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Aviation History
1987
1987 - 0874.PDF
accurate, four computers. The throttle levers also "speak" to computers. There is a considerable psychological problem among pilots and certificators towards electronic control, says senior vice-president engineering Bernard Ziegler. The test programme is intended as much as anything else to work toward dispelling this automatic mental attitude which believes that, as Ziegler puts it, "hardware is more reliable than the electronic". He is fond of using parables involving animals to explain the concep tual change: there is his now-famous likening of the advance in the A320's control to the one major advantage that a horse and cart has over the motor car—the horse follows the road automatically while the car has constantly to be steered. Now he explains the advance from mechanical connections to electronic instructions with this simile: "We are moving from the physical to the mind. If we had not gone this way we would still be monkeys". With electronics, he adds, "we know exactly what we are doing. There is no guesswork any more". The psychological problem Ziegler refers to tends to disappear very quickly among those who become involved with operating the A320, he says. But he confesses to being concerned about a lack of media comprehension. About 75 per cent of all air transport accidents have pilot error as a significant or originating component, says Ziegler. The A320 is designed to reduce pilot error not just by the doubtfully-effective method of reducing workload, but by actual prevention of the pilot from acci dentally exiting the flight envelope, and by allowing him to fly the aircraft right to the limits of the aircraft's performance safely when the situation demands it. What about the problem that a pilot once converted to the A320 will take con siderable conversion back to a tradi tionally controlled aircraft? "Do we have to refrain from advance because of a difference in handling and problems with conversion back to the old system?" asks Ziegler. And he points out that the new Airbuses, the A340 and A330, will be fly- by-wire. Boeing plans it for its 7J7. At the end of this month the A320 will have production standard Snecma/ General Electric CFM56-5 engines fitted to it and the programme from there onward will be in a position to guarantee engine performance. Two engines have been shipped back to Cincinnati to have the batch of improvements installed which brings them up to production stan dard. These include installation of a reshaped fan stator which improves flow stability; turbine modifications; and the light alloy engine cowling which was tending to distort slightly in flight is being replaced with the production carbonfibre one. There are two aircraft involved now in the A320 test programme. The first aero plane had made the type's maiden flight on February 22, and the second joined it on April 27. Airbus had positively lunged into the programme, hitting height and speed maxima during the maiden flight on a day where instrument meteorological Right Test pilots are pleased with the A320's handling characteristics. They say "it is not like a computer to fly". Below On the approach at Toulouse. Below right The flight- deck is all-digital, with sidestick controls conditions prevailed between 300 and 12,000ft (Flight, March 7, page 26). Test ing is some 20 per cent ahead of schedule. It is probably self-evident that a great deal of the A320 is basically the same as any other airliner and therefore has to undergo the same testing; but the part of the programme which is exercising every one's minds to the limit is definition and final adjustment of the flight control laws, which ultimately means the software which determines how the aeroplane reacts to the pilots' sidestick inputs via the flight management system (FMS) com puters to the controls. Various control modes can be selected during flight, which determine just how much protection the pilot has against flight envelope exit conditions like stall and overspeed. When Normal is selected full protection applies. There is no ques tioning of normal mode flight control laws as they apply during almost all of any flight. An example of an area where there is still discussion about finalisation of software is the landing phase. Normal mode, when the aircraft is well clear of the ground, means that the A320 responds to sidestick input according to a programmed manoeuvre demand law. In pitch it delivers a flightpath, not merely a rotation about an axis. Sidestick pitch 112 deflection is proportional to g demand, with pitch rate feedback giving stabilisation. In normal mode, roll rate is proportional to manual deflection of the sidestick, but is limited to 15 "/see. Yaw normal gives auto matic yaw damping, turn co-ordination, and rudder application limitation, even though the rudder link is mechanical, not fly-by-wire, but is linked with the flight management guidance computer (FMGC). All of this software is defined but for possible minor adjustments. Problems emerge in deciding what the pilot wants to feel during the last part of the final approach and during the landing flare. Remember that the sidestick has no artificial feel, it is simply spring-loaded central. Letting the stick go so that it takes up its neutral position does, during flight, deliver a straight-line flight path whatever you subsequently do with the speed or the throttles (until the aircraft approaches either the stall or maximum operating speed, when appropriate atti tude changes automatically start to be applied for protection), because stick neutral means no demand for g. Imagine the pilot is using the sidestick under normal mode as he starts the land ing flare; as soon as he had brought the attitude and the rate of descent to what he wanted he would have to begin to reduce FLIGHT INTERNATIONAL, 13 June 1987
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