The Role Of Computers In Fly By Wire Aircraft Essay, Research Paper

THE ROLE OF COMPUTERS IN FLY BY WIRE AIRCRAFT.

AN INTRODUCTION TO FLY BY WIRE(FBW):

Traditionally pilots use an inceptor (a wheel, control column, or sidestick) to manipulate cables that control pumps and hydraulic actuators. The actuators in turn operate surfaces such as flaps, ailerons and rudders to provide the necessary effect (e.g. roll left, yaw right etc).

The FBW inceptor is usually a sidestick (so called because it is mounted on the pilot s side). Moving the stick generates electrical signals that are processed by the onboard Flight Control Computer (henceforth called FCC) and relayed as control signals to energise pumps, move actuators and position control surfaces. The set-up, inceptor to Relayer to Actuators constitutes a Flight Control System (henceforth called FCS).

The fundamental difference between conventional- mechanical FCS and FBW FCS is the relayers. Whereas conventional FCS connect inceptor to actuators mechanically via cables and mechanical linkages etc, FBW FCS use electrical signals sent down wires (hence FBW) from inceptor to actuators to achieve the desired effect. The FCC therefore is central to a FBW FCS and acts as a nerve centre where mechanical inputs from the inceptor are interpreted into electrical signals to be relayed to actuators. Here the electrical signals in turn cause mechanical movement of actuators and thus the flight control surfaces and ultimately the aircraft.

EXPLANATION OF TECHNICAL TERMS:

Inceptor: Joystick, control column or sidestick where command input is made.

Actuator: A device imparting mechanical motion.

Flap: An auxiliary wing surface basically designed to increase lift or drag as necessary located on trailing edge or wings.

Ailerons: Horizontal control surfaces on trailing edge of wings.

Rudder: Vertical control surface on trailing edge of vertical tail.

Angle of attack (AOA): Angle at which the cord of an aircraft wing meets the relative airflow.

Roll: Movement of an aircraft about the horizontal axis.

Bank angle: The angle of the aircraft wing measured from the horizontal during a roll.

Yaw: The rotation of an aircraft about its vertical axis.

Yaw angle: The angle between the longitudinal axis of the aircraft and its initial, undisturbed direction of flight.

WHY A FLY BY WIRE FLIGHT CONTROL SYSTEM?

Efficiency

The major advantage of computer-controlled flight is greater efficiency. The computer precisely controls all the flight control surfaces on the plane and in the process tailors the performance of these control surfaces to flying conditions for maximum efficiency. This improves handling significantly and cuts fuel consumption as compared to conventional aircraft. This advantage is even more marked in long haul, commercial aircraft like the Airbus A320, A340 and the Boeing 777 where operating efficiency is key to the survival of an airline.

A FBW FCS is also simple and confers a weight advantage over a conventional FCS since it has fewer parts and all the mechanical cables, linkages are eliminated and are replaced by lightweight wires and a FCC this in turn also saves further fuel. A lightweight is also critical to the performance of a combat aircraft, the lower the weight of the airframe etc. the faster the plane can go for the same thrust from the engines. Also more weapons can be carried.

The inherent simplicity of a FBW aircraft also renders it much easier to maintain. In-flight malfunctions and anomalies can be identified by the Central Maintenance Computer, which both identifies a failed component and assesses the integrity of repairs. A record is kept of such instances available for study by an on-ground engineer on landing, this makes a FBW FCS even more efficient as it improves troubleshooting capability, further simplifying maintenance and producing quicker turnarounds.

Reliability

FBW FCS is also considered by most to be more reliable than a conventional FCS. Because the FCC is so critical to the stability of the aircraft a malfunction would have catastrophic consequences. For this reason the primary FCC has independent backup computers which kick in in case of a total primary systems failure. This is known as Multiple Redundancy. A majority of FBW aircraft have quadruple redundancy (four backup computers). The Lockheed Martin F-16 Fighting Falcon, the Eurofighter Typhoon, the Airbus Industrie A340 fall in this category. But the more recent Boeing 777 incorporates 5 backup FCC s, as does the Airbus A320. Multiple redundancy drastically reduces chances of a total systems failure (in the case of the A320 it is equivalent to only a 1 in 10 chance of failure in a fleet of 1000 A320 s throughout their 30 year service life). Multiple redundancy confers failsafe/fail operative features to a FBW aircraft and thus increases reliability.

