IFR Operations in Turbine Aircraft

In most respects, operations under IFR (Instrument Flight Rules) are similar for all civil aircraft. However, there are a few IFR procedures specific to flight at high altitudes (the realm of most turbine aircraft) and to turbine aircraft themselves. Note that in some cases, regulations and procedures specify turbine-powered aircraft; in other cases, turboprops are differentiated from turbojet aircraft. For purposes of IFR operating regulations, the term “turbojet” includes both turbojet and turbofan-powered aircraft and means any “pure jet” or nonpropellerdriven turbine aircraft.

Profile Descents

A profile descent is simply a published transition procedure from cruise flight to an instrument approach. Courses, headings, and altitudes are prescribed, usually from a high-altitude cruise structure intersection or navaid down to interception of an ILS (instrument landing system) or nonprecision approach. Profile descents may begin as far out as 100 miles or more from the destination airport, depending on the aircraft’s cruising altitude. A profile descent normally allows a continuous descent from cruise, interrupted only by a brief level-off to slow to 250 kias (knots indicated airspeed) by 10,000 feet MSL (mean sea level).

Jet Routes

Turbine aircraft routinely travel at high altitudes above 18,000 feet. Altitudes at and above 18,000 feet are known as flight levels. Twenty-one thousand feet, for example, is known as “FL 210” and pronounced as “flight level two-one-zero.” At these altitudes, aircraft no longer travel on victor airways but instead use jet routes, or J-routes. The jet route system is a VOR-based route structure, similar to the victor airway system. J-routes are established from FL 180 up to and including FL 450.

On high-altitude enroute charts jet routes are depicted in black on NOS charts and blue on Jeppesen charts. They are identified on the charts by the letter “J” and the J-route number. When examining a high-altitude chart, the first thing you’ll notice is the length of airway legs between VOR stations. Most legs are over 100 NM in length, due to the high speeds at which turbine aircraft travel. If J-routes had short legs like victor airways, flight crews would be continually changing frequencies and adjusting the course knob. Because of the high altitudes involved with J-routes, VOR reception at long ranges is possible.

Above FL 450, aircraft typically navigate directly from point to point, often with some type of RNAV system. (See Chapter 13, Navigation, Communication, and Electronic Flight Control Systems.) ATC approval and radar monitoring are required on these types of routings.

Altimetry and IFR Cruising Altitudes at Flight Levels

A few more words are in order regarding operations up in the flight levels. All operations at or above 18,000 feet must be under IFR flight rules, and all aircraft must set their altimeters to 29.92 inches Hg. One reason for the standard altimeter setting is that high-altitude weather, at any given location, doesn’t necessarily correspond to the surface weather below where altimeter settings are established; it would be dangerous to project surface altimeter settings all the way up to very high altitudes. Another reason is that it would be impractical to require flight crews to reset altimeters to local reporting stations within 100 NM of the aircraft, as is required below 18,000 feet. At jet speeds, that would require an altimeter change roughly every ten minutes.

Requiring a standard altimeter setting above 18,000 feet does cause a few complications, however. If the surface altimeter setting is very low, aircraft flying up at 17,000 or 17,500 feet could start bumping into those flying at FL 180 with 29.92 altimeters. Therefore, the FAA has defined limitations as to what minimum flight levels may be used under given situations. For example, FL 180 may not be used when the local altimeter setting falls below 29.92. As the altimeter continues to fall, more flight levels are restricted. (See 14 CFR 91.121 for a full breakdown of minimum flight levels.)

IFR cruising altitudes between FL 180 and FL 410 are set up similarly to IFR cruising altitudes below FL 180. Above FL 180, we continue the use of odd altitude assignments for eastbound magnetic

courses of 000–179 degrees, and even altitudes for westbound magnetic courses of 180–359 degrees. For many years, IFR cruising altitudes above FL 290 were spaced 2,000 feet apart. This additional spacing was necessary because aircraft equipped with older, less accurate pressure altimeters needed additional separation at higher altitudes where altimetry errors are more pronounced. However, with the availability of more accurate air data computers and altimeters, and the need to increase the airspace capacity, the FAA deemed it safe to reduce the flight level separation to the current 1,000-foot spacing.

This reduced vertical separation minimum (RVSM) has given pilots and controllers a number of operational benefits. RVSM has doubled the number of usable flight levels between FL 290 and FL 410, which has helped relieve traffic congestion at cruising altitudes. Additionally, the greater number of available flight levels offers flight crews the flexibility to optimize each aircraft’s cruising altitude for better performance and fuel economy. More flight levels also allow the flight crew greater ability to change altitudes to find a better ride. In the past, aircraft were often “stuck” at inefficient or turbulent flight levels for hundreds of miles at a time due to traffic congestion.

Due to the importance of accurate altitude measurement, however, certain equipment is required in order to operate in the RVSM flight levels. Qualifying aircraft must have RVSM-certified altimeters, two independent altitude measuring systems, an altitude alerting system, at least one altitude control system (autopilot), and of course a transponder with altitude reporting capability.

From flight level 410 and above, the spacing of usable flight levels is increased to 4,000-foot intervals, and only odd flight levels can be assigned regardless of direction of flight. If your magnetic heading is from 0–179 degrees, use flight levels 410, 450, and 490. For magnetic headings of 180–359 degrees, flight levels 430, 470, and an amazing 510 are used!

Category I/II/III Approaches

As you instrument-rated pilots know, standard ILS approach procedures are certified by the FAA, based on specific installation, down to lowest minima of 200 feet DH (decision height) and 1/2-mile visibility. These CAT I approaches (Category I) may be flown by any IFR operator meeting basic IFR licensing, currency, and equipment requirements. However, the FAA has established that with proper crew training, installed equipment, and aircraft certification, lower precision approach minima may be individually approved for specific operators and airports.

Special low-minimum operations may be approved as (from highest minima to lowest) CAT II, CAT IIIa, CAT IIIb, and CAT IIIc. Each step to lower minima requires additional training and equipment. CAT II allows operations down to 100 feet decision altitude and 1,200 feet runway visual range (RVR), while CAT III approaches have, in most cases, no decision altitudes, only visibility limits (e.g., CAT IIIa RVR must not be less than 700 feet, while CAT IIIb RVR must not be less than 150 feet). Although most CAT III approaches do not have a decision altitude per se, most operators use an “Alert Height” in its place. An alert height is typically 50 feet above the runway. If an aircraft or ground-based equipment failure occurs prior to this alert height, the approach is discontinued and a missed approach initiated.

Until recently, it was required that all approaches to lower than basic CAT I minima be flown with an autopilot with approach coupler or flight director system. However, with the latest-generation headsup displays (HUDs) or heads-up guidance systems (HGSs), CAT IIIa approaches are now permitted with a DH of 50 feet and an RVR as low as 700 feet, hand-flown!

Under CAT IIIc, aircraft can land under “zerozero” conditions without reference to ceiling or visibility. These operators must use aircraft with full “auto-land” capability, including redundant autopilots that cross-check each other throughout approach and landing. Procedures are highly refined and specialized and are not approved in some countries for zero-visibility landings. (Paris has a DH of 20 feet.) Most low-minima operators are major airlines, due to equipment, certification, and training costs. Only a few aircraft and operators are certified down to CAT IIIc; most of those fly extremely long international routes where weather diversion at the destination is highly undesirable.