I understand why this is -- you don't want to have the stall warning go off while the plane is parked at the gate or slowly taxiing.
However, this seems like clearly bad design for two reasons (that should have been obvious to the designers):
- This introduces an extra point of failure. If the pitot-static system fails, then the stall warning may be disabled due to erroneously low reported airspeed.
- As demonstrated by AF447, a stalled plane may have very low forward airspeed in an extreme stall.
Moreover, it seems like the solution is easy: Only disable the stall warning if a number of criteria are met. For example, the following logic could be used.
The the stall warning (based on AoA vanes) is disabled if and only if (i) the forward airspeed is below 60 knots AND (ii) the landing gear is down/on the ground AND (iii) the radar altimeter reports a height of 0.
This logic would still prevent erroneous stall warnings on the ground, but it is more unlikely to suppress "real" stall warnings, as it would require multiple systems to fail.
Alternatively, if you are worried about one broken sensor resulting in erroneous stall warnings, you could require only two of the three conditions to hold for the stall warning to be disabled. That's still better than only one condition.
My question: Why do aircraft not use multiple criteria, such as suggested above, for disabling the stall warning?
The point of my question is that you want the stall warning disabled when the plane is on the ground. Whether or not the plane is on the ground should be rather obvious and you should not need to try guessing whether the plane is on the ground based on airspeed alone. There are plenty of other sources of information that can indicate whether the plane is on the ground.