Safety

The FCC acts a the pilots eyes, ears and generally, a best friend keeping him/her out of sticky situations and keeping him/her informed about flight conditions at other times. This serves to alleviate the workload on the pilot considerably. The FCC also automatically damps out /trims turbulence or gusts allowing for a more comfortable flight. FBW also eliminates the danger of stall and overspeeding (an aircraft stalls when it loses lift, stalling depends on g , AOA, speed etc). Total structural protection results. Because the FCC is programmed to prevent manoeuvres for which it was not designed the aircraft structure cannot theoretically be overstressed.

A case study of the A320 FBW airliner:

The A320 is a commercial FBW airliner which will serve to demonstrate some of the safety feature of computer-controlled flight.

The A320 FCC is programmed with a maximum bank angle of 65 degrees. Beyond 33 degrees bank it is necessary to maintain pressure on the sidestick in the direction of turn because the FCC is programmed to return the aircraft to 33 degrees bank when the stick is released at higher bank angles. The FCC does not permit the aircraft to be rolled beyond 65 degrees bank, which would be dangerous.

The maximum safe speed at an altitude of 12000 ft is 345 knots (kt). Past this speed the FCC issues oral overspeed warnings and tries to raise the nose to slow the aircraft down. It becomes necessary to apply increasing forward pressure on the sidestick to keep the nose down and the speed above 345 kt. The FCC then maintains this attitude to prevent airspeed from increasing above 360 kt no matter how hard the stick is pushed forward. When the stick is released at 360 kt the FCC automatically raises the nose to maintain airspeed of 345 kt.

If the stick is pushed forward to drop the nose 15 degrees below the horizon and the aircraft is also rolled 45 degrees then as the plane approaches 345 kt the maximum bank angle is reduced from 65 degrees to 45 degrees. Speeds above 345 kt in such situations causes the aircraft to return to a wing-level situation as the FCC begins to exert pressure on the stick to raise the nose and slow the air speed.

The A320 also has stall protection through the incorporation of AOA limits beyond which the FCC will not permit the aircraft to go no matter what control inputs are made by the pilot. An angle of 15 degrees has been designated as the AOA floor at which the throttles /engines advance automatically to full power in order to avoid stall. 17 degrees is AOA limit and the point which the FCC will stabilise the aircraft for a maximum power/maximum climb situation.

An additional safety feature is employed to stabilise the aircraft in case of the failure of one of the engines in such a case the remaining engine (e.g. right engine) goes to full power and the FCC holds the aircraft in a 10 degree right bank and sufficient rudder deflection to counter the thrust from the right engine.

Other features of FBW:

A FBW FCS also has other interesting consequences in that aircraft that are aerodynamically unstable (can t fly) because of their aerodynamic design can, to put it simply fly by incorporating such a FCS. Examples of such aircraft are the Lockheed Martin F-117A Nighthawk (the black, stealth bomber) and the Eurofighter Typhoon. Both these aircraft are inherently unstable in flight and no human pilot could posses reactions rapid enough to prevent the aircraft from going violently out of control and breaking up. Only the constant intervention of the FCC can feed in controls fast enough to keep the aircraft stabilised, thus conferring artificial stability to these aircraft.

Because now the aerospace engineers don t have to worry about making the aircraft aerodynamically stable they can concentrate on other performance enhancing features which would not otherwise have been possible. This is exactly how the awkward looking F-117A stealth bomber was produced. The FBW FCS gave the aircraft artificial stability so that the engineers could configure the surface of the aircraft to achieve stealth by a lower radar cross section.

THE CASE AGAINST FBW:

Critics of FBW argue that the FCC has ultimate control over the aircraft leaving the pilot as a mere supervisor. They argue that because the pilot is less involved with flying the aircraft his situation awareness suffers. Also the FCC imposes non-transgressable limits to a combat aircraft s performance. They point out that Multiple Redundancy is a myth since computer systems tend to crash/malfunction under the same parameters. This means that on the failure of the primary FCC the backup systems would not respond either, this would lead to a total loss of control and potential disintegration of the aircraft (such a failure would be even more dangerous in aerodynamically unstable aircraft). They are also keen to point out that in the period 1987-1993, 4 A320 s crashed because- as they see it of the unreliability FBW

THE FUTURE FOR FBW:

It is evident that a FBW FCS has its faults but as my limited research has shown the advantages far outweigh the disadvantages and risks. It can be argued, I think, convincingly that indeed FBW FCS will replace conventional FCS. The trend has already started, all new airliners and combat aircraft have incorporated FBW technology. Indeed I am surprised as to why it took so long to gain recognition as the next generation in FCS.

But FBW is not in any way restricted to the air-going type traffic, a cunning variant has already been incorporated into Formula 1 racing cars, and soon we will probably be going for a spin in Drive By Wire saloons. I for one can t wait.