Aircraft flight controls are the means by which a pilot controls the direction and attitude of an aircraft in flight.
Flight control systems are subdivided into what are referred to as primary and secondary flight controls. Primary flight controls are required to safely control an aircraft during flight and consist of ailerons, elevators (or, in some installations, stabilizer) and rudder. Secondary flight controls are intended to improve the aircraft performance characteristics or to relieve excessive control loading, and consist of high lift devices such as slats and flaps as well as flight spoilers and trim systems.
Movement of any of the primary flight controls causes the aircraft to rotate around the axis of rotation associated with the control surface. The ailerons control motion around the longitudinal axis (roll), the elevator controls rotation around the lateral axis (pitch) and the rudder controls movement around the vertical axis (yaw).
The most basic flight control systems are mechanical and, although they date back to the earliest aircraft types, are in use in the majority of light, general aviation aircraft. In this design, a collection of mechanical components such as cables, pulleys, rods and chains transmit the movement of the flight deck controls to the appropriate control surface(s). In larger and faster aircraft, the aerodynamic forces become too great for the pilot to overcome without assistance so hydraulic systems are often incorporated to move the flight control surface. In some newer aircraft models, designers have replaced most of the mechanical components with computers and fiber optics to produce control systems which are referred to as Fly-By-Wire.
In conventional aircraft, control column movement is transferred along cables, pulleys, and levers, to hydraulic servos that move the flight control surfaces.
In A320 series aircraft, the control yokes are replaced by sidesticks.
Flight control inputs are transferred along electrical wires to the hydraulic servos that move the flight control surfaces.
The fly-by-wire system provides several advantages. Some of which are:
- Reduces weight
- Provides easier and more precise maintenance
- Allows for the programming of flight control limitations
- Allows for a more pilot friendly cockpit
- Provides a more precise autopilot
- Provides more advanced flight control capabilities
Seven flight control computers process pilot and autopilot inputs according to programming parameters based on level of aircraft capability.
The seven computers are:
- 2 ELAC’S
- 3 SEC’S
- 2 FAC’S
There are multiple computers of each type to provide redundancy.
For example, if ELAC 1 fails ELAC 2 can takeover all of the failed computer’s functions without interruption of aircraft operation.
As will all computers, the flight control computers operate with a given set of programmed instructions and within certain parameters.
These instructions and parameters are called laws.
The law in use varies depending on the mechanical or computer degradation the aircraft is experiencing.
The flight control laws, in order of priority, are:
- Normal law
- Alternate law
- Direct law
- Mechanical backup