maintained.

Once an aircraft enters a holding pattern, the main tenance of track and groundspeed records in the computer is not really necessary. Instead the computer will issue a

leaving clearance in terms of time and heading with which the pilot will be expected to comply. Any errors produced by this fairly difficult piece of flying v/ill be readily ab sorbed OS they occur very early in the procedure.

ments, we must decide how the operational requirements

In the best of regulated systems, overshoots will occur,

and procedures can be integrated with the current manual

and aircraft in emergency will demand to be fitted into the

system. From these considerations we can derive the man/

system out of turn. Some method of entering a resequenc-

machine communications requirements and then data and data arrangement and display needs.

The first assumption is that the system is working to the runway capacity. This means a regulation of flow at some point or points such as holding areas either inside or out side the TMA. Consider first the procedures which do not

involve holding within the TMA. As each aircraft enters the system., its position in the sequence is allocated by the No. 1 director and the callsign and type entered by means of a

keyboard and positionally identified by Rolling Ball. The computer then allocates an SSR identification mode which is passed to the aircraft, and a landing time slot. The com puter then stores the aircraft position derived from its SSR returns and computes its groundspeed and track. It esti mates from stored performance and meteorological data,

the flying distances required to make good the time slot and divides this into three sections — continuation on pre sent heading, intermediate approach and final approach.

It is absolutely essential that the aircraft be flown very accurately with regard to heading and speed so that this track distance can be accurately related to landing time.

The computer assumes a mean controller/pilot reaction time and, on the tabular data display, gives a countdown from 30 seconds before displaying a heading change from one section of the flight path to the next.

ing requirement will therefore be necessary. In the case of an overshoot, the aircraft can be reintroduced in a fairly

leisurely way at a point which will delay but not require rerouting of other traffic. In other words it can follow the last aircraft to hove been turned from its heading towards the clearance limit.

For an emergency, however, the controller must first inform the computer of the point at which he wishes to break the current sequence. The aircraft following this point will require to have their flight paths stretched, pos sibly by an orbiting maneouvre. It could well be that this aircraft in an emergency has just taken off, so that the computer has no knowledge of it, and it would hardly be desirable to bother the pilot with the selecting of an SSR code. This recovery will therefore be carried out manually and the controller can readily make an estimate of the delay. This suggests the method that can be used to intro duce the appropriate delays into the system. Having select ed the first of the aircraft to be delayed, the controller will

input with his keyboard, two digits to identify the aircraft (the final two of the SSR identity) three digits to indicate the new time slot, and a "Delay Until" function key. The time slots of all subsequent aircraft will be correspondingly adjusted and displayed, and the appropriate control actions required.

29

DVORS-System

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Comparison between signals

Transmission and

received from a VOR and - after modification - from a

Navigation Division 42, Hellmuth-Hirth-Str.

DVOR on same location, course,

7000 Stuttgart 40 (West Germany)

altitude and distance.

standard Elektrik Lorenz AG Germany

I T T

The International Federation o f A i r Tr a f fi c C o n t r o l l e r s A s s o c i a t i o n s A d d r e s s e s a n d O f fi c e r s AUSTRIA

Secretary

R. Jdrvinen

Verband Osterreichischer Flugverkehrsleiter A 1300, Wien Flughafen, Austria, Postfoch 36

Treasurer

H. Pullinen

I F A T C A L i a i s o n O f fi c e r

F. L e h t o

President

A.

Nogy

Vice-President H. Secretary H.

Kihr Bauer

D e p u t y S e c r e t a r y W. S e i d I

Treasurer

W.

Chrystoph

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BELGIUM

President

Francis Zammith

Belgian Guild of Air Traffic Controllers

First Vice-President

J. M. Lefranc

Airport Brussels National

Second Vice-President

M. Pinon

General Secretary

J.Lesueur

Zaventem 1, Belgium President

A. Maziers

Treasurer

J. Bocard

Vi c e - P r e s i d e n t

M. van der Straate

R. Philipeau

Secretary

C. Scheers

Deputy Secretary Deputy Treasurer

Secretary General

A. Davister

I F A T C A L i a i s o n O f fi c e r

A. Clerc

Treasurer Editor

H. Campsteyn J. Meulenbergs

IFATCA Liaison Officer

J. Aelbrecht

M. Imbert

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CANADA

Canadian Air Traffic Control Association 56, Sparks Street Room 305

J.

D.

Lyon

First Vice-President R. McFarlane

Second Vice-President D. M. Diffley Managing Director G.J.Williams Treasurer

President

W.

Vice-President

Ottawa 4, Canada

President

Verband Deutscher Flugleiter e. V. 3 Hannover-Flughafen, Germany Postlagernd

A.

Cockrem

Chairman

AT C

Chairman Chairman

Kassebohm H.

MIL AIS

von

W. W.

Secretary Treasurer

Guddat

E.

