I can imagine there are several different ways to measure angle of attack (accelerometers, relative pressure measurements between pitot tubes, etc), but I'm curious which sensors the 737 MAX uses for its MCAS system since it's been in the news recently.


The pitot tube has nothing to do with the angle of attack. It is used for measuring pressure which in turn gives you the airspeed.

Accelerometers are used for the Inertial Navigation System.

The angle of attack is another sensor (shown below): the principle is that it rotates and reads the angle between the current position and the reference position.

angle of attack sensor

Regarding the 737 MAX, there are two AoA sensors one on the captain side and one on the first officer side.

The MCAS reads the AoA from the captain side.

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    $\begingroup$ appreciate the info! Just to not seem like an idiot, there have been studies as far back as 1955 showing that pitot tubes could be used for AoA measurement: apps.dtic.mil/dtic/tr/fