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Aviation History
1985
1985 - 2195.PDF
The flight control revolution Avionics has traditionally been the poor cousin of aircraft design. The k hardware associated with naviga tion, communications, and warnings systems has always had to be accommo dated after the structure and performance of the aircraft was settled. Today, because of phenomenal in creases in computing power, things are different. Digital computers can now make decisions critical to the control of the aircraft, such that the traditional interface between pilot and machine has fundamentally altered. Avionics has matured, and now ranks equally with all other design considerations from day one. The state-of-the-art in passenger aircraft avionics will be seen in the Airbus Industrie A320, due to enter service in 1988. It will be the most "performance optimised" aircraft flying, by virtue of the consistent, high-speed decision-making in which digital computers excel. Computers will also be used to improve the safety and comfort of the passengers to an unprece dented extent, says Airbus Industrie. The use of computers for critical flight control work in passenger carrying aircraft dates back to the mid-1960s, when Tridents were fitted with a Smiths triplex redundant autoland system. This highly successful effort, repeated on the VC-10 with a quadruplex Elliot Automation (now GEC Avionics) automatic landing system, undoubtedly did much to give the UK a head start in the development of elec tronic flight control technology. Another important milestone was achieved with Concorde, which has a fully automatic flight control system developed by GEC Avionics and Sfena. This has yet to fail in service, and remains unsurpassed in the level of automation it provides for a FLIGHT International, 6 July 1985 The control of airliners in flight is becoming increasingly dependent on digital com puters. The A320, which flies in 1987, and Boeing's 1992 airliner represent enormous advances over aircraft now in service. Julian Moxon reports on the state-of-the-art in flight control, and Harry Hopkins assesses what the A320 means to the pilot. civil aircraft. It was among the first of its type to exploit microcircuit technology, half of which was based on analogue computing elements, and the remainder on digital computing. Today, digital computers rule in all new flight control system designs, bringing lower weight, faster and more flexible computation, and a high degree of self-test and monitoring ability. The goals, as in all civil aircraft technology, are the reduction of the cost of flying from A to B, and the improvement of safety and despatch reliability. Digital electronics enable all of these aims to be achieved, although the evolution of flight control technology as represented by the A320 is such that safety considerations still limit the ulti mate exploitation of a fully fly-by-wire system. The correct meaning of the term fly-by- The A320 will be the world's first all-digital fly- by-wire passenger carrying airliner wire is lengthy, but should be stated. It is "the continuous adaptation and modi fication of control signals by computers to provide the optimum setting of the flying surfaces". In other words, signals gener ated and distributed electronically can be tailored to match exactly the flight criteria set by the aircraft's designers. In the A320, for example, the system is designed to make it impossible for pilots to exceed pre set flight envelope limits. They will thus be unable to stall, roll, overspeed, or overload the aircraft (see pages 27 and 27). In the UK, work on advanced flight control systems for civil aircraft has been carried out principally by the Royal Aircraft Establishment, Bedford, and British Aerospace aboard BAe One- Eleven testbeds, and by GEC Avionics. The RAE One-Eleven has been fitted with a Smiths Industries duplex autopilot (providing full automatic landing capabil ity) and a digital computer which has been used (among other things) for investiga tions into direct lift control and relaxed static stability. Direct lift control was achieved by linking speed brakes to eleva tors to enable much more accurate glide- slope tracking during the descent, along with reduced throttle cycling and fuel burn. Flight trials were also carried out to examine the handling characteristics of the One-Eleven with reduced stability margins. British Aerospace's work (on its own One-Eleven) had indicated that rela tively small reductions in static stability gave useful reductions in trim drag. This was confirmed by the RAE, which used the direct lift system to destabilise the aircraft, maintaining handling character istics artificially. Use of the DLC system meant that relaxed stability could be 23
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