around on the runway and taxi back to the exit in order to leave the runway, L-T and L-L intervals will be large, because of the exce·ssive runway occupancy time of the landing aircraft. This runway occupancy time can be reduced by installing a full-length parallel runway with an exit at each end. The installation of high-speed exits at strategic locations along the runway will allow aircraft to leave the runway with a gentle turn, at a somewhat higher speed (and thus sooner) than would be possible in using a conventional right-angle exit. High-speed exits are particularly useful in shortening the average L-T interval. Obviously, high-speed exit's cannot improve traffic flow if pilots elect not to use them. For this reason, the exit taxiway beyond any high-speed exit must provide enough roll-out distance to let the pilot continue deceleration to a low speed, well before he has to make any sharp turns. In addition, all exits must be adequately marked and lighted.
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be so designed that the flow of arriving traffic does not interrupt or block the flow of departure traffic - and vice versa - regardless of the runway in use.
ILS Interference When instrument approaches are in progress, taxiing or parked aircraft must not shadow, or interfere with, \he ILS glideslope signals. At some airports this requires that departures be held a long distance away from the runway entry point, as shown in Fig 4. If it then takes longer for the departure to taxi from the holding point to the takeoff position than it takes for the landing aircraft to decelerate and vacate the active runway, then the L-T intervals will be excessively large, and capacity will be limited accordingly. This problem can often be solved by installing the ILS glideslope antenna on the opposite side of the runway, away from the taxiway used by departing aircraft. As shown in Fig 5, a departure can ·safely hold much closer to the runway entry point. As soon as the landing aircraft passes this point, the departure can taxi into position and be ready for takeoff by the time the landing aircraft is off the runway.
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Bypasses One way to prevent interruptions in the departure stream is to provide a bypass on the taxiway, as shown in Fig 6, wide enough for one aircraft to taxi around another. This will prevent delay in a case where the first aircraft in line is not ready for takeoff by the time the ·second aircraft is ready to go. For best results, this bypass should be as close to the takeoff threshold as is practicable.
Runway Crossings A runway crossing blocks the runway during the time required to taxi the entire distance shown in Fig 7. If 20 aircraft or vehicles per hour have to cross the active runway, and if the average crossing takes 45 seconds, then the runway can be blocked up to 20 X 45 = 900 seconds during the hour. As there are 60 X 60 = 3600 seconds in an hour, the crossings would reduce the theoretical runway capacity up to 900/3600, or 25 per cent. One way to reduce such losses is to move the crossing point closer to the takeoff end of the runway. As shown in Fig 8, the cros'sing can begin as soon as the takeoff or landing aircraft has passed the crossing point; the crossing operation continues while the runway is still being used by the takeoff or landing aircraft. This reduces the amount of time that the runway is being blocked exclusively ';Jythe crossing operation. To minimise the effects of runway crossings on airport capacity, it is necessary to design the crossover itself to minimise the time required to cross. For example, any crossover which looks like Fig 9 should be redesigned to look more like Fig 10. Further reductions in runway occupancy time can be made, if necessary, by increasing the number of crossing lanes so that more than one aircraft or vehicle can cross at the same time.
Conclusion Fig 6 A bypass allows an aircraft not ready for takeoff to pull out of the way. Fig 7 A runway crossing blocks the runway during the time taken to taxi the distance shown. If 20 aircraft or vehicles cross per hour, capacity may be reduced by 25 per cent. Fig 8 A crossing near the threshold enables aircraft or vehicles to cross while the takeoff or landing is still in progress. Fig 9 A staggered crossing should if possible be reconfigured as shown in Fig 10.
To achieve efficient utilisation of the runway, an efficient taxiway layout is necessary. Sometimes relatively simple lowcost changes to the airport layout, based on the concepts of (a) reducing the average runway interval, (b) minimising runwayoccupancy time, and (c) separating the arrival and departure taxi routes, can increase runway capacity and reduce air traffic delays. Side benefits will include a reduction in air traffic control workload per aircraft, with a re·sulting increase in safety. ■
Arrival and Departure Streams High capacity requires the capability to sustain an uninterrupted flow of arrivals and departures. The taxiway layout ·should
Reprinted with the kind permission of the Journal of ATC. The article first appeared in AIRPORTS INTERNATIONAL.
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