A rare thing...
Someone confessing ignorance here on R&N, so enjoy it...
You know, I really don't know anything much about this "tail stall" phenomenon. (Well, same as I am waiting to hear just which t-tail jet it is that needs the nose pushed down to recover from a full-breaking stall yet is certified without a stick-pusher, really!)
Where can I find an authoritative account of exactly what a "tail stall" is, please? I think I know what is under discussion but I never saw it exactly, officially defined.
I take this to be an ice-contaminated tail stalling well before the wing reaches the critical AOA (Angle Of Attack) perhaps due to the change in airflow direction from flap extension. The Twin Otter, for instance, has a prohibition on using more than Flap 10 when icing has been encountered, which must be for this reason but you cannot see the term "tail stall" itself used in the aircraft manual or checklist.
I can see the logic in commanding a pitch-up and/or removing flap, when the tail AOA would decrease for recovery from its stalled condition, when these are both actions that would be contra-indicated for a normal stall.
The funny thing is, I do hold a current FAA Flight Instructor Airplane Single and Multiengine; Instrument Airplane Gold Seal licence but there are still areas of personal ignorance I come across now and then, hence this question.
FTC, above... Hope I come up to your demanding professional standards, probably not but here goes anyway:
I would have thought that autopilot disconnect would see the aircraft trimmed to stay at the speed for the last attitude that the autopilot had commanded while holding altitude, assuming here that it was in "Altitude" mode. That is the way an autopilot system works: it senses a pitch control force and tells the trimmer to run until the force is gone so that its servo is not continuously activated.
Say you want to hold 3 thousand feet, when you then reduce power. You should see the speed decay while the autopilot holds altitude and the trimmer runs "nose up." When you get to the stall regime the autopilot will kick off, giving you a warning tone but when it disconnects you shouldn't get the airplane doing a sudden pitch-up; it should just sit there properly trimmed if you still have enough power to sustain level flight or else exhibit a nose-down pitch if the speed is decaying, since pitch trim is really trimming to a speed and not an attitude.
Given all of that then where should this sudden nose-up input come from except from the crew manipulating the controls?
Last edited by chuks; 27th Mar 2009 at 09:02.