Originally Posted by
grity
"AAIB Bulletin No: 6/2001: Consequently, in turbulence the speed scale will probably be oscillating, the aircraft pitch angle could also be oscillating..."
if the bird flow for sume time nearly in a balistic curve, the AoA vane is in a stable normal position... no AoA protection will start working
AoA protection alone seems not very good for stable flight
Not sure I'd call it 'stable', but it is 'flying' below 'stall speed'.
the climb-input is interesting, after the stopp of the autopilot they moved the elevator up to +4 for ten sec. but in the following seconds the G falls to 0.5 and the pitch drops a little down (!) IMO this needs good downwind. then they hold the elevator between 0 and -3 and pushed the 4 engines this must be the climb-input, mayby together with the autotrim
Note 'they' is not pilot input, elevator input may be from Normal law controlling 1g, but is also almost coincident with the second MMO exceed event, so could be
"High speed protection" pitch up?
ar you shure AoA reached alpha max? nighter the trim nor the AoA is shown in the diagram. the climb after the startinput could also happen with a lower AoA....
The report states that "the
corrected or phase-advanced 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)."
Which suggests it wasn't the real AoA, but it is what the instrumentation reported, and if the signal is noisy you do not want to "phase advance" it by differentiation and extrapolation. If the AoA sensor suffers friction and moves in 'stick-slip' steps this sensor processing would give AoA spikes!
But does that explain the pitch response before the pilot inputs knocked it out of the protection mode? To control to alpha prot with falling airspeed why would it pitch up 10 to 16 degrees? Perhaps that's just what happens when it tries to AoA with very noisy AoA sensor inputs due to the turbulence? As the speed falls does the target alpha prot increase?