Human Factors And The Airbus Fly-By-Wire Control Laws

One of the first fly-by-wire (FBY) control laws was developed by Boeing for experimental use on a B-47. It had a sidestick control and was straight stick to control surface. “Stick to control surface” means there is a proportional input to the control surfaces for inputs to the controller, in this case, a sidestick. Boeing named this the C* (C star) law.

Airbus claimed to use the C* law, but made changes in the way the sidestick worked. They incorporated it as a quasi automatic system like control wheel steering mode of an autopilot, which  will hold roll and pitch levels as input by the sidestick in increments. In addition, Airbus changed the pitch channel to control flight path angle with airspeed selected by digital dialed input and incorporated non moving autothrottles and automatic trimming. These features removed important tactile and visual feedback information from pilots.

Aircraft are required to prove positive longitudinal stability throughout the flight envelope during certification. From a trimmed angle of attack, pitch is abused to change the angle of attack (the angle between the airstream and the wing). An aircraft is allowed to oscillate, but not diverge from the trimmed condition. This means that in manual control, the aircraft will have a feel of trimmed condition and will not diverge far without control input against the trimmed condition or re-trimming. This tactile feel of trimmed condition is not present on an Airbus system. Banking the aircraft in level flight requires a pitch and roll input which is another important tactile feedback, i.e., in normal aircraft,  without a continued input, the aircraft should attempt to return to level flight.

In automatic flight, the autothrottles move on all other designs which are important visual and tactile feedback cues for pilots to monitor autothrottle operation. On an Airbus, even when the autopilot is disconnected, it is normal to operate with the autothrottle connected so there are no visual or tactile feedback cues of thrust except instrument indications which the manufacturer says are adequate. Thrust requirements for different flight paths at the same angle of attack become obscure.

The sidestick control used by Boeing for the B-47 was for a single pilot. With two pilots, another important sensory function is removed. In conventional aircraft with control wheels, the pilot not flying (PNF) sees that the pilot flying (PF) is making control inputs. Not so on the Airbus. Also, a control for only one hand creates a problem when a pilot may want to change hands, say to reach across his body to take a cup of coffee. When only one hand is used for control, the pilot would have to engage the autopilot, unless control wheel steering is always engaged as on an Airbus in its normal operating modes.

Another important sensory clue on other aircraft is visual and audible perception that the elevator is being trimmed. In an Airbus, a pilot may observe auto trim, but will soon become accustomed to the fact the aircraft will automatically keep the proper trim. An out of trim condition could be obscure and unnoticed on an Airbus in Normal and Alternate Law.

Airbus marketed their system with the claim that it was like any other, but better. However, with; 1) control wheel steering and 2) flight path hold in pseudo-manual mode; 3) dialed airspeed with 4) non moving autothrottles; 5) automatic trim; and 6) non visual indication of other pilot’s control inputs, the pilot interface is significantly different from other aircraft. Whether better or not is open to conjecture.

August, 2011