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I have read the wikipedia entry about the accident but I don't understand why did the stall happen and why the plane couldn't recover from the stall.

Correct me if I am wrong but I would think a plane would naturally recover from a stall situation by pitching down the noise by itself with no control inputs needed and gaining speed (due to the fall). The AoA would be back to safe values to generate lift again and the plan would be stabilized.

I don't get how a plane like that would fall from 35,000 feet with its engines set at 100% thrust.

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I‘ll attempt a very brief answer based on the report against your questions:

1) Why did the aircraft stall? The aircraft stalled because crew pitched the aircraft up beyond the performance limit at that altitude (they pitched into a climb which the available engine thrust could not sustain, bleeding off speed until the aircraft stalled).

2) Why was power not reduced? I don’t know, but in my view that didn’t play a crucial role, either.

3) Why did the aircraft not pitch down by itself? Because crew didn’t realise they were stalled and continued to apply „nose-up“ control input. Seemingly inconsistent behaviour of the stall warning system and unexpected parameters on their instruments (i.e. a combination of speed and vertical speed not ever seen in normal flight) contributed to the situation.

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    $\begingroup$ @lmaxd Without trying to sound bitter or anything, at some point I‘d be interested in some constructive feedback as to why my answer did not work as well as the accepted one - this might help me in future answers. $\endgroup$ – Cpt Reynolds Mar 16 '19 at 16:31
  • $\begingroup$ The accepted answer gave me a better understanding of the physics of why the the plane did not at some point during the (free ?) fall pitch down by itself. I should have put more emphasis on that particular aspect in my question. Thank you. $\endgroup$ – Imaxd Mar 16 '19 at 18:30
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    $\begingroup$ @Imaxd Fair enough. Didn’t see that particular relevance originally and tried to answer your questions as I understood them. Thanks for commenting! $\endgroup$ – Cpt Reynolds Mar 16 '19 at 18:37
  • $\begingroup$ Thanks for the report. $\endgroup$ – Robert DiGiovanni Mar 17 '19 at 13:29
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In the larger sense, the aircraft didn’t recover from the stall because the pilots were confused on what operating mode they were in (mode confusion).

So much functionality has been placed into software, that flying in different modes can literally be like flying different aircraft. The relationship between control inputs and control surface movements can completely change.

In short, they thought they were flying Aircraft X when they were actually flying Aircraft Y. They flew a perfectly operable aircraft into the water because of it.

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This ties in with another current question about why we do not have larger elevators on airliners, and highlights the potential of accelerated stalls.

Aircraft need proper air flow over the wing to produce adequate lift. This is a function not only of speed, but also of angle of attack. The lift to angle of attack chart illustrates this.

One of the key functions of a properly designed horizontal stabilizer is to push the nose down when the plane sinks. The design parameter is to have this have adequate area on the H stab to lower the nose as it sinks before it stalls. The pilot adds power to resume level flight.

The other critical factor is center of gravity. A forward set CG again pulls the nose down as the plane slows. Forward set CG acts as a counter balance to elevator trim to stabilize air speed to a desired range: slower - nose down, faster - nose up.

It is the position of the elevator that creates the AOA to stall the aircraft. If the elevator is stronger than the torques created by Hstab while sinking and forward CG, the plane cannot sufficiently reduce AOA.

Sadly, even with power applied, the stall can not be broken until the elevator is released. In the report of this incident, although pitch did not exceed 15 degrees relative to the horizon, AOA exceeded 35 degrees even with throttle at TO/GA, and the aircraft was falling at over 10,000 feet per minute. Weight and balance were within limits.

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  • $\begingroup$ Thank you ! This answers my question about why the plane did not pitch the nose down by itself (thus reducing the angle of attack and gaining lift back) during the fall. Maybe the engines being at 100% thrust compensated the HStab torque ? $\endgroup$ – Imaxd Mar 16 '19 at 15:26
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    $\begingroup$ It did not pitch down because full down elevator has to be sustained for a period of time and the right seater was holding full aft stick almost the entire time. The left seater applied nose down stick several times - he had the right instincts - but he didn't hit the override button on his side stick. Full aft on one side and full forward on the other is summed by the FBW computers and you get neutral elevator unless the left seater hits override. Then the left seater gave up on that and they were back to full up elevator. $\endgroup$ – John K Mar 16 '19 at 16:00
  • $\begingroup$ @JohnK The airplane was in a full stall according to the report. Computer systems preventing stall were inoperative during the accident sequence due to initial disagreement between sensors. Angle of attack was beyond 35° which is much higher than stall angle of attack and much in excess of what flight control computers would allow. $\endgroup$ – Cpt Reynolds Mar 16 '19 at 16:15
  • $\begingroup$ You're right I had a different scenario in mind and didn't go back and review the report. $\endgroup$ – John K Mar 16 '19 at 16:24
  • $\begingroup$ @Imaxd possibly, if the engines were underslung. But even at full power, with the elevator full aft, a plane will either loop or stall trying. Even if it is moving forward at 250 knots, if it is sinking at 125 knots, the AOA will be around 35 degrees even with the nose near the horizon. The elevator authority is key. Also, as John K stated, for that particular type of plane, the controls were not linked. $\endgroup$ – Robert DiGiovanni Mar 16 '19 at 18:55

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