Fly-by-Wire Flight Control

Fly-by-wire replaces direct mechanical transmission of pilot inputs with electrical signals processed by flight-control computers. In an aircraft with relaxed static stability, automatic feedback continuously counters any tendency of attitude or flight path to diverge, thereby creating artificial stability.

Why active stabilisation is needed

A combat aircraft may be designed with low or negative static stability, especially in pitch, to improve manoeuvrability and reduce the control effort needed for rapid changes. After a disturbance, however, such an aircraft may depart further from its original condition unless corrected immediately.

• The pilot cannot reliably make the small and rapid corrections required to stabilise such an aircraft manually.
• The flight-control system therefore provides continuous stability augmentation rather than waiting for pilot intervention.

Closed-loop stabilisation

Sensors measure variables such as angular rates, attitude, acceleration and air data. The flight-control computer uses programmed control laws to compare the pilot's commanded response with the measured motion and commands hydraulic or electro-hydraulic actuators to move the control surfaces.

• This closed feedback loop detects an incipient divergence and applies an opposing control-surface deflection before it becomes large.
• Pilot stick movements can represent a desired roll rate, pitch rate or load factor; the computer determines the required combination of control-surface movements.
• Control laws may also schedule responses with speed and altitude and impose flight-envelope limits against unsafe manoeuvres.

Reliability and indigenous capability

Because loss of computation or signalling can be catastrophic, combat-aircraft systems use redundant channels, sensors, power supplies and fault-detection logic. Independent channels can cross-check outputs and isolate a failed channel.

• India's Tejas uses a digital, quadruplex fly-by-wire flight-control system developed through the Aeronautical Development Agency programme.
• Indigenisation requires mastery of control-law design, real-time computing, sensors, actuators, system integration and rigorous verification and flight testing.