b773, If you have not seen that one
Airbus A330 C-GGWD, Airbus A340 TC-JDN very instructive reading in that only 9 pages report.
Five seconds after the autopilot disengaged, the thrust levers were closed and then the autothrust was disconnected, probably by the handling pilot in an effort to prevent another overspeed condition. Ten seconds after the autopilot disengaged, the corrected or phase-advanced angle of attack (a computed parameter which is not recorded but can be calculated by Airbus Industrie from the DFDR data) reached the 'alpha prot' value. This angle of attack excursion beyond alpha prot caused a change in the pitch flight control law from normal law (NZ law) to angle of attack protection law (AoA law). If both sidesticks are at neutral, the AoA protection law seeks to hold the angle of attack constant at alpha prot until a sidestick pitch command is made. If the stick is pulled fully aft then the angle of attack increases to alpha max. If the sidestick is not moved aft, AoA protection law remains active until a nose-down command greater than half forward travel is made or until a nose down sidestick input has been applied for more than one second. The first recorded sidestick input was made at 14:22:08 which was some 28 seconds after the commencement of the Master Warning.
Angle of Attack protection law
Once AoA law is active, rearward movement of the sidestick controls angle of attack between alpha prot (neutral sidestick) and alpha max (full aft sidestick). Forward movement of the sidestick disengages AoA protection law and the system reverts to normal pitch law. However, there is no aural or text message which informs a crew that AoA protection law has been invoked. If the sidestick is not moved from its neutral position, the pitch flight control system is programmed to capture alpha prot and not the airspeed that corresponds to alpha prot in 1g flight. Consequently, in
turbulence the speed scale will probably be oscillating, the aircraft pitch angle could also be oscillating, and the change from normal pitch law to AoA protection law could be difficult to detect.
The commander's reported sighting of an 'Alpha Lock' message was probably an alpha floor warning on the flight mode annunciator portion of the PFDs. Alpha floor is an autothrottle function which applies full thrust, irrespective of the position of the thrust levers, if the airspeed is likely to reduce to a value approaching alpha max. In this incident, the A340's calibrated airspeed decreased from around 270 kt before the turbulence encounter to 205 kt at the apogee of the climb.
Originally Posted by bobrun
In normal law you can't stall the airplane since it won't let you go below alpha max, but it doesn't mean you can't be in a very low energy state. You won't be stalled, but you'll be descending like a brick!
Correct, but only if Alpha Floor has been either automatically inhibited or manually inhibited.
Once the speed is below alpha prot, the sidestick controls the angle of attack directly instead of doing the normal pitch control. With full back stick held, the speed will go to alpha max and stay there.
And AoA will reach alpha max and be maintained at that value ... but it never did at Habsheim !?