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I'm watching video RV 10 N783V My 1st Emergency

At 8:45 a stall alarm goes off and is a horrible piercing whistle. Why? Surely the pilot has enough distractions without also being deafened while trying to talk to ATC.

Isn't there a better way of indicating a continuing alert, and why can't pilots switch the the thing off when they need to?


EDIT: The video shows that it is a false-positive warning. The pilot eventually manages to switch it off by resetting all his electronics.

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    $\begingroup$ @ymb1 While I think it may be relevant if the OP were asking about this specific situation, it seems the OP is more interested in why they can't be turned off. $\endgroup$
    – Ron Beyer
    Apr 25 at 0:56
  • $\begingroup$ I never liked the gear horn in the CRJs (Rockwell Collins GPWS). It sounds exactly like the old steady dual frequency tone you got on TV stations when they went off the air overnight. I found that particular sound very easy to blend into background when under some high stress situation in the sim. The saving grace was you'd get "Too Low - Gear" if you ignored the warning horn because you were saturated with crises and got too close the ground. $\endgroup$
    – John K
    Apr 25 at 2:28
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    $\begingroup$ "...while trying to talk to ATC". In a situation with a stall warning, talking to ATC should be the LAST priority. Aviate - Navigate - Communicate. A stall warning should absolutely override the radio. $\endgroup$ Apr 25 at 6:25
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    $\begingroup$ For those that didn't watch the video, the stall warning was caused by an iced-over pitot tube, supposedly. The pilot realized he wasn't stalling before he realized the icing condition. Not sure how the stall warning actually works on this aircraft - the pilot also mentioned the aoa indicator not working. Would that have separate icing protection? Or is it somehow driven from the pitot tube, by factoring in the g-loading? $\endgroup$ Apr 25 at 12:50
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    $\begingroup$ Because the opposite problem can be far worse $\endgroup$
    – Machavity
    Apr 26 at 13:27
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A stall condition needs to be handled now, and once it's handled, the alarm will go away. What you don't want to happen is you are in an approaching stall condition, the pilot hits a "silence" button while the situation gets worse without the plane telling them.

An emergency is "AVIATE, NAVIGATE, COMMUNICATE" in that order. Fly the plane first, point it where you want to go second, and third, only if you have time/resources, is to talk to ATC about it. The pilot should be fixing the problem first, focus on flying the plane, ATC can wait.

They can't be turned off for that reason. Deal with the problem and the alarm goes away, you don't want to silence the alarm and forget about it. There is some merit to "overloading", where multiple alarms are happening at the same time, or when the pilot focuses on a low priority item and forgets to fly the airplane (Eastern Air 401).

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    $\begingroup$ "AVIATE, NAVIGATE, COMMUNICATE" - All the time, not just in emergencies. $\endgroup$
    – jamesqf
    Apr 25 at 5:08
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    $\begingroup$ "Deal with the problem and the alarm goes away" was unfortunately part of the accident chain for AF447, as in that accident, the alarm went away while the problem was being made worse, due to very low airspeed, while it started sounding again when the crew dealt with the problem and the airspeed began to increase. But that goes to show how ingrained "deal with the problem and the alarm goes away" is, to the point the crew continued to exacerbate the situation, perhaps because making the problem worse did happen to silence the alarm. $\endgroup$ Apr 26 at 2:51
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    $\begingroup$ @ZachLipton I would trust the airplane tech way longer than any human - and so did the pilots. It was the wrong call in this situation, but how many accidents would happen, if there were no tech and only humans to fly the plane? $\endgroup$
    – Christian
    Apr 26 at 12:56
  • $\begingroup$ @ZachLipton the issue is dealing correctly with the problem, not a general rant on doing things just to turn off alarms :) $\endgroup$
    – EarlGrey
    Apr 26 at 14:03
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Things may be different for small airplanes, but for the transport-category, i.e. Part 25 certification, your concern is taken into account:*

(d) The alert function must be designed to minimize the effects of false and nuisance alerts. In particular, it must be designed to:

...

(2) Provide a means to suppress an attention-getting component of an alert caused by a failure of the alerting function that interferes with the flightcrew's ability to safely operate the airplane. This means must not be readily available to the flightcrew so that it could be operated inadvertently or by habitual reflexive action. When an alert is suppressed, there must be a clear and unmistakable annunciation to the flightcrew that the alert has been suppressed.

14 CFR § 25.1322 - Flightcrew alerting.

As the text shows, suppressing alerts must not be readily available, but the means to suppress must be provided – once the crew for example confirms the false-positive. In the linked video the pilot reset a few circuit breakers and the warning was gone.


