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Aviation History
1972
1972 - 1631.PDF
932a Inquiry briefing Trident G-ARPI crashed on June 18 following retrac tion of the leading-edge droop sections at a speed lower than normal. On June 19 it was announced that a public inquiry into the accident would be held. "Flight" suggests certain courses of action for this inquiry and we highlight factors which have a bearing on the behaviour of the aircraft and crew during the emergency. Questions for the inquiry BEA PILOTS are not normally checked out "in anger" on stall-recovery procedures. This regime is not inherently dangerous on the Trident, but too many training accidents have happened in the airline industry as a whole. The flight simulator does give an absolutely accurate representation of the Trident's stall, including the action of the stick pusher. In the case of this accident it is doubtful whether, at 1,750ft and in cloud, any pilot however trained or experienced could have recovered. The inquiry will have to discover whether premature droop-retraction has ever happened before in Trident operations; and, if so, whether the incident-reporting system was used effectively. The inquiry must also seek to discover how an item could possibly be taken out of sequence from the drill card if, as in BEA, that sequence is practised time and time again in training and in service. While it is mechanically impossible for droop to be retrac ted before flaps have been selected fully up, and while the linkage was designed to be mechanically irreversible (see diagram), could the droop have retracted untouched by human hand? This will have to be probed too. Five amber lights flash if droop should be prematurely retracted. The inquiry will try to discover if these lights went on, and whether they could have been overlooked. The Trident, in common with all other aircraft, if given a sudden pronounced change of aerofoil camber 20kt below the appropriate stalling speed will stall immediately, and descend at a rate probably of at least 8,000ft/min. Should the droop have been engineered not to retract below a safe speed, or would this have been legislating against an improbability so extreme that it was rightly not just discounted, but not even considered? The Trident was in fact never flown in such a configuration on test, nor was it required to do so by the airworthiness authorities. Could the stick pusher have saved the situation? The Trident was the first aircraft in the world to be certificated with this protection against the deep-stall characteristics of high-tailed aircraft, which have caused at least four fatal accidents. The recorder shows that in this case the stick pusher operated, as it would have been expected to do in the circumstances. Indeed, it appears to have operated more than once. The Trident stall was squared off to the satisfaction of its manufacturers and the ARB in the autumn of 1963, after completing what must have been one of the most comprehensive stall programmes on record—some 3,500 stalls were performed by John Cunningham and his team. The stick pusher operates after the stall-warning light and stick shaker. It can be dumped by operation of a lever (see diagram). The inquiry will want to establish the position of this lever and whether—if it was found in the dumped position—this was the result of impact. Whatever this finding, it is probable that the sequence of events in this case would have been the same whether the aircraft had a stick pusher or not—as in the case of the Boeing 727 and Douglas DC-9. Whatever protective devices are installed, retraction of droop or slats at low speed and low altitude must lead to a stall and high sink rate, regardless of the type of aircraft. FLIGHT International, 29/une (972 Flight path Departure routes from London Heathrow are determined to suit the needs of vertical and horizontal aircraft separation and of noise abatement. The vertical profile is the combination of the initial cleared altitude of the aircraft and the need to cross the noise-monitoring point at a height of not less than 1,000ft. The latter requirement necessitates a steep initial climb, normally with take-off flap retained throughout, and in the case of PI this climb was maintained until the aircraft had reached its initial cleared altitude of 1,500ft (London QNH setting). After changing radio frequency from Heathrow departure to London Centre, PI was given clearance to climb to flight level 60 (6,000ft on standard setting) and this climb had been started at the time of the change of leading- edge profile. The horizontal departure profile is specified in the United Kingdom Air Pilot and this instance is referred to as a Dover One departure. The aircraft is required to follow the extended centreline of the runway (28R) until the middle marker of the reciprocal ILS is heard. At this point, 34 mile from the end of the runway, a turn should be started to bring the aircraft on to a track of 145° to the Epsom beacon. This turn at Trident speeds is flown with 20° bank. PI followed the standard departure and was turning steadily up to the moment when the emergency began. It had not completed the turn to a heading for Epsom and at impact the aircraft was heading approximately 210°. The inboard section of leading edge and the adjacent Kruger flap can be seen below in the extended position. The outboard leading edge has vortex generators on a closing plate which fills the top-surface gap when the nose is lowered
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