Powered servo controls

Powered controls may take two forms, servo-assisted, or fully power operated. In the former type, hydraulic pressure is transmitted via pipes to a servo – actuator which helps the mechanical linkage to move the surface. The mechan­ical linkage can be used to operate the control surface, even if power is lost, although the controls will then feel very heavy. The system is similar to the servo-assisted steering and braking system of a car.

Power control, fly-by-wire and fly-by-light

In pure power operation, no mechanical override is provided. Control signals may be transmitted hydraulically, directly from valves attached to the control column, or electrically to actuators, which move the control surfaces. The lat­ter system is known as fly-by-wire. The actuators are electrically or hydraulic­ally operated rams or motors.

As an alternative to electrical signal transmission, modulated light signals may be transmitted along optical fibres. This system is known as fly-by-light

and overcomes problems due to electromagnetic interference. The detonation of nuclear weapons would cause very strong electromagnetic signals capable of upsetting, if not destroying, conventional electronic circuits. The deliberate jamming of electronic circuitry by means of powerful electromagnetic beams is also a possibility, and some military aircraft have been found to be very vulnerable in this respect.

Once control by electrical signals is accepted, it becomes convenient to incorporate sophisticated electronic processing into the circuit, with increas­ing emphasis on digital systems. Such processing can be used to alter the response to control inputs, and can allow for manoeuvres such as flying in a stalled or an unstable condition, or approaching very close to the stall on landing.

Fly-by-wire can thus dramatically improve the performance, efficiency and even safety of aircraft. It also allows for co-ordinated control surface move­ments that would be too complex for a pilot to manage unaided. Such systems have demonstrated a high level of reliability and are being increasingly used. On military aircraft, the flight control, autostabilisation, navigation, radar and weapons control systems are all integrated in varying degrees.