* Thanks to @Gerry for his insight and @Bianfable for tracking down the similar European rule, that regulation applies to aircraft made since 2011. It may be worth mentioning that at least the Airbus A320 have had the ability to suppress spurious alerts since the mid-80s.

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    $\begingroup$ This version of the rule, Amdt. 25-131, Eff. 1/3/2011 was first applied to the B787 as Special Conditions and then codified in 2011. Prior to that (Amdt. 25-38, Eff. 2/1/77) 25.1322 only specified the color of 'Warning, caution, and advisory lights'. So the rule you reference only applies to new and relatively recent Part 25 aircraft. See the link for the explanations. $\endgroup$
    – Gerry
    Apr 25 at 12:28
  • $\begingroup$ As a non-pilot, I'm wondering if the alarm should have come back on after resetting the circuit breaker - his pitot tube was still frozen up at that point, he still hadn't figured out to turn on the heating for it. Why did it not come back on? (Also, why is there no hygrometer/thermometer combo inside a/that pitot tube?) $\endgroup$
    – Sixtyfive
    Apr 25 at 21:17
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    $\begingroup$ @Sixtyfive: My analysis: when the airspeed dropped on its way to an invalid reading, this triggered the digital stall warning. When the system was rebooted, there was no speed reading to use for the stall algorithm (this is worthy of its own question BTW). More complex aircraft have ice detectors, but temperature alone would not be a good indicator; altitude and visible moisture come into play (again, very valid question worthy of more visibility). $\endgroup$
    – ymb1
    Apr 25 at 21:23
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    $\begingroup$ If you want more insight to the Flightcrew Alerting rule and what is acceptable, I suggest reading AC 25.1322-1. It provide examples on how to categorize alerts and how they should be presented. $\endgroup$
    – Gerry
    Apr 27 at 12:59
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A Stall horn will tell you, you are (close to) no longer flying, but actually falling.

Since this is of paramount importance for every airplane, it will sound until recovery is completed. Often it is only the silencing of the stall horn that can give a pilot certainty about that. Sometimes even this is ignored, because pilots treat it as just as alarming as any other device, which it isn't. Gear malfunction, terrain alert, traffic alert, burning engine, a hijacker holding you at gun point, all these things are of minor importance compared to a stall.

As dictated by law, a stall horn works on every winged aircraft, even if all power is lost and all other instruments are dead. (EDIT: this is not correct. It should be, but it ain't. See comments)

A winged aircraft with a continuously sounding stall horn is in the process of crashing. It is not going there, it is there. Its falling, like a brick.

Tell me, why would anybody in his right mind ever want to even just be able to shut that down?

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    $\begingroup$ "As dictated by law, a stall horn works on every winged aircraft" I've flown aircraft without stall horns, they are not dictated by law (usually you find this in retractable gear aircraft where the stall horn can be mistaken for the gear horn). What is required by law is an indication, which can be a pronounced aerodynamic indication or a horn (but a light is not allowed by itself). $\endgroup$
    – Ron Beyer
    Apr 25 at 19:43
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    $\begingroup$ "Tell me, why would anybody in his right mind ever want to even just be able to shut that down?" Because, as indicated and shown in the video, the pilot discovered it was a false indication. Also because it was an unpleasant continuous whine that lasted for minutes. $\endgroup$ Apr 25 at 19:45
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    $\begingroup$ @Ron Beyer The reason why stall horns are called "horns" and not "alarm" or "indicator" is because of the simplicity of their construction. They are not just activated by a stall, they are actually powered by it. As far as I know, true stall horns or an equivalent with a comparable reliability are mandatory in all certified winged aircraft. Whether they are mandatory in powered gliders, rotor planes or experimental aircraft I admit I do not know. My bad $\endgroup$
    – Berend
    Apr 25 at 21:04
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    $\begingroup$ @Berend "As far as I know, true stall horns or an equivalent with a comparable reliability are mandatory in all certified winged aircraft"... That's the false part. I know what a stall "horn" is and how it works, see CFR 25.207(b). An audible device is not required for "other than transport category aircraft". It absolutely did not exist in the RG PA-24-180 Comanchee I flew, a certified aircraft. $\endgroup$
    – Ron Beyer
    Apr 25 at 21:32
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    $\begingroup$ @Berend : I've flown a few acrobatic-certified aircraft and none had a stall warning. Makes sense, as they often get into deliberate stalls and stall-like conditions, and they aren't used for long-distance travel anyway. Also in case of gliders, a stall horn doesn't make any sense, as gliders are flown at near-stall for extended time periods in a thermal. I haven't seen any in any glider I've been in. $\endgroup$
    – vsz
    Apr 26 at 12:31

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