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Light Cessna aircraft - at least the high wing single engine line - have commonly made use of a solid state pneumatic stall warning system. The system is fairly unique to Cessna airplanes and makes use of the fact that negative pressures form over the leading edge of the wing near the critical angle of attack, causing a reverse flow of air through the warning system and causing a horn in the cockpit to sound. Most other light aircraft make use of a movable tab with activates an electric switch when depressed by airflow near the critical AoA, causing a chime, horn or other aural cue in the cockpit alerting the pilot to an impending stall. Here's the question: What are the pros and cons of each of these systems? From brainstorming, I can come up with the following:

Pneumatic Stall Warning Systems:

Pros:

  • Solid State; has no moving parts which can break or become immobilized or altered by neglect, damage or airframe icing.
  • Does not require electrical or other sources of power from engine or elsewhere to work.

Cons:

  • Orifice can become blocked by FOD or airframe icing, causing the system to become inoperative.

Mechanical Stall Warning Systems:

Pros:

  • Simple operating and mature technology.
  • Can be electrically heated to prevent immobilization by airframe icing, though this is not always the case depending on the aircraft.

Cons:

  • Moving parts which can break.
  • Tab can be bent by maintenance or neglect, causing system to operate erroneously and give false readings.
  • Can be immobilized by airframe icing, rendering the system inoperative.

Can anybody think of other pros and cons for each of these systems?

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  • $\begingroup$ Pneumatic one can also be heated, perhaps even easier. (Though I've never seen it done). $\endgroup$
    – Zeus
    Nov 1, 2018 at 23:51

3 Answers 3

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Electromechanical:

Pro - can be tested easily during preflight by turning the mains on and manually actuating the tab

Pro - Potentially lighter (although not my much)

Pro - easy to install multiple switches in parallel if you want to trigger based on a stall of differing characteristics as is done on the Piper Seneca

Pro - Easy to wire into other systems. Maybe you want to add a stick shaker or some kind of feed back to the autopilot, much easier if its already generating an electrical signal.

Pneumatic:

Con - can only be tested by creating a low pressure on the horn, not as easy as the tab option

Many of the above pros become Cons for solid state units, like being harder to connect to other systems with out a transducer, harder to add multiple units without multiple horns, and the weight.

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  • $\begingroup$ I doubt the electromechanical one is lighter, even potentially, if you consider all the wiring required. $\endgroup$
    – Zeus
    Nov 1, 2018 at 23:49
  • $\begingroup$ Cons for pneumatic systems from a maintenance perspective also include decoupling of pneumatic tubing and mechanical failure of the horn reed. $\endgroup$
    – J W
    Nov 18, 2018 at 18:54
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One pro of the pneumatic one not yet mentioned is that it is somewhat 'analog'. It's not just on/off; it starts faintly and intermittently and then raises to a solid loud signal as the AoA increases. This gives a somewhat better picture of what's happening, esp. on landing. Perhaps even prevents the pilot from freaking out when it happens unexpectedly.

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  • $\begingroup$ Some modern electronic stall indicators will give a similar indication via intermittent, increasing, then finally continuous tones as AoA approaches critical. These are somewhat more complex than the basic on/off tab based stall warning as they sense differential air pressure (dual port pitot), but far more useful for the same reasons you state above. $\endgroup$
    – madscientist159
    Aug 2, 2020 at 23:26
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Main problem with the Cessna one is all you get is that whistle. When I've flown Cessnas in the past I never paid much attention to it as it just blends in with other sounds like whistles coming from flaps or a door that leaks ("That funny whistle is the stall warning? Really?"). Not as effective as the BLAAAAAAAAAAAAAAAAAAAAAAAA coming from an electric one.

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  • $\begingroup$ I have no trouble hearing the pheumatic horn, even with active noise cancelling headsets. I've not noticed any whistles from flaps, that's a new one. Door seals, that's maybe more of a whooshing sound. Guess it can vary from airplane to airplane. $\endgroup$
    – CrossRoads
    Nov 1, 2018 at 12:58
  • $\begingroup$ I've flown good number of different types of airplanes, and they can make all kinds of noises especially when they're old and beat up lol. $\endgroup$
    – John K
    Nov 1, 2018 at 17:27
  • $\begingroup$ My experience is again the opposite... Never had a problem hearing the horn, it's so distinctive and very unlike any other sound potentially emitted by the avionics. I posted a separate answer explaining some specifics. $\endgroup$
    – Zeus
    Nov 2, 2018 at 0:04

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