From the
QF72 (AoA spikes) final report:
Table 19: Characteristics of elevator control mechanisms
High AOA protection
Control law: Normal law only
Speed: Any
Altitude:Any (must exceed threshold for at least 2 seconds when aircraft below 500 ft)
Configuration: Any
Maximum authority: 4° elevator movement (at time of pitch-downs)
Anti pitch-up compensation
Control law: Normal or alternate law
Speed: Mach 0.65 or more
Altitude: Any
Configuration: Landing gear retracted, flaps up
Maximum authority: 6° elevator movement
High angle of attack protection
Aerodynamic stall in large aircraft is a potentially dangerous condition and aircraft manufacturers incorporate design techniques to prevent it.
On the A330/340, the FCPCs continually monitored the AOAFCPC input.
If the master FCPC detected that this value exceeded a predefined threshold (alpha max), then it issued control orders for a nose-down elevator movement to reduce the AOA and prevent a stall.
High AOA protection was only available when the aircraft was in normal law.
If AOAFCPC input was outside the range of -10° to +30°, the control law reverted from normal law to alternate law, and the protection was therefore no longer available.
In addition, when the aircraft was more than 500 ft above ground level, the protection was effective immediately; when the aircraft was below 500 ft, it was only active after AOAFCPC input exceeded the threshold for 2 seconds or more.
The maximum authority or change in elevator movement that could result from the the high AOA protection varied depending on several factors.
The aircraft manufacturer reported that, at the time of the two in-flight upsets, the maximum authority was about 4° of elevator movement.
The protection would be applied until the aircraft’s AOA was reduced below the stall angle.
Anti pitch-up compensation
Anti pitch-up was a mechanism included in the A330’s control laws to compensate for a pitch-up* tendency at high Mach numbers and high AOA.
The compensation was only available above Mach 0.65 and when the aircraft was in a ‘clean’ configuration (that is, with the landing gear and flaps retracted).
Its maximum authority was 6° of elevator movement.
*
On a statically stable aircraft, the centre of lift is situated behind the aircraft’s centre of gravity and an increase in AOA would lead to an increase in lift and a restorative tendency to pitch the aircraft nose down.
However, at higher Mach numbers and AOAs, it is possible to stall the wing tips.
On a swept-wing aircraft, the centre of lift will then move forward, leading to a reduced nose-down reaction to increasing AOA, reduced stability, and a tendency to pitch up.
Might not this constant AOA vane position, as displayed as characteristic speed, indicate an AOA condition at, or even beyond, alpha max?
IMO alpha prot can be @ alpha max (in this case and normal for 5s @TO) but never beyond, with normal operating AoA vanes the alpha protect will kick in before alpha max.