Fly by wire

Fly-by-wire (FBW) is a system that replaces the conventional manual flight controls of an aircraft with an electronic interface. The movements of flight controls are converted to electronic signals transmitted by wires (hence the fly-by-wire term), and flight control computers determine how to move the actuators at each control surface to provide the ordered response. The fly-by-wire system also allows automatic signals sent by the aircraft's computers to perform functions without the pilot's input, as in systems that automatically help stabilize the aircraft, or prevent unsafe operation of the aircraft outside of its performance envelope.

Mechanical and hydro-mechanical flight control systems are relatively heavy and require careful routing of flight control cables through the aircraft by systems of pulleys, cranks, tension cables and hydraulic pipes. Both systems often require redundant backup to deal with failures, which increases weight. Both have limited ability to compensate for changing aerodynamic conditions. Dangerous characteristics such as stalling, spinning and pilot-induced oscillation (PIO), which depend mainly on the stability and structure of the aircraft concerned rather than the control system itself, can still occur with these systems.

The term "fly-by-wire" implies a purely electrically signaled control system. It is used in the general sense of computer-configured controls, where a computer system is interposed between the operator and the final control actuators or surfaces. This modifies the manual inputs of the pilot in accordance with control parameters.

Side-sticks, centre sticks, or conventional flight control yokes can be used to fly FBW aircraft.

Fly-by wire systems are quite complex, but their operation can be explained in simple terms. When a pilot moves the control column (or sidestick), a signal is sent to a computer (analogous to moving a game controller) the signal is sent through multiple wires (channels) to ensure that the signal reaches the computer. A 'Triplex' is when there are three channels being used. In an Analog system, the computer receives the signals, performs a calculation (adds the signal voltages and divides by the number of signals received to find the mean average voltage) and adds another channel. These four 'Quadruplex' signals are then sent to the control surface actuator, and the surface begins to move. Potentiometers in the actuator send a signal back to the computer (usually a negative voltage) reporting the position of the actuator. When the actuator reaches the desired position, the two signals (incoming and outgoing) cancel each other out and the actuator stops moving (completing a feedback loop). In a Digital Fly By Wire Flight Control System complex software interprets digital signals from the pilots control input sensors and performs calculations based on the Flight Control Laws programmed into the Flight Control Computers and input from the Air Data Inertial Reference Units and other sensors. The computer then commands the flight control surfaces to adopt a configuration that will achieve the desired flight path.

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