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Aviation History
1976
1976 - 0076.PDF
102 AIR TRANSPORT FLIGHT International, (7 January 1976 When nobody has control By Capt H. A. HOPKINS, technical secretary, British Air Line Pilots' Association THERE are still few things, even in these complex days, that are vital to the safe control of an otherwise mechanically sound and safely navigated aeroplane. The pilot is concerned principally with controlling attitude and keeping the aircraft within its speed and altitude envelope. The significance of attitude and speed information can be gauged from the degree to which the relevant flight instruments are duplicated or triplicated in heavy transport aircraft. Given these provisions, how then can a crew lose control in flight? Primary flight parameters are of course interdependent, and an immediate control problem need not arise should, say, a failure of all speed indications occur—provided that sensible use is made of attitude and power. If the pilot has had adequate and recent experience of "limited-panel" flying, an aircraft can be kept the right way up on little instrumentation. Regrettably, great store is not always set by the value of such exercises in routine crew refresher training. Reliance is largely placed instead on the redundancy built into the instrument panel. After all, the chances of a complete loss of speed information when three separate airspeed indicators are installed are surely quite remote. This would certainly be so were it not for the vulnerability of pitot probes to ice formation. This problem has long since been recognised, and such probes, among others, are required to be heated. The record shows however that unfavourable combinations of rain and temperature profile during a flight can still lead to temporary pitot-pressure obstruction in spite of the heating provided. While one such failure can be an inconvenience, a multiple loss can be a hazard, and neglecting to switch on the pitot heaters is a direct way of provoking such a situation. The December 1974 Roeing 727 accident near New York would appear to have been caused by such an omission. Attention should be drawn to the fact that a wide variety of operating practices apply to the in-flight use of pitot heat. While modern equipment operates satisfactorily on the ground, less sophisticated equipment used to suffer high failure rates unless non-flight use was kept to a minimum and pre-flight drills designed to delay pitot-heat switch-on until just before take-off. This type of accident could not take place if pitot heaters were switched on automatically in the air. It can be argued that crew drills should be able to take care of such a simple issue. Yet since it is common practice to provide automatic initiation of airframe or engine ice- protection when ice is detected, why not provide automatic pitot heating? There is no reason why the undercarriage weight-off switches should not be used to initiate heating if it has not already been selected. Moreover, it is difficult to conceive of any circumstances in which pitot heating is not advisable throughout the whole flight. The exact nature of the pitot-malfunction warnings is important when speed data are processed, together with other parameters, through an air-data computer system. The airspeed indicator itself may incorporate warnings of both discrepancy and ADC malfunction; add the fact that other speed-related warnings may arise from any central warning system and you have a high potential for distraction. Changes in speed indication as a result, of pitot mal function can be quite subtle and are, of course, dependent on height as well as time. What is more, when an instrument is displaying incorrect information rather than nothing, its reading can still be terribly compelling. Speed is but one parameter among many, and erroneous ASI indications should not under normal circumstances interfere oatastrophically with a crew's scanning pro cedures. It is widely recognised, however, that "normal" disciplines face a heavy test when things start to go even a little wrong. When considering the 727 accident one is also led to question the use that was made of the attitude information provided by both main and standby instruments. The dominance of speed as a parameter in current operating practice must be appreciated. Speed is basic to performance, configuration, noise-abatement and ATC procedures; autopilot control in both climb and descent commonly relies on the speed-hold mode. This latter can lead to situations in which the autopilot acts like a highly skilled idiot—it can only think in terms of one parameter (speed, attitude or descent rate) at a time in the vertical profile—and an autopilot locked onto a spurious speed input will inevitably cause a pitch upset if left unattended. Only in the large, well-scaled attitude display driven from inertial-platform data do we have an instrument that can be used in pitch as anything more than a rough aiming point. Conventional instruments play a much more precise role in bank-angle adjustment for turn rate than they do in the management of pitch angle for vertical profile. The instrument fitted to my current aircraft only warns of a discrepancy of more than 4°, indicating the low accuracy to be expected at present. There is a lot of talk about "attitude flying," but do most current instru ments warrant being used for anything more than confirmatory cross-reference during a normal flight? I'll wager that a pilot who says that he is flying attitude has his eyes tracking back and forth across speed, altitude and descent rate as well. History of pitch upsets The history of pitch upsets affecting UK-registered aircraft includes the Comet accident at Ankara in December 1961 and the Vanguard accident at Heathrow in October 1965. In the first case it was said that too much reliance had been placed on pitch information, and on barometric instruments in the second. In August 1968 a complete loss of electrical power caused a Viscount, deprived of attitude information and pitot heating, to crash in Germany. It was subsequently ruled that the standby attitude Indicator should be provided with an independent power source. I feel that if the authorities and aircraft operators were to recognise more clearly the psychological dominance of speed throughout normal operations, more training time might be devoted to instrument-failure procedures, even at the expense of other instrument-flying tests. The standby horizon is a reliable if not too accurate source of attitude information even in the worst circumstances of failure. How much flying on this instrument is regularly practised? The wide range of primary instrumentation and systems provides the enormous benefit of often making the single failure a matter of no consequence. Automatic changeover of air-data and attitude inputs are frequently available. Strangely, however, while warning of excess or insufficient speed must be provided, direct warning of excess pitch or roll is not required. The fact that a pilot will rarely be faced with a major loss of speed and attitude informa tion makes this situation, unlikely though it is, a potentially hazardous circumstance. The discussion could be further complicated by consider ing whether the stick-pusher is likely to be a safeguard in cases of false speed indication, as opposed to upsets due to turbulence or pilot error. The Trident accident of June 1972 would appear to place its value in doubt. Safety depends as much on the survival of failure as on the quest for failure prevention. Imaginative training —rather than formal or routine instruction—must surely be the best foundation for this.
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