Fly-by-wire Primer

Fly-by-wire Primer

Edited From Airbus FAST publication

March 2000


From the early days of aviation until the times of the Stratocruiser and Super Guppy, flying an airplane was often hard, physical work. Battling against the elements, pilots had to navigate their flying machines by manually operating control cables that were connected to the surfaces of flaps, ailerons, elevators and rudders.

Larger and faster aircraft required more than human strength to control them, so having powerful hydraulic systems that the pilot operated via the controls, cables, and pulleys, were introduced. In the early 1980s, secondary flight control design began to utilize electrical signals from the control lever via computers to the hydraulic actuators of the surfaces.

The new fly-by-wire system extended this technology to primary flight controls. The conventional yoke was no longer needed because the flight deck commands were transmitted electronically and was replaced by a smaller lever, the sidestick. The new system reduced the aircraft's weight, mechanical complexity, and cut costs. For the pilot, the system enhances advantages mainly in terms of precision, safety, and ergonomy.

Fly-by-wire aircraft from Airbus Industrie have now been in airline service for more than seven years and over 700 are currently in service. Over seven million flight hours and over four million flight cycles have been reached. The experience gained in the process of conversion to this technology has been well analyzed.

Yet, there has been misinformation circulated about these systems. Examples of A320 folklore include stories of incidents such as the "stuck-in-the-hold" and "unable to descend." Extensive research has been carried out with many A320 operators and reveals no recorded evidence that these incidents ever occurred. Indeed, from a technical point of view, it is impossible to understand how either incident could have occurred because the basic modes, Heading and Vertical Speed, are always available. However, these unsubstantiated stories continue to circulate.

Through the mediating role of the computers that monitor the full scope of the technical and aerodynamic capabilities of the aircraft, the pilot can exploit the system to the full without the risk of exceeding the flight envelope. The envelope part of the fly-by-wire computers is pre-programmed to limit aircraft attitudes (in Normal Law) to 67 degrees of bank (2.5G in level flight) and usually +30 to -15 degrees of pitch. Violations of speed limits (Vmo/Mmo, low speeds), are also protected against, regardless of pilot sidestick input.

Technical
Of all alternatives thought of or tried out in a long development process, the designers, together with experienced airline engineering and test pilots, retained the sidestick as it is today.

The sidestick provides no direct feedback through the grip. Feedback is indirect via the results of the application. The sidestick is moved against spring pressure and damping elements. The designers wanted to avoid complex back-driven feedback systems, sidestick linking, jam or feedback monitoring devices, and control-splitting systems, all of which increase friction, weight, complexity and cost and finally reduce system reliability.

The sidestick has no direct mechanical connection to the control surface. The means of transmission from sidestick to computers to control surfaces is via shielded low impedance electric cables. As part of the A320 European and US certification process, the system was bombarded by radiation from military radars and the aircraft was deliberately flown into multiple lightning strikes.

There are no recorded cases in airline service where electromagnetic interference has affected the A319, A320, A321, A330 or A340 fly-by-wire systems. In fact, it is understood that the US FAA electromagnetic protection standards for fly-by-wire transports have now been reduced. If an incapacitated pilot should freeze his sidestick into full deflection, the other pilot simply presses his instinctive take-over pushbutton on his sidestick and immediately takes control.

After holding the button depressed for 30 seconds, he can lock out the other sidestick completely. However, the last pilot to press and hold this button always takes control. If both pilots make a sidestick input together, the result is the algebraic sum of both inputs. It is, therefore, important in the training environment to give priority to the other cues that measure trainee inputs, such as the visual cues used in the past. It is important for pilots to be clear about the allocation of control.

Control in Pitch
Control is via the computers. Throughout the flight, the elevators move under the control of the flight computers with no pilot input needed to maintain a 1.0G flight. In normal or alternate law, the sidestick does not select a control deflection or attitude directly, as would be the case with a conventional aircraft, and the elevator deflection is not proportional to sidestick movement.

A fore or aft sidestick application selects "G." If a pitch input is made and held, the aircraft will pitch at a constant G until the flight envelope limits are met. Moving the sidestick back creates a demand greater than 1.0G, and forward creates a demand less than 1.0G. When the sidestick is released (stickfree), the demand fed to the computers is to maintain flight at 1.0G (relative to the earth).

One can, therefore, consider a selected input as a selected vector through space, which the computers will maintain even through turbulence. There is no need to ride the sidestick as may be done with conventional controls. In normal law, there is no requirement to trim. Without autotrim, the fly-by-wire aircraft would be no different from a conventional aircraft in that as it slows down, it would try to maintain its intrim speed, and as a result would pitch nose down — losing altitude.

However, in normal law, the flight control computers now detect a pitch-down tendency as a G less than 1.0G and so cause the elevators to move up, returning the aircraft to flight at 1.0G. As a result, the aircraft will decelerate in level flight with no pilot input, maintaining 1.0G to the earth and continuously adjusting the trim until it reaches the flight envelope protection.

Control in Roll
In normal law in roll, the sidestick demands roll rate. If the sidestick input in roll is held, the aircraft will roll until the flight envelope limits are met. This is apparent during a crosswind take-off, if a normal control input is made into wind and held after rotation. While on the runway, the sidestick applies aileron directly, and then when airborne as the flight control laws blend in, the aircraft will roll into the crosswind at a rate proportional to the sidestick deflection.

Up to 33 degrees of bank, the aircraft is automatically trimmed and maintains level flight (no nose drop). Above 33 degrees bank, when releasing the stick, it returns to 33 degrees. To perform a steep turn at 45 degrees or 60 degrees of bank, the stick must be held into the turn and pulled in order to maintain level flight. In alternate law in roll, the sidestick commands control surfaces directly, which is virtually the same as a conventional aircraft. It may be found that alternate law roll is rather more sensitive than normal law.

This is a very broad-brushed primer on fly-by-wire and is meant only as an introduction. New generation aircraft (A319 through A330/A340) have now accumulated large amounts of in-service experience — over seven million flight hours. Look for the technology to spread rapidly to other aicraft designs and manufacturers and for articles that will help you troubleshoot these systems in future issues of AMT magazine.

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