Fly By Wire

Fly-by-wire is a system that replaces the conventional manual-mechanical flight controls of an aircraft with an electronic interface. A computer system is interposed between the pilots and the final actuators-surfaces. The term “Fly-By-Wire” implies a purely electrically-signalled control system..

The movements of flight controls(which commanded by Pilots) are converted to electronic signals and these signals are transmitted by wires to the flight control computer, and flight control computers determine how to move actuators at each control surface.

Fly by wire aircraft incorporate flight-envelope protection system into its flight control software(inside flight control computer), It is used in all modern commercial fly-by-wire aircraft. Flight envelope protection system prevents the pilot of an aircraft from making control commands that would force the aircraft to exceed its structural and aerodynamic operating limits.

The flight data which used by flight computers for prevent dangerous actions-maneuras, stabilize the plane and control the plane  are;

• Pitch, yaw, roll rate and linear acceleration
• Angle of attact and sideslip
• Airspeed, pressure, altitude and radio altimeter indications
• Sidestick-Yoke and pedal demands
• Other cabin commands such as landing gear condition, thrust lever position etc.

Fly By Wire – Basic Operation

• When the pilot move the sidestick to command the flight control computer to make a certain action such as pitch the aircraft up or roll to one side, this demand is first of all transduced into electrical signal in the cabin and sent to flight control computer
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  , the signal is sent through multiple wires (channels) to ensure that the signal reaches the computers.

• The flight control computer also receives the data signals concerning the flight conditions and servo-valves and actuators positions . Then the flight control computer evaluates and calculates (in accordance with the position of aircraft, airspeed, actuator positions, pressure, altitude, landing gear condition and signals come from the pilot’s sidestick-yoke and pedal demands) what control surface movements will cause the plane to perform that action, and then the computer chose the best action and convert it to electrical signals.
• These signals are then sent to command the electronic controllers for each surface. The controllers at each surface receive these commands and then move the actuators attached to the control surfaces until it has moved to where the flight control computer commanded it to.
• The controllers measure the position of the flight control surface with potentiometers in the actuators and then send a signal back to the flight 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 and the actuator stops moving (completing a feedback loop).

This process is repeated continuously as the aircraft is flying.

Fly By Wire – Advantages

• Flight-Envelope Protection software automatically help to stabilize the aircraft and prevent the unsafe actions.
• Turbulence suppression and consequent decrease of fatigue loads and increase of passenger comfort
• Drag reduction by an optimised trim setting
• Easier interfacing to auto-pilot and other automatic flight control systems
• Maintenance reduction
• Reduction of airlines’ pilot training costs (flight handling becomes very similar in an whole aircraft family)
• Flight-control computers continuously “fly” the aircraft, pilot’s workloads can be reduced
• Fly-by-wire control systems also improve economy in flight because Fly By Wire Aircraft can be lighter due to they eliminate the need for many mechanical, and heavy, flight-control mechanisms and wires except of hydrolic systems cover less space, less complex and reliable.

Differences between Airbus and Boeing in using Fly By Wire system

The Airbus A320 was the first commercial aircraft to incorporate full flight-envelope protection into its flight-control software.Airbus give full-authority to FBW controls, the flight envelope protection cannot be overridden completely, although the crew can fly beyond flight envelope limits by selecting an alternate “control law”. But in the Boeing 777 has taken a different approach by allowing the crew to override flight envelope limits using excessive force on the flight controls. Airbus used full-authority FBW controls. Boeing followed with their 777 and later designs. (Just B777 and B787) Boeing uses yoke on the B777, with  fly by wire primary flight controls. Airbus does the same with a sidestick. Both provide full envelope protection.