Alternate law
The are four reconfiguration modes for the Airbus fly-by-wire aircraft, two
Alternate Law (1 and 2),
Direct Law and
Mechanical Law. The ground mode and flare modes for
Alternate Law are identical to those modes for
Normal Law.
Alternate law 1 (ALT1) mode combines a
Normal Law lateral mode with the load factor, bank angle protections retained. High angle of attack protection may be lost and low energy (level flight stall) protection is lost. High speed and high angle of attack protections enter alternative law mode.
[5]
ALT1 maybe entered if there are faults in the horizontal stabilizer, an elevator, yaw-damper actuation, slat or flap sensor, or a single air data reference fault.
[4]
Alternate law 2 (ALT2) loses
Normal Law lateral mode (replaced by roll direct mode and yaw alternate mode) along with pitch attitude protection, bank angle protection and low energy protection. Load factor protection is retained. High angle of attack and high speed protections are retained unless the reason for
Alternate 2 Law mode is the failure of two air-data references or if the two remaining air data references disagree.
[5]
ALT2 mode is entered when 2 engines flame out (on dual engine aircraft), faults in two inertial or air-data references, with the autopilot being lost, except with an ADR disagree. This mode may also be entered with an all spoilers fault, certain ailerons fault, or pedal transducers fault.
[4]