PPRuNe Forums - View Single Post - Air Asia Indonesia Lost Contact from Surabaya to Singapore
Old 3rd Feb 2015, 22:28
  #3009 (permalink)  
RetiredF4
 
Join Date: Jun 2009
Location: Germany
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silverstrata (Bolding by me)

In normal law, an Airbus should never enter this state, but in alternate law it certainly can (as can Boeing twins). There is no reason why a stick pusher or a thrust reducer cannot be applied in these cases, where the aircraft is in alternate law. A thrust reduction at the point of stall, will lower the nose quite sharply (if the stick is neutral or forward of neutral) and make the exit of the stall relatively simple.
That statement of yours is imho misleading and does not reflect the correct procedure. A neutral stick in the bus commands a loadfactor of 1g, and would therfore force the aircraft in a decreasing speed environment due to power reduction in a higher AOA, the elevators and the THS may drive all the way nose up to achieve the commanded 1g. Same would happen with slight nose down input, as the increasing rate of descent in a stall would be felt by the sensors as less than 1 g, and the commanded g by a slight SS forward might be bigger, thus still the flight surfaces would be positioned for a nose up flight path. With such SS inputs the aircraft would be forced deeper into the stall and into high AOA. Bonin demonstrated how it would end. I'm not saying it might not drop the nose finally, but it might do it only in a very high stall state. And such a mistrimmed aircraft with the elevators and THS all nose up will be a definite handicap during recovery. It may even be the recipe for secondary stalls, as the increasing speed after a possible nosedrop could cause a pitchup again if not countered by manually trimming nose down and using amounts of SS forward to counter it.

So wy would you let this happen in the first place?

There is only one correct stick position as early in the stall as possible concerning the SS and especially in a flightpath stable aircraft like AB: Put the SS forward (and i would even say full forward) until the AOA is reduced to normal values and speed has reached flying airspeed.

We discussed that in the AF447 thread quite extensively, but this misjudgement of the effects of the flight computers under C* striving for the SS commanded g on the elevators and the THS trim from AB qualified crews is astonishing or even alarming.
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