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Aviation History
1984
1984 - 0474.PDF
AIRBUS UPDATE Pioneering FBW controls When it comes to certifica tion, the A320's fly-by-wire (FBW) controls are bound to be a major issue. Airbus Industrie points out that FBW is already used in a limited way aboard A310, however, on which it is used to command slat, flap, and airbrake movement. There is also the precedent set by fighters such as the all-FBW F-16, and the ground broken by Concorde—albeit cush ioned by manual reversion. The A320 will be the first airliner to have FBW primary flying controls. Its ailerons and elevators will only respond to electrical signals generated by the pilot's side- stick or the automatic flight system—there is no backup path via mechanical cables and pulleys. So if the four electronic channels linking sidestick and aileron actu ators fail, all aileron move ment is lost. The same applies with the elevators. Airbus Industrie's answer is to provide an alternative way to control aircraft pitch and direction through mechani cally operated secondary controls. Loss of elevator movement is compensated for by moving the whole tail- plane, for example, via mechanical trim (the tail- plane also has electric trim). Total aileron failure is covered by A320's mechani cally operated rudder, which offers an alternative way to "point" the aircraft. A320 will also have FBW spoilers, and, like the A310, electrically signalled slats, flaps, and speed brakes. The A320's use of FBW includes the link between throttles and engines, allow ing maximum benefit from the specified full-authority digital engine control (Fadec). The Fadec system will be dual-channel on each engine, with dissimilar software to minimise the effects of unforeseen "bugs" in the computer programs. Even if there is a complete failure of all FBW links between engines and throttles, all is not lost. Instead, each engine will automatically select and maintain 45 per cent power. Airbus Industrie admits that Fadec was not part of the original CFM56-4 specifica tion, but it has now persuaded CFM International to include it in the engine, and will do the same for V.2500. All of the A320's FBW links will be electrical—there are no plans to use fibre-optics. The links to primary flying controls will be quadruplex. The company cites weight saving as the main factor in its decision to adopt FBW, but there are other important advantages. These include maintenance savings, easier integration of the automatic flight system (AFS), and the ability to introduce active controls in the future. Provision for future active controls extends to the hy draulic jacks that operate the control surfaces. These jacks will be designed for high rates of movement, for example, plus superior reliability. Colin Stuart, manager of sales technology, explains that provision for active controls, and hence artificial stability, offers a way to "grow" A320 performance in future versions. This is part icularly important given Airbus Industrie's extensive optimisation of A320 for the 150-seater role. Take the A320 wing, for example, which has been made just big enough to do its job—there is no "penalising" additional area to provide for growth versions. Instead, Airbus Industrie expects to raise the lifting power of the wing by techniques such as load- alleviation. Ailerons and spoilers can be made to deflect rapidly upward when the aircraft flies through a gust, for example, thus keeping the loads "felt" by the wing within limits. A similar technique has already been used by Lockheed to increase the wingspan of its TriStar 500, without reinforcing the wing. Active controls could also lead to a reduction in fin and tailplane area, currently sized by the need to provide natural stability. Alternatively, they could allow the same fin and tailplane to stabilise a larger aircraft. The pilots' office Each pilot is confronted by his Efis (electronic flight instru ment system) panels—two side-by-side CRT displays. His left-hand display is the PFD (primary flight display), the right-hand one the navi gation display. The PFD carries all the information which a pilot gets from the standard "T" arrangement of primary instruments in a conventional cockpit. Moreover, the infor mation is laid out in "T" format; artificial horizon/ flight director central on the screen; vertical scales showing IAS to the left and altitude to the right; and horizontal heading scale at the bottom, extending 50° from left to right with the heading pointer in its centre. Other basic information, shown digitally only, is the Mach number below the speed scale and the altimeter pressure setting below the altitude scale. Each pilot's right-hand display, the navigation display, will be similar to that which an A310 pilot can call up on his navigation display today. Normally the aircraft symbol is bottom centre on the screen with a 90° compass arc at the top and the course and waypoints between shown like a map. Weather radar returns are super imposed. Shown digitally at top left is ground speed and TAS; at top right the time and distance to go to the next waypoint; at bottom left the wind velocity (with arrow); and at bottom right the radar tilt and the time. The pilots can select a new compass rose display, which puts the air craft symbol at the centre of a complete 360° compass rose (all of the above information is on the display). The pilots can call up to the display any airport arrival or departure procedure which the FMS has in its memory. "Plan mode" puts a selected waypoint in the centre with north upper most. Those two side-by-side displays on each pilot's panel constitute the aircrew's complete flying/flight man agement information display, but just to the right of the captain's screens, exactly where they normally are, are three refreshingly familiar mechanical flight instru ments—the standby flight instruments. The absence of a between- the-knees control column for either pilot means that the aircrew can have "map tables" which slide out of the main A320 will be the first airliner with fly-by-wire primary flying controls (ailerons and elevators). Mechani cal operation of rudder and tailplane-trim provide an indirect backup FLIGHT International. 24 March 1984 u
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