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Aviation History
1964
1964 - 1985.PDF
10 FLIGHT International 2 July 196- Approach controllers at London Heathrow Airport AIR COMMERCE . . . APPROACH SPACING and its Effect on Airline and Airport Economics BY A. L. RACKHAM I| N the years since radar was first applied to civil aviation arevolution has taken place in air traffic control techniqueswhich has passed almost unnoticed. The application has been gradual and large areas of Europe still have inadequate cover; but the use of radar has greatly improved airline economics by enabling aircraft to be handled much more rapidly than was possible with procedural control. It is in the terminal areas, during the arrival and departure phases, that the biggest advances have been made; but, with traffic figures increasing annually, some terminal areas at peak periods are already under severe pressure, and it is possible that some of the economic advantages gained by the use of radar may be lost unless traffic control techniques and equipment are improved. A key position is occupied by the men who position aircraft on to final approach, because runway utilization is the ultimate limiting factor for a particular airport, regardless of any effort applied elsewhere in the system. Assuming still-air conditions and a constant groundspeed on final approach of 140kt, if it were possible to space aircraft accur- ately at three, four or five nautical mile intervals as at touchdown, then the hourly landing rate on a single runway would be 47, 35 and 28 aircraft per hour respectively. Whether any particular spacing is achieved depends on safety factors, equipment limitations and the ability of the individual radar director. Provided there is a constant traffic offering, the controllers who take aircraft from the stacks and position them on to final approach carry not only the heavy responsibility for the safety of the aircraft concerned, but also to a large extent for the economics of the operators and airport authorities. At a hypothetical airport with landing fees of £50 per aircraft, with constant traffic every day of the year, the annual income from landing fees alone at the hourly movement rates quoted would reach the staggering sums of £20m, £15m and £12m. However, landing fees are by no means the only source of income. A modest airport tax on each departing passenger would yield as much as landing fees, while the amount of other income would also have a direct relationship to traffic figures. Such traffic offerings will, obviously, never be achieved, but the figures serve to illustrate the importance of spacing and its effect on runway utilization. A decrease in the spacing distance of only half a mile between aircraft would have a profound effect on the econo- mics of both airline operators and airport authorities. Because safe spacing on final approach depends on the skill and judgment of a handful of air traffic control officers it follows that the operators and airport authorities have a very real financial interest in the capabilities of the control staff and control system. This interest may not be significant on a low-density field, but where traffic peaks approach maximum runway utilization, the economic factors are very considerable. The runway utilization achieved may be the deciding factor in whether or not a new airport should be developed, at a cost of many millions of pounds, to accommodate excess traffic demands. A few hours spent at London Heathrow Airport in the summer will confirm that aircraft land and depart with monotonous regu- larity ; but, as the significant factor is accuracy of spacing on final approach, let us examine some of the problems of the radar directors whose responsibility it is to achieve this accuracy, taking Heathrow as an example. The first and most obvious difficulty to be overcome is the marked difference in approach speed of various aircraft types. This diffi- culty is aggravated by the fact that even the same type of aircraft will be flown at a variety of different speeds depending on operating weight and conditions, and even on the whims or particular tech- niques of the aircrews. This difficulty can be reduced to some extent by requiring aircraft to remain within certain speed ranges, but difference in speed remains a major problem in accurate spacing. The second difficulty is that of wind effect during intermediate and final approach. Since aircraft converge on Heathrow from many points of the compass and wind speed and direction vary with change of altitude, wind effect is impossible to assess with any accuracy. It will depend on the heading flown, the speed of the aircraft, the altitude from which it is descending and even the time taken to complete the approach. Numerous other factors affect spacing on final approach. The radius of turn depends on the rate of turn and aircraft speed, while the effect of wind during the turn, which also depends on the number of degrees through which the turn is made, must also be assessed. Yet another factor is pilot reaction time, or the time taken to initiate a turn, as this may be affected by aircraft speed and/or control characteristics. There is also a possibility that the controller may be prevented from passing instructions at the critical time be- cause of other aircraft transmissions, or that instructions may be wrongly given or misunderstood. Aircraft may also be prevented
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