Quote from HN39:
Yes, there was an overspeed condition that caused a warning an disconnection of the A/P. However, that overspeed condition was very brief and not really a problem. Shortly after that the airplane encountered a vertical gust that increased the AoA to alpha-prot and put the FCS in high-AoA protection mode which caused the zoom climb.
Yes, and I've always been curious about the implementation (in pitch) of AoA Protection Law ("Alpha Prot"), which can only happen if the AP is off (caused here by the windshear-induced overspeed). Clearly it is advantageous to stop the AoA increasing unless the pilot commands such by "pulling" the sidestick (aft of neutral). The bit I don't understand is why the FBW is programmed to maintain Alpha Prot even if normal-G exceeds 1 (I'm referring to the situation at cruise speeds; not low speeds). To put it another way: on the face of it, it seems bizarre for the FBW to "grab" a transient, higher-than-normal AoA even if it results in more than 1G with a neutral sidestick (assuming wings level). No doubt there are good reasons for doing so - perhaps the likes of OwainGlyndwr may explain?
In this case, the AAIB and AI decided that a "phase advanced" value of AoA - caused by a vertical gust - triggered Alpha Prot Law (mode). "Phase advanced" means that the AoA was rising rapidly, and going into Alpha Prot Law was a pre-emptive move (as-per stall protection systems, including stick-shakers, on old a/c like the BAC 1-11 and VC10). Once the FBW system has engaged in Alpha Prot, the AoA presumably rises further to the stable value of Alpha Prot. If so, that would further exacerbate the situation in this particular scenario. Forward (down) sidestick is ncessary to restore Pitch-Normal Law. (Does anyone know how much?)
There is no message to inform the crew of the change of the Pitch Law (mode). In this case, attempts to re-engage the AP seem to have caused the delay in sidestick intervention by the PF.
BTW, in the AAIB's DFDR trace, the sidestick parameter seems to give negative values for "up" inputs, i.e., stick back.
The common factor this incident had with the much later AF447 accident is that of the unexpected transition from automatics to "manual": specifically from AP to FBW, in a pitch law other than Normal. In this AIRPROX case, the FBW system forced the a/c into an undesired zoom-climb which needed positive crew intervention to counter. In AF447, the FBW made no such input; the PF forcing the a/c into a zoom-climb by inappropriate sidestick inputs.