By Fred George fred_george@aviationweek.com

Digital electronic flight control systems, commonly known as fly-by-wire (FBW) flight controls, increasingly are being used aboard business aircraft because they reduce pilot workload, increase safety margins, and prevent structural and aerodynamic limits from being inadvertently exceeded. Most airframers also include many other proprietary high-level functions and features in their FBW controls to make the aircraft easier to handle during abnormal or emergency conditions and thereby gain an advantage over other manufacturers.

In addition to the foregoing, “Fly-by-wire offers redundancy above and beyond what's necessary for certification. It's a standout in that respect,” says Glenn Zwicker, chief engineer at Parker Aerospace, a leading provider of digital flight control hardware and software.

Airworthiness certification authorities typically require a one in 10 million probability of a catastrophic failure of a hydromechanical flight control system. FBW systems, in contrast, must meet a one in one billion probability of failure. Parker, among other leading FBW manufacturers, targets one in 10 billion probability of failure, notes Zwicker.

“This allows margin for common cause [catastrophic damage] associated with rotor burst or tire failure, along with bird strike or bomb blast. We design in multiple paths and multiple actuators to isolate local damage,” he says.

FBW control systems have no mechanical connections between the cockpit controls and the flight control actuators. Instead, as the name implies, FBW systems have electrical links between the cockpit flight controls and the power control actuators attached to the flight control surfaces. The cockpit hand and foot controls have force and/or motion sensors that measure pilot inputs. The inputs are transmitted as electrical signals to the FBW system computers. Those boxes then send electrical signals to command the movement of the power control actuators that move the control surfaces.

In the most basic FBW Direct Law mode, control inputs by the flight crew result in direct and proportionate movement of the flight control surfaces. The only FBW components required are the electrical cockpit control position or force transducers, actuator control units and the electrohydraulic or all-electric power control actuators attached to the flight control surfaces. There also is a feedback loop that senses when the desired control surface deflection has been attained to tell the system when to stop commanding more movement.

Direct Law, along with other FBW modes, also requires uninterruptable electrical and hydraulic power supplies as there are no backup mechanical links between the cockpit controls and flight control surfaces or power control actuators. If all electrical and hydraulic power is lost, the aircraft will not respond to stick, yoke or rudder pedal inputs.