How it works

Architecture

Two ELACs (elevator and aileron computers), three SECs (spoiler and elevator computers) and two FACs (flight augmentation computers) form the flight control system. Each computer has command and monitor channels that continually compare each other, so a computer that disagrees with itself isolates. Multiple computers and three hydraulic systems (Green, Blue, Yellow) give high redundancy.

Sidestick input is a request for load factor in pitch and roll rate in roll, not a direct surface deflection. Release the stick and the aircraft holds the flight path in pitch, with auto-trim keeping it in 1G flight.

Normal law

Normal law provides protections: load-factor limits, pitch and bank-angle limits, high angle-of-attack protection (alpha protection) and overspeed protection. The aim is to let the pilot give full-stick input in an emergency without exceeding structural or aerodynamic limits.

Alternate and direct law

Certain failures (for example loss of two hydraulic systems or multiple air data sources) degrade the system to alternate law, where some protections are lost, and pitch control becomes more like conventional. In direct law the sidestick commands surface deflection directly, and manual trim is required. The ECAM shows the active law and any associated speed or configuration restrictions.

Mechanical backup via the trimmable horizontal stabiliser (using the manual pitch trim wheels) and the rudder is available if all computers fail.

Technical detail — computers and actuators

Seven flight control computers: two ELACs (elevator aileron computers), three SECs (spoiler elevator computers) and two FACs (flight augmentation computers, rudder and yaw damper). Each computer has a command and a monitor channel; if they disagree, the computer drops out. The ELACs normally control the elevators, ailerons and the stabiliser; the SECs control the spoilers and are backup for the elevators.

Each control surface is moved by one or two hydraulic servo-actuators supplied by different hydraulic systems (Green, Blue, Yellow). Each elevator has two actuators — one active, one damping. The rudder has mechanical backup through cables from the pedals, and the trimmable horizontal stabiliser can be moved by the manual pitch trim wheel.

Technical detail — control laws

Normal law: sidestick commands load factor in pitch (1 g with stick neutral, auto-trim) and roll rate in roll. Protections: high angle of attack (alpha floor triggers TOGA thrust), load factor (+2.5 g / −1 g clean), pitch attitude (30° nose up, 15° down), bank angle (67°), high speed.

Alternate law: most protections are lost (replaced by stability features); 'ALTN LAW: PROT LOST' shows on ECAM. Direct law: surface deflection is proportional to stick position, no auto-trim — use the trim wheel; 'USE MAN PITCH TRIM' shows on the PFD. Mechanical backup: pitch through the trim wheel and lateral through the rudder.