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I've read many accident reports, and seen reconstructions, where instrument failures, disagreeing sensor sources and auto pilot faults have led pilots into believing that they are over speeding when they are not (think AF447), are descending when they are actually climbing etc.

It seems that in all cases where this has led to a stall, the stick shaker has activated but the PF does not take the correct immediate action for stalls, instead continuing to believe the erroneous information they are being fed.

I understand the psychology of why this might be but is does strike me that many lives would have been saved if power up nose down had been applied. I've not found a case yet where the shaker was wrong. This leads me to wonder...

How reliable are stick shakers? Should they always be trusted?

Some older types even had an automatic stick pusher but this does not seem to be present in modern designs. Is this a consequence of shakers not being infallible?

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    $\begingroup$ The stick pusher is still there, it didn't go away. Stall recovery practice on the EMB-145 was to take it to the shaker and recover with minimal altitude loss without the pusher activating or secondary shaker activation. $\endgroup$ – casey Oct 21 '14 at 20:39
  • $\begingroup$ @ratchetfreak I edited my question slightly. In all cases I have read, the shaker was right. $\endgroup$ – Simon Oct 21 '14 at 21:03
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    $\begingroup$ I'm not sure there are stick shaker on airbus (AF447 you cite). $\endgroup$ – Manu H Oct 22 '14 at 14:52
  • $\begingroup$ @ManuH I did not use 447 as an example of a shaker, but as an example of how crews but can fooled into believing something is happening when the opposite is true. $\endgroup$ – Simon Oct 22 '14 at 17:37
  • $\begingroup$ PC-12 has a pusher, too $\endgroup$ – rbp Oct 27 '14 at 14:52
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The Australian Transport Safety Bureau recently published a comprehensive research paper on reported stall warnings in high capacity aircraft - http://www.atsb.gov.au/publications/2012/ar-2012-172.aspx

The report says that 30% of all stall warnings were false. Note that stall warnings aren't limited to the stick shaker.

The report says that of all reported stick shaker events, 16% were false warnings. Have a read of it, you'll find lots of interesting bits of information.

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  • $\begingroup$ False positives are better than false negatives in a shaker. I'm curious how many times the shaker should have gone off and didn't. $\endgroup$ – rbp Oct 27 '14 at 14:52
  • $\begingroup$ The difference between stall buffet and mach buffer is known as the "coffin corner." I thought most modern aircraft had fairly large (50+kt) differences, and was surprised to read that it is as low as 20kts. ( en.wikipedia.org/wiki/Coffin_corner_(aerodynamics) ) $\endgroup$ – rbp Oct 27 '14 at 14:56
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How reliable are stick shakers? Should they always be trusted?

I think the answer to this is can be covered by one of the great aviation mantras:

"always trust your instruments"

This is drilled into you when learning to fly on instruments. Pilots very rarely rely solely on one source of information. If you get a stick shaker, you'll normally immediately look for the reason as part of your scan. Stall warnings are rare in large aircraft but you sometimes get transient warning, e.g. when an aircraft is in turbulence or at low speed with a higher angle of attack and manoeuvring.

The crew of AF447 had a number of conflicting indications in the flight deck and were relying on the protection system of the aircraft to prevent the stall. Sadly the aircraft ended up stalled as the protection systems were not functioning correctly due to an erroneous air data source. One of the pilots held positive back pressure on the controls which maintained the stall condition of the aircraft. The recovery from the stall in large aircraft is the same as any with the most important part - Lower the nose.

Good article by Boeing here

Stick pushers are rare now as aircraft designers tend to shy away from producing aircraft which require one. Aircraft such as the HS Trident had stick pushers as they could end up in a deep stall - which is unrecoverable flight condition.

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  • $\begingroup$ Roger Roger. I think this confirms what I'm thinking. Stick shakes, no matter what anyone else is telling you, push it. I did rather complicate things by referencing 447 which I used as an example of fixation, rather than of stick shake. $\endgroup$ – Simon Oct 22 '14 at 17:41
  • $\begingroup$ I think it's important to analyse all of the information you are presented with. If the stick shaker is going - why? Justify your actions, act and then feedback the results to reanalyse the condition. $\endgroup$ – vectorVictor Oct 22 '14 at 18:57
  • $\begingroup$ The flight envelope protection of Airbus and Boeing 787 is a stick pusher and more (it allows you to fly reliably on the edge of stall—e.g. when you find yourself in downburst and need to stay as high as possible). $\endgroup$ – Jan Hudec Oct 22 '14 at 20:20
  • $\begingroup$ I don't think they have stick pushers in the traditional sense. The old stick pushers literally pushed the yoke forward to lower the nose and help recover the stall. Modern fly-by-wire aircraft have Alpha protection, called alpha floor on the Airbus. This limits the angle of attack and applies Take Go / Go Around thrust. $\endgroup$ – vectorVictor Oct 22 '14 at 20:29
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I'm going to lump stick shakers with all stall warning devices as they all have the same purpose. Stall warning devices are extremely reliable, and often more reliable than many pilots' judgement. When they go off it is because the conditions for a stall exist, and action should be taken. Occasionally a stall warning will go off momentarily in level flight because of turbulence, that usually is so short no action could be taken before the warning goes away.

Humans often have trouble in re-evaluating situations and can become fixated on one theory of events, especially under stress. People like to have confidence in their beliefs and judgement which leads to confirmation bias, where people "cherry pick" details which fit their model of events. This can lead people to ignore information that is literally in their face, as in AF447, Northwest 6231, and other similar accidents. The airplane can be shouting "STALL! STALL!", with an air horn blaring, stick shaker vibrating the pilot's hands until numb and the pilot will still be wondering why there's a high rate of descent.

So always trust your stall warning. Personally (I'm a light aircraft, not a commercial pilot) I practice stall recovery often where I immediately recover on the buzzer/horn, I do this so that it will become an instinctive reaction which I will do even if I'm overloaded and confused.

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    $\begingroup$ In a fully developed stall no pilot should be wondering about a high sink rate, or she/he better hands back her/his license. Immediately. $\endgroup$ – Peter Kämpf Oct 22 '14 at 15:56
  • $\begingroup$ Thanks GdD. This is pretty much what I figured. I'm a heli pilot so stall warnings are alien to me. If you stall a helicopter, there is NO recovery ;) $\endgroup$ – Simon Oct 22 '14 at 17:40

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