Originally Posted by
Chris Scott
Thanks for grasping the nettle and reminding me and others that the designers have to make provisions for the complications of phugoid oscillations, as confirmed by the Airbus input to the NTSB report on the Hudson River accident. That seems to be the explanation of the apparent shortfall of achieved AoA at Habsheim. My tentative suggestion that it might be a measure to allow sudden bank applications without the need to drop the nose can probably be discounted.
Not necessarily. If I understand the concept correctly, the design will carry a certain amount of leeway for any potential change in dynamic loads (or "all of the above"). The graph posted earlier provides a qualitative view of how the system should behave, but that graph is not annotated with specific values - an omission which I'm convinced is deliberate.
I can't say for certain, but if I were designing such a system, I'd be inclined to make the behaviour consistent - but make specific attainable values dependent on the prevailing conditions. In particular, if the aircraft has airspeed or thrust in reserve, then reaching the optimum values should happen sooner than if the aircraft is already on the edge of the envelope.
While the deviation below 100ft RA made Alpha Floor moot in this case, its existence indicates an implicit understanding at the design stage that AoA protection can only get you so far. Without sufficient airspeed and/or thrust in reserve, attaining optimum AoA is likely to be doubtful.