Bismarck

Ehrhard Kroncke

H.J.KIinke K.

Piotrowski

CYPRUS

GREECE

The Cyprus Air Traffic Controllers Association Civil Aviation Dept.

Nicosia, Cyprus

A i r Tr a f fi c C o n t r o l l e r s A s s o c i a t i o n o f G r e e c e

DENMARK D a n i s h A i r Tr a f fi c C o n t r o l l e r s A s s o c i a t i o n

10, Agios Zonis Street, Athens 804, Greece President C. Theodoropoulos Vice-President N. Protopapas General Secretary E. Petroulias Treasurer

S.

Sotiriades

Copenhagen Airport — Kastrup Denmark

HONGKONG

Chairman

E . T. L a r s e n

Vice-Chairman

O. Christiansen

Secretary

E. Christiansen

Treasurer

M. Jensen

I FAT C A L i a i s o n O f fi c e r

V. F r e d e r i k s e n

Hongkong Air Traffic Control Association Hongkong Airport President

Secretary

K.

M.

Malcolm

A.

I F A T C A L i a i s o n O f fi c e r

Wightman P. Leung E. Collier

F i n l a n d

A s s o c i a t i o n o f F i n n i s h A i r Tr a f fi c C o n t r o l O f fi c e r s

Suomen Lennonjohtajien Yhdistys r. y.

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A i r T r a f fi c C o n t r o l A s s o c i a t i o n o f I c e l a n d

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Finland

Chairman

G.

Secretary Treasurer

S. K.

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V.

Suhonen N.

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Kristinsson

Trampe Sigurosson 31

IRAN

N O R W A Y

I r a n i a n A i r T r a f fi c C o n t r o l l e r s A s s o c i a t i o n

Airport

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Lufttrafikkledelsens Forening Box 51, 1330 Oslo Lufthavn, Norway Chairman

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Vice-Chairman

K. Christiansen

Secretary

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AT S S h a n n o n

R h o d e s i a n A i r Tr a f fi c C o n t r o l A s s o c i a t i o n

Private Bag 2, Salisbury Airport Rhodesia

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J. E. Murphy

President

C . W. D r a k e

Gen. Secretary

J. Kerin

C . P. F l a v e l l

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P. J.O'Herlihy

Secretary Treasurer

W.

Asst. Gen. Secretary

M. Durrack

Va n d e w a a l

SWEDEN S w e d i s h A i r T r a f fi c C o n t r o l l e r s A s s o c i a t i o n ISRAEL

Fack 22, 1 90 30 Sigtuna, Sweden

A i r T r a f fi c C o n t r o l l e r s A s s o c i a t i o n o f I s r a e l

P. O. B. 33

Lod Airport, Israel Chairman

W.

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E. Medina

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Katz

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Fl. Jelveus

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A. Karlahog

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I TA LY

CH 1215, Geneva Airport, Switzerland

A s s o c i a z i o n e N a z i o n a l e Assistenti e Controllori

Chairman

J. D. Monin

della Civil Navigazione

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T. R o u l i n

Aerea Italia

L i a i s o n O f fi c e r

Via Cola di Rienzo 28

for Zurich Airport

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J. Gubelmann

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Dr. G. Bertoldi, M. P.

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T r a f fi c C o n t r o l l e r s

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President

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Secretary

H. Trierweiler

14, South Street, Park Lane London W 1, England

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J. Ronk

Master

W. E. J. Groves

Executive Secretary

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U R U G U AY

Schiphol Airport Centra 1 , N e t h e r l a n d s

Aeropuerto Nacional de G□

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Th. M. van Gaalen

To r r e d e C o n t r o l

Asociacion de Controladore

s rrasco

Secretary

F. M . J . M e n t e

Montevideo, Uruguay

Treasurer

P. K a l f f

Chairman

U. Pallares

Member, Publicity Member, IFATCA-affairs

A. Vink

Secretary

J. Beder

B. H. van Ommen

Treasurer

M. Puchkoff

Y U G O S L AV I A NEW ZEALAND

Jugoslovensko Udruzenje Kontrolora Letenja

A i r Tr a f fi c C o n t r o l A s s o ie l a t i o n

Direkcija Za Civiinu Vazdu s n u P l o v i d b u Novi Beograd, Lenjinov Bui evar 2, Yugoslavia

Dept. of Civil Aviation, i3th Floor, Dept. BIdgs.

Stout Street

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A. Stefanovic

Wellington, New Zealari d

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Z . Ve r e s

President

E.Meachen

Secretary

D. Zivkovic

Secretary

C. Latham

Treasurer

D. Zivkovic

G. N. McLindon

Member

B. Budimirovic

I FAT C A L i a i s o n O f fi c e r

32

control in training Elliott airspace controi -

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AIR TRAFFIC CONFUSION OR...

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