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Aviation History
1964
1964 - 1066.PDF
FLIGHT International, 16 April 1964 603 LOWER LANDING LIMITS Some Pertinent Questions by CAPTAIN LINCOLN LEE THOUGH considerable improvements have been made duringthe past decade in the equipment and techniques relating toinstrument approaches there have been few changes in the landing limits for transport aircraft. The most obvious explanation is that these improvements have been counterbalanced by the increased landing speeds of jet aircraft, coupled with the relatively reduced margins for error which their operation permits. However, certainly in this country and in America, pressures are now being brought to bear towards lowering the landing limits. The primary reason is economic: diversions and delays have become increasingly expensive, while the cost to an airline if its main base is closed can be excessive. A secondary reason is the impending advent of the supersonic airliner: like every new type of aircraft before it goes into service, the SST promises to be critical in some phases of its operation, and it is generally agreed that the less it becomes involved in low-level holding and diversion manoeuvres the better on every count. In any case, not only is it now appreciated that auto-approach and auto-flare are quite practicable, but demonstrations—such as those given by BLEU—have shown that robot-controlled blind landings can be safe and efficient. If zero-zero landings are already feasible, many people would judge that the present limits (which average something like 350ft critical height and 1,000yd visibility) could be lowered. If blind landings are eventually to become routine, a slow progression towards them would seem inevitable. If this line of reasoning is to be pursued, the sensible course of events would be to improve transition aids such as lights—which is being done—and also to ask pilots to demonstrate their ability to land under conditions of say 100ft/400yd, then to lower the limits progressively, first to say 200ft, and subsequently to something less. This is, in effect, what is happening, and already many pilots have been asked to give such demonstrations. In the writer's opinion there is no doubt that the holder of an ALTPL should, under favourable circumstances, be capable of delivering his aircraft in the slot at 100ft, especially if he can take advantage of modern director systems or auto-coupling. However, most holders of ALTPLs are employed in route flying, and are not closely associated with the development work involved in a step such as the lowering of limits. They may feel confident of placing their aircraft in the right position at 100ft, yet be uneasy about the safe completion of either the landing manoeuvre or the overshoot. In this article the writer attempts to pose a few of the questions which the average route pilot would like to have answered before further steps are taken to reduce minima which some pilots would consider already low enough. Altimeter Accuracy (1) Are the altimeters currently in use, and the associated alti- meter settings, accurate enough? If an aircraft breaks cloud at 300ft an altimeter error of 50ft is only a slight embarrassment; but if the cloud-break is at 100ft the same error is not merely three times more serious—it may well be calamitous. Yet the writer's experience is that an error of 50ft is not unusual. The ground calibration error permitted by regulations is surprisingly large, while there are sometimes ATC delays in reporting what may seem to be insignificant pressure changes. Other circumstances may cause the addition of further small errors: for example, under squally con- ditions there can be small pressure fluctuations over approach paths which are sometimes several miles from the airport pressure indic- ator. Eventually radio altimeters will no doubt be relied upon for greater accuracy, but as yet these are not widely used. Larger errors, in excess of a millibar, can obviously be more serious. These tend to occur in international operation, and are usually caused by the misreading of settings, failure to pass a QFE, the passing of a QFE for the wrong end of a sloping runway, or trom sloppy procedures sueh as "the setting is 007." (2) Can enough reliability be placed upon ILS equipment? It is a stale joke among airline pilots that when ceiling and visibility are unlimited the ILS is excellent, but in marginal weather it is uncertain. If one of the two ILS beams fails completely, then low limits will obviously not apply. But all too often the very weather that reduces the ceiling—be it monsoon, typhoon, snow, or penetrating damp—seems to affect the accuracy of transmission. Theoretically this should never happen, and at some airports it seldom occurs; others cannot make the same claim. At present, monitoring by PAR provides some protection against ILS faults. At lower heights this may prove more impracticable. System Reliability (3) In discussing the true blind landing of the future, it is normal to consider, in relation to autopilot and ILS equipment, a statistical failure rate; is it fair to extend this practice to the individual low- limits landing case? Because of their very nature, autopilot, ILS, and flight director systems seem prone to partial transient failures. In their present state of reliability they do not seem to imbue pilots with more than 99 per cent confidence. (4) Are the terms critical height, obstacle clearance limit, cloud ceiling, cloud base and break-off height precisely enough defined? The definitions, if one knows where to find them, are quite good enough for the 300ft case, but at really low limits a new problem arises. The long-range jet is a big aircraft, and its wheels, its static head, its glide-slope receiver and its windscreen arc at different levels. Under extreme conditions there could be 20ft or more in vertical distance between one and the other, a gross inaccuracy in the 100ft OCL case. If the cloud base is 100ft, the wheels may be at 80ft before the pilot obtains visual reference. (5) Compared with out-dated "visibility," RVR gives a more accurate picture of conditions, but is it accurate enough for the sort of landings envisaged ? Visibility along a 10,000ft runway can often change rapidly from point to point, and almost instantly in time, especially in the wake of a departing jet, or if rain or snow are falling. Even if these changes are measured by a series of electronic RVR meters and the information is relayed to the pilot, until he is com- mitted to the landing a deterioration can still occur before the aircraft comes to a stop. (6) After the transition from instruments, the pilot depends on visual aids for the flare and landing; can anything be done to standardize these? During recent years the tendency has been to add more and more lights: centre-line approach lights, touchdown- zone lights, bars and barrettes, VASIs and strobes, extended run- way edge approach lights, and so forth; but no two airports seem to have quite the same selection or pattern. When he breaks cloud the pilot is often faced with a bewildering array of dazzle which he must interpret, and if fails to see the green threshold lights he may even be left uneasily uncertain as to whether he has reached the runway proper. If, for example, he is landing a Boeing 707 at maximum landing weight his target threshold speed is 146kt, so that if the RVR is 500yd, the first lights which he sees may be passing abeam in less than 6sec. The effective time available to him for identifying an unusual pattern and gauging its significance is a fraction of this time. Six seconds is about one-third of the time it takes to read the next paragraph. (7) From what height is it safe to overshoot a jet aircraft on instruments ? The jet may have greater reserves of power, but its absence of slipstream and its swept-back characteristics complicate the situation. Furthermore, when approaches are made to a low level it will normally be considered advisable to have full flap extended, which again aggravates the overshoot condition, especially in those aircraft where retraction from the full position causes a marked change of trim. Except on rare training details, it is unlikely that pilots will get any practice at this manoeuvre. (8) If low limits are in force, will they be amended in unfavourable
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