_Phoenix;
#
3634 (
permalink) Does anyone of you know?
The video below shows an interesting simulation of high altitude stall.
Stall begins at 1:30, then trim wheels freeze, for g <0.5...
Now look at 2:15, while the aeroplane increases speed, the trim wheels start to roll back
, then finally at 2:32, trim wheels spin forward and the aeroplane exits stall in about 10 seconds.
https://youtu.be/Tyg_qFbnnUU?t=85
The Trim Horizontal Stabilizer, (THS) trim position is a function of elevator movement and position, nothing else.
The concept behind the design, and actuality of trim position is to render elevator position neutral. In the Airbus this is done automatically. In conventional aircraft this is manually done by the crew when the autopilot is not engaged.
This neutralizing does not occur immediately; - the THS does not "chase" elevator position so much as follow it up over a short period of time.
The THS did respond to stick position in the AF447 accident about 40 seconds after the loss of airspeed information when the stick was essentially held in the full-up stick position until impact.
In simulator exercises, the THS responds to the full-forward (ND) stick position and returns to it's cruise setting within about 13 seconds.
For simulator exercises, please be aware that there is no "stall data" for these aircraft and the simulator, rather the software programmer and/or aerodynamicist is mathematically estimating performance of aircraft behaviour during the full stall. This is changing as many here would know.
The key in stall recovery is to unload the wing...reduce the AoA and start the wing flying again. This means getting the nose down as quickly (aggressively) as possible and keeping it there until the wing is flying again, (absence of buffet, reducing rate of descent). This results in very high rates of descent and considerable loss of altitude, (up to 18,000fpm, 15,000ft of height lost).
This doesn't necessarily require an AoA indicator although that would be a valuable addition to the kit, but I doubt if it would have made a difference here, or in AF447.
Stall AoA at high altitude, high-Mach numbers can be as low as 6°. Typical cruise AoAs are around 2°to 2.5°
The stick input is a 'g' demand - it is not an actual elevator-position "request", (in Direct Law, it behaves conventionally just like say, a B737's elevator system).
Because the THS "follows up" elevator position over time, it's movement is related to and corresponds to stick position. Remember, in Normal Law, (and Alternate Law) the elevator is positioning itself to sustain 1g flight, (1g being perpendicular to the aircraft's longitudinal axis!).
The stall warning was sounding in this video so the aircraft was in Alternate Law, (with reduced protections).
A full description of re-configuration when certain system or autoflight components are lost is provided below. While the schematic may appear complex if one doesn't fly or more particularly, fly the Airbus, to those who do fly the aircraft, it is, (or should be, depending upon the air carrier's training and standards group), second nature:
I hope this helps further understanding.