All this chat about alpha margin is totally irrelevant without knowledge of the extra manoeuvre capability to be gained by an alpha increase. We all know that the speed was way below a safe speed and I would guess that all the swept wing jet pilots here would know that any similar aircraft would have extremely limited manoeuvre capability at this speed.
Without a total CL/Alpha curve for the aircraft on the day (and total drag curve) most of this discussion is speculation. The only thing that really matters to the pilot is how much extra G he can pull without making the situation even worse.
With reference to the phugoid damping:
1. In alpha protect Airbus found that that phugoid could be excited, at an alpha=alpha protect (phugoid is essentially a constant alpha oscillation).
2. For whatever reason, Airbus decided this characteristic was unacceptable and included a damping term in the flight control laws.
3. While, generally speaking, the phugoid is a non event because there wlll be adequate longitudinal control authority to stop the oscillation during either the pitch up or pitch down phase.
4. However, if you put in a hard controllability limit (like alpha protect) the pilot will be unable to stop the oscillation during the pitch down phase - he is denied the ability to increase alpha by the control laws.
5. Best to allow the flight control protection laws (but not the pilot) a little extra authority in terms of alpha capabilty so this oscillation can be prevented.
6. It has been suggested to give the pilot a "little more authority" to be able to demand a slightly higher alpha by removing the phugoid damping term. Seem to me this just moves the phugoid to a higher alpha and in no way guarantees that the pilot will be able to stop the phugoid if it triggers in the nose down mode, at the new alpha protect, when on final approach to his ditching.
7. Seems to me that if you think alpha protect is a good idea, but you don't wish to damp the phugoid you must accept that the nose down pitch phase will not be controllable.
8. If you give the pilot sufficient authority to stop the phugoid at any time, by definition you no longer have alpha protect.
Last edited by zzuf; 23rd May 2017 at 01:40.