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Just an enthusiast here; pardon the dumb question. Im a little confused between the difference of a stall and airspeed low.

“An aircraft Stall Warning System is that system which provides the pilot with advance warning of an impending stall.”

A stall occurs when the wings are not able to produce sufficient lift, and therefore, the aircraft will begin to drop.

‘Airspeed Low’ is telling the pilots that you have very low airspeed but if the airspeed is too low, won’t the plane then stall?

What’s the difference then between AIRSPEED LOW warning and a STALL warning? For they both lead to the same result. An aircraft that stops flying and starts falling. Is one more advanced of the other, perhaps? If you get an AIRSPEED LOW warning do you have more time to recover than if you receive a STALL warning? If that’s the case, they both take the amount of time to recover, though. Max thrust and point the nose down.

Do you perform difference actions for these two warnings?

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  • $\begingroup$ What aircraft produces these warnings? $\endgroup$
    – Organic Marble
    Commented Jun 16, 2021 at 22:12
  • $\begingroup$ doesn’t all airbus fleet? apologies, added the tag…anyway, the 330 definitely does…. $\endgroup$
    – cmp
    Commented Jun 16, 2021 at 22:16
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    $\begingroup$ @Organic Marble, most current day Boeings. $\endgroup$
    – skipper44
    Commented Jun 17, 2021 at 0:07
  • $\begingroup$ @cmp, are you sure it is an Airbus warning at all? As far as I remember, Airbus only gives you a “SPEED! SPEED! SPEED” aural warning and then you hit the alpha-min, alpha-floor and alpha-limit, which won't let you proceed to STALL unless in alternate mode. Also the fact skipper44 answers for Boeing suggests it isn't in Airbus, because the low-speed and auto-thrust logics are different between Airbus and Boeing. $\endgroup$
    – Jan Hudec
    Commented Jun 17, 2021 at 18:49
  • $\begingroup$ Related for Airbuses. This is A320 specifically but other have similar function. aviation.stackexchange.com/questions/66646/… $\endgroup$
    – busdriver
    Commented Jun 18, 2021 at 8:42

2 Answers 2

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Referring to one manufacturer's nomenclature and systems:

The "AIRSPEED LOW" caution occurs shortly after the speed drops below the Minimum maneuver speed. The Min Maneuv spd is ~30% above stalling speed for slat/flap config. Even if the auto-throttle is not engaged in any mode, as long as the auto-throttle is armed, this is the speed at which "auto-throttle wake-up" takes place, ie the auto-throttle pushes the thrust levers up so as to regain speed without any pilot action. At typical weights and maneuvering speeds this will occur well above the speed at which stick-shaker (stall warning) takes place.

Immediate action is to be taken by the pilot when "AIRSPEED LOW" is displayed. It is considered that the airplane is entering the 'approach to stall' phase. Here's an actual extract (maybe not the latest though)

The first indication of an approach to stall is the AIRSPEED LOW EICAS message. A stall warning should be readily identifiable by the pilot, either by an artificial indication (stick shaker) or natural indication (initial buffet). During the initial stages of a stall, local airflow separation results in buffeting, giving a natural warning of an approach to stall. Stick shaker operation will usually precede initial buffet as a stall warning indication. In some cases, near cruise altitude and cruise Mach, stick shaker may be simultaneous with initial buffet. Recovery from an approach to stall should be initiated at the earliest recognizable stall warning, either AIRSPEED LOW EICAS message, stick shaker or initial buffet.

Thus the procedure is the same for both the AIRSPEED LOW and SS and it is called, "Approach to stall or stall recovery". Here are the first few steps only:

Pilot Flying:
Initiate the recovery:

  • Smoothly apply nose down elevator to reduce the angle of attack until buffet or stick shaker stops.

Continue the recovery:

  • Roll in the shortest direction to wings level if needed
  • Advance thrust levers as needed
  • Retract the speedbrakes
  • etc

note 1. When the AIRSPEED LOW caution first appears, there would be no SS and if there's no adverse bank, the first action would be to advance thrust levers.
note 2. The procedure quoted by @cmp - If that’s the case, they both take the amount of time to recover, though. Max thrust and point the nose down - has now been changed. The increase in thrust is to be made after this initial procedure of reducing the AoA and leveling wings. This is based on the consensus between industry, regulator, professional pilot's groups etc., who made the study of the altitude loss/ ground contact issues during LOC-I (Loss of Control - Inflight) study.

Actual procedures must nevertheless be checked for the aircraft type being flown and as the actual situation may demand, for e.g. 'ground contact is a factor'.

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  • $\begingroup$ So the issue is not only threat of a stall but not having enough air over the tail surfaces for a turn or some other movement from level flight? $\endgroup$
    – MacGuffin
    Commented Jun 17, 2021 at 12:49
  • $\begingroup$ This is an interesting answer, although clearly not applicable in the GA aircraft I fly. Can someone scope this answer? Are the features described common to all transport category aircraft? $\endgroup$
    – 2NinerRomeo
    Commented Jun 17, 2021 at 17:42
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    $\begingroup$ @MacGuffin, no, it's still about the wing, mainly. When you turn, the centripetal force is also provided by wings by banking into the turn, but that means the wings need to produce more lift in a turn and therefore the stall speed increases. $\endgroup$
    – Jan Hudec
    Commented Jun 17, 2021 at 19:02
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    $\begingroup$ @MacGuffin, there is also minimum control speed, which is minimum speed when rudder has enough airflow over it to compensate for one engine running at maximum power while the other being shut down, but that does not depend on flap setting and this warning does, so this warning is just about margin above stall to allow turning. $\endgroup$
    – Jan Hudec
    Commented Jun 17, 2021 at 19:04
  • $\begingroup$ @JanHudec I recall some kind of minimum speed for 2-engine propeller planes to maintain controlled flight. The speed was different depending on which engine was out if both props spun the same direction. I thought there might be something similar with jets, just without the speed being different depending on which engine is out. This adds another element, thanks. $\endgroup$
    – MacGuffin
    Commented Jun 17, 2021 at 20:12
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From a physics point of view (which may or may not agree with manufacturers’ terminology) a stall is when the angle of attack is steep enough that the airflow breaks away from the upper surface of the wing. Low airspeed is just what you would expect. In level flight at a known air pressure the relation between these two is well understood, but in other conditions this relationship can break down; a radical manoeuvre could induce a stall at relatively high speed, whereas a wing with zero airspeed will never be stalled. So both warnings inform the pilot, and hopefully they can interpret the information appropriately.

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