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Old 20th Mar 2019, 20:38
  #2170 (permalink)  
BrandonSoMD
 
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Originally Posted by yanrair
AoA detects a stall by measuring the angle of attack with two sensors, one each side. Measuring the angle of airflow.
Speed is measured by sensing air pressure in a tube facing forwards (pitot)and converting that into Indicated Airpspeed. Not the same as real speed.
GPS measures real speed and is almost never wrong.
Say you have blocked pitot tubes as per aF 447, the AoA still works and stall warning still works.
Say you have frozen AoA vanes so that they are not able to detect stall, or incorrectly detect a stall (as is suggested in these Max incidents) then the Airspeed should still work.
Another scenario is that one airspeed is faulty, but the other two work - there are three airspeed indicators. That one is easy since you go for the two that agree. Backed up with GPS.
There is a lot wrong here. I understand the point you're trying to make - we can cross-check a lot of things and make smarter decisions about what might be wrong - but let's be accurate, since a lot of non-pilots and non-aero-engineers are viewing this thread.

AOA sensors are not always duplicated. Yes, on the 737 they are, as with nearly all commercial aircraft, but that's not a universal fact.

Pitot *AND* static pressures are both required for measuring airspeed. In fact, airspeed is a function of the difference between the two.

Pitot/static speed as shown to the pilot on his airspeed indicator *IS* a real speed. Yes, it's got some assumptions wrapped in it - specifically, standard day pressure - but it's far more useful to a pilot than GPS speed. The airplane cares not a whit how fast over the ground it's moving; it flies according to the pressure field around it, which is a function of altitude and temperature and humidity.

GPS speed is not "almost never wrong". Its accuracy is strongly dependent on the relative position of the satellites. Yes, it's fairly reliable. But onboard air data sensors are far more predictable and reliable.

GPS speed by itself is fairly useless unless you know (with some certainty) the speed and direction of the wind and the airplane altitude and the ambient temperature and humidity. To get back to a useful airspeed, you need quite a bit of additional data. In fact, calibrating airplane Pitot/static systems using GPS data is astoundingly difficult. I should know - I developed several test methods for doing exactly that.

Some airplanes do use Pitot/static data for stall warning. In fact, most commercial airliners (like the 737) show a "zipper" of minimum speed which is also used to activate stall prevention systems; this is computed directly from Pitot/static data.

AOA data is used to correct Pitot/static errors. An incorrect AOA can definitely screw up the indications of altitude and airspeed, and cause changes in automation that depends on those values.
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