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.

Direct Law doesn't provide any of the higher level FBW functions that distinguish digital flight controls from conventional hydromechanical-powered flight controls. As with conventional flight controls, it's up to the flight crews to avoid inadvertent stall, or over-speed, over-control and over-stress of the aircraft.

Higher level FBW functions, commonly known as Normal Law or Alternate Law functions, require a second set of computers that are capable of using inputs from several sensors, such as angle-of-attack (AOA), flap/slat and landing gear position sensors, air data and IRS/AHRS, weight-on-wheels and perhaps also radio altitude, as well as cockpit controls. The computers then shape, smooth and calculate the best aircraft behavior in response to pilot inputs, speed and configuration changes and autopilot commands, as well as other factors. The higher level law computers team with the actuator control units to determine optimum flight control response.

However, the term “optimum” is subject to a wide variety of interpretations depending upon the FBW flight control design philosophies of each aircraft manufacturer. Some high-level FBW functions, however, are common to virtually all civil aircraft and include yaw, spiral, static and dynamic pitch stability augmentation to reduce pilot workload. Most FBW systems also have soft or hard limiting for maximum AOA, Vmo/Mmo over-speed and over-stress.

“Once you have the algorithms, it's not difficult for the control laws to accomplish such functions,” says David McLaughlin, Parker Aerospace's chief engineer for systems.

As noted, airframe manufacturers each add their own high-level FBW functions in order to differentiate their digital flight control systems from their competitors' designs.