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Home | Update | Latest Issue | Gallery | FR Profile | Datafiles | FR 9/97 AIRBUS A320 FAMILY STILL GOING STRONG by Norbert Burgner In none of the airliners before (and after) was the concept of a digital flight-control and information presentation realized more uncompromisingly than in the Airbus A320. And, never before had experts discussed so intensely the aptness of the new technologies. "Did your computer fly you home again today?" was one of the standard greetings from Lufthansa Boeing 737 crews direted to their A320 colleagues. Fly-by-wire (FBW), digital flight control, and side-stick control are the known attributes of the Airbus twin. However, the A320 was not the first application of FBW and digital system controls. The Concorde, which entered airline service in 1976, had, from the beginning, a digital control of its engine inlets, allowing to optimize the air flow into the engine depending on the atmospheric conditions. Furthermore, the Concorde was equipped with a FBW system for stabilization around all three axis. The system, however, was an analogue one. In the Airbus A300FF, that entered the market in 1981, the aircraft's communication/navigation equipment communicated with the auto-flight system via digital data buses. The Boeing 757 and 767 followed in 1982, already offering digitally controlled CRTs which replaced several conventional electro-mechanical flight instruments. By introducing the Airbus A310 and A300-600, the application of the glass-cockpit concept was continued. For the first time, these aircraft featured a digital control of the secondary flight controls (flaps, slats, spoiler). Based on the experience with the electric signaling, Airbus made the decision to fit its new twin with an entire fly-by-wire flight control. FBW in the A320 means, that control inputs are relayed through wires and hydraulic servos to the flight controls. In the A320, not only the secondary flight controls are electrically activated but, also the primary flight controls (ailerons and elevator). Only the rudder and the elevator trim are mechanically controlled, such creating a safety system which allows to fly the A320 in case of an emergency. The A320's digital electronic flight control system (EFCS) is the real difference to all other preceding designs. The system is based on a total of five main flight-control computers (three spoiler/elevator computers and two elevator/aileron computers). All computers are laid out in a two-channel configuration, meaning that the active channel is checked by the other, slightly delayed operating channel. In order to insure safety, both computers and the software are produced by two different manufacturers. Also, redundancy of the electric wiring is achieved by using separate cables. The system is coupled to the autopilot. According to the Airbus engineers, the big advantage of EFCS is the always present control which protects the aircraft from dangerous flight conditions such as stalls, over-speeds or uncontrolled flight. The computers are continuously comparing the existing conditions with the programmed flight envelope. Furthermore, the software makes it possible to store certain characteristics, allowing, for example, to program an automatic pitch compensation which is activated when the pilot banks the aircraft. Just one control input is sufficient to make a level turn, while with conventional flight control systems, the pilot must make two inputs, the bank, as well as, the pitch correction to maintain altitude. Furthermore, with each input into the A320's flight control system, the computers automatically generate a trimmed condition. Once the input on the side-stick is stopped, the aircraft maintains the selected flight attitude until a new input follows. If the aircraft is flown a too high angle of attack and maneuvered close to a stall, the computers automatically generate a pitch down movement, allowing the crew to fly the aircraft safely within a computer supervised flight envelope. The A320 was fitted with an efficient ARINC 700 package with a digital automatic flight control system, which integrates the functions of the above mentioned autopilot (coupled to the FBW system) and the digital flight management system (FMS). According to the control input and air data processing, the information is displayed digitally also. For this task, the cockpit was suited with an electronic flight instrument system (EFIS), comprising of a total of six 18,5 x 18,5 centimeter displays from Thomson-CSF/VDO. Carrying on the design stages of the A300-600's and A310's electronic instrument system (EIS), the color CRT's in the A320 allow to present all relevant data, while minimizing the use of electromechanical instruments. The so called primary flight display (PFD) presents the usual attitude indicator in connection with other data, such as airspeed, altitude, vertical speed, heading or coupled landing systems. The next display over is the navigation display (ND) which can display route information, a compass card or the weather radar picture. The displays of the electronic centralized aircraft monitor, which are vertically arranged in the center of the instrument panel, present the technical condition of the aircraft. The top of the two displays shows data of the primary engine parameters (N1/fan speed, EGT, N2/high pressure turbine speed),as well as, the fuel flow, the operational status (flap and slat position), along with other information. The lower system display presents system malfunctions and possible consequences in clear writing. Checklists give instructions to correct the malfunctions. The A320's digital structure is completed by the FADEC engine control. With the full authority digital engine control, the CFM56 (or V2500) turbofans are not controlled via hydraulic systems any more but, like most of the other aircraft systems, via digital electronics. This allows for the computation of the optimum power setting for each condition of flight. The throttles are set by the crew to the desired position (take-off, climb, cruise, descent, or reverse) and the computer sets the appropriate power. But what happens if the A320 looses its electrical power? In cockpits with a conventional electromechanical instrumentation, a power loss leads most often to a failure of most of the instruments. The maneuverability of such mechanical aircraft can normally be maintained. However, since the way to a digital signal transmission has been consequently gone by the Airbus engineers, a conventional mechanical back-up system is missing in the A320. However, the aircraft is still flyable in the case of a power loss. The energy supply is backed up in several ways. Along with one generator per engine, a third generator is powered by the APU. A fourth power supply is available by extending a little fan generator into the airstream. The chances of experiencing a complete power loss in an A320 are calculated to be at around one in one trillion. Should this happen, the above mentioned mechanical system of the rudder and the elevator trim ensure a safe landing. From page 36 of FLUG REVUE 9/97 Home | Update | Latest Issue | Gallery | FR Profile | Datafiles | FR 9/97 Copyright 1997 by Motor-Presse Stuttgart. All rights reserved. Last updated August 5, 1997 FLUG REVUE, Ubierstr. 83, 53173 Bonn, Germany