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Aviation History
1972
1972 - 1633.PDF
934 INQUIRY BRIEFING Stall protection The stall-protection system adopted on all versions of the Trident consists of an electric stick shaker and a pneumatic stick pusher. In the landing configuration artificial warning will begin at a speed 8 per cent above the natural stall, but with the aircraft clean and leading edge retracted the margin is increased to 17 per cent. The system is duplicated off two incidence probes and, while either can initiate the action of the shaker, both must actuate separate valves in the air supply to the pusher before it will operate. The pusher consists of a pneumatic ram supplied with air at 70-901b/sq in, the supply bottle being topped up by bleed air from No 2 engine. The move ment of the control column is fast enough to take it from the pilot's hands, but the response from the aircraft is smooth and not of sufficient severity to require the incor poration of a negative-g switch. The action of the pusher imposes a force sufficient to give an autopilot overload and cause it to disconnect. To reduce the risk of the system operating because of a fault, and thus producing a dangerous situation, it is automatically isolated at speeds below 50kt and above 250kt. Additional audio warning is not fitted since the out-of-balance motor driving the shaker is inherently noisy. Warnings to the crew include an amber light indicating "stall recovery operating" and a light on the centre pedestal to show low air pressure in the pusher supply. Stall- recovery fail lights mounted above each pilot's airspeed indicator can be pushed to isolate the system electrically, but the recommended procedure for isolating the pusher is by means of the override lever at the rear of the centre console which dumps the air pressure to the system. The two< air valves which open to supply the pusher are self-closing when the aircraft has been restored to a low enough angle of attack. The variation of stalling speed with aircraft configuration on the Trident is wide. At the take-off weight of G-ARPI, approximately 50,000kg, with leading edges and flap lowered the shaker would operate at 126kt and if speed were reduced further the pusher would take over at 116kt. With the leading edges retracted and flaps up the pusher should operate when the speed reduces to 166kt. According to. published information the airspeed was 160kt when the leading edge retracted and the change of configuration would have caused the pusher to operate almost imme diately. llliH'H, il Continued from page 927 airline customers, and they, as well as private lenders, have demonstrated their willingness to put their own money at risk to cover a significant part of the total cost of the project. The Bill is not intended to make federal loan guarantees a way of life in the civil aviation industry. Most projects can be, and should be, developed and financed in the traditional method, by private capital at risk. The Bill would cover only those projects which, because of their size and scope, cannot be financed in the traditional way but which are clearly of major importance. The Bill's criteria for eligible loans are therefore such that relatively few projects would qualify. Under the Bill, an application for a loan guarantee would have to meet the following standards: (1) At least three airline customers (including foreign airlines) must have committed themselves to the project. FLIGHT International, 29 June 1972 Flight recorders BEA Tridents are equipped with two flight recorders, a conventional crash-proof equipment installed in the base of the fin, and a cassette recorder on the flight deck whose purpose is to monitor automatic approaches and landings. The Plessey-Davall Type 726 fin-mounted recorder is a refined version of the original Type 710 equipment and monitors 32 parameters per second, compared with its 24-channel predecessor. The latest requirements for recorders include measurement of engine thrust and of flap position and BEA has a development programme in hand which will include this information; it was not, however, available on the PI installation. The survival capability of this equipment is shown in the photograph of the wire removed from another accident. The Davall 1089 cassette recorder, although not required to be crashproof, is well protected by its location at the rear of the flight deck and that on PI survived the impact. This equipment, which is designed for quick removal and replay, monitors 64 parameters. It is not proposed at present that leading-edge-droop position should be a mandatory parameter, but this informa tion was being recorded on PI through a sensor on the selector lever, and this enabled the investigation team to reach a quick conclusion as to the cause of the accident. The recorders have also shown the sequence of operation of the stall-protection devices. • (2) The aircraft or aircraft engine would have to receive, or be judged by competent authority as likely to receive, appropriate certification from the FAA. (If an application is sought before certification is issued, the loan guarantee agreement could be made conditional on the later issuance of the certification.) (3) The aircraft or engine would have to (a) incorporate significant improvements in design, construction, or bene fits to the public; (b) be likely to compete with or be superior to comparable aircraft, if any, manufactured wholly outside the United States; and (c) be necessary to meet the reasonably predictable need of the public for efficient, economical, and "environmentally compatible" air transport. The second criterion would not rule out meritorious projects involving joint ventures between US and foreign manufacturers. (4) There would have to be committed to the project from the manufacturer, the airline customers, and lending insti tutions enough private funds without federal guarantee to satisfy the loan guarantee agency that the project has been adequately reviewed, and its risks shared, by the private sector. The Bill suggests that the amount of such non-guaranteed private funds be not less than 15 per cent of the total projected cost o the project. •
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