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West Caribbean Airways Flight 708 which crashed in 2005, fell victim when their plane encountered a deep-stall. From my understanding, only certain planes can 'deep-stall'

How can pilots recover their plane to recover from a deep stall?

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Are you asking only how to recover, or are you also asking about what a deep stall is? (I ask this because of the title of the question) –  Steve V. Aug 5 at 15:08
    
I'm asking both but more importantly about how the recovery is performed –  Dv8r Aug 5 at 15:16

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up vote 22 down vote accepted

A deep stall or a super stall is a condition where the wake of the wing impinges on the tail surface and renders it almost ineffective. The wing is fully stalled, so the airflow on its upper surface separates right after the leading edge, which produces a wide wake of decelerated, turbulent air. Consequently, the dynamic pressure at the tail surface is much smaller than in unstalled flight, which is the main reason for the reduced effectiveness.

Consider this case: The aircraft flies in a nose-up attitude, but on a downward flight path. Consequently, the angle of attack of the wing $\alpha$ is much beyond its normal operating range, causing a fully separated upper surface flow. Due to the T-tail configuration, the wake is hitting the elevator such that all of it is shielded from the regular airflow. enter image description here

The pitching moment of this configuration over angle of attack (blue line) looks something like this. First there is a stable region with a negative gradient at low values of $\alpha$, followed by a minimum when the wing stalls, and then a region with a positive gradient, where the separation unfolds and the tail moves into the wake from above. This region is unstable in pitch, so without control input the aircraft will not stay there, but either pitch down or up until it reaches a stable region again. At high angles of attack follows another stable region with a negative gradient: enter image description here Note that we have two trim points, one in the regular range of the angle of attack, and one way out to the right. In both cases the aircraft has a stable trim condition, so small disturbances are answered with force changes which will keep the aircraft at one of these points. Between the two, there is another equilibrium point, but here the airplane is unstable. If it pitches up slightly there, the pitch up will accelerate until it reaches the upper trim point.

Now consider the control power of the horizontal tail. When it flies in undisturbed air, it can trim a wide range of angles of attack. In the deep stall condition, however, its control power is much reduced, resulting in a much smaller range of trimmable angles of attack. If the lower end of this range is right of the point where the pitching moment crosses into positive values (here at $\alpha$ = 24°), the aircraft cannot escape with elevator deflections! enter image description here

Please note that the control power is not sufficient to enter the deep stall with quasi-stationary trim changes. The pilot needs to pitch up quickly and has to overshoot the static trim range in order to cross into the stable region above 30° angle of attack. There, his range of trimmable $\alpha$s is too small to achieve the same overshoot backwards.

To get out of this trap needs other changes: Either shift the center of gravity forward, or try to drop one wing. Unfortunately, both the ailerons and the rudder will also be much less effective due to the massive separation and the wake. In a number of cases, even experienced test pilots could not escape this condition.

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Would asymmetric thrust in multi-engine aircraft be a useful technique to counter the degraded control surfaces? –  Hugh Aug 6 at 4:14
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@Hugh: Possibly, but I would expect that tail-mounted engines would not produce enough yawing moment. Wing-mounted engines, however, should. In the end, it depends on the details of the configuration. –  Peter Kämpf Aug 6 at 5:16

A deep stall is a stall where the pilot is unable to pitch down due to either loss of clean airflow over the elevator (typical in T-tails) or the canards still producing lift while the wing behind it is stalled.

Depending on the aircraft the pilot may be able to bank and use the rudder to get the nose down to correct the stall. If possible the pilot may be able to move/ditch cargo to move the Center of Mass forward.

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Cycling the landing gear is also a suggested technique if you are in a deep stall as the drag below the center of thrust may pull the nose down enough to break the stall. –  GdD Aug 5 at 15:51
    
So, is a tail stall a subset of deep stalls? –  user2168 Aug 5 at 16:12
    
@Articuno: Under "tail stall" I'd understand condition where the tail itself is stalled and therefore ineffective rather than being ineffective due to disturbed airflow behind the (stalled) wings. It is slightly different, but similarly difficult to recover. –  Jan Hudec Aug 5 at 19:12
    
@JanHudec So, in a deep stall, the tail is not stalled? –  user2168 Aug 5 at 19:17
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@Articuno: I'd say it is not really well defined. The flow is already turbulent and won't attach to it at any angle of attack. –  Jan Hudec Aug 5 at 19:21

The article on stall in Wikipedia talks of one instance where a B727 recovered from a deep stall by "rocking the plane to higher bank angles" till the nose dropped and normal control response was recovered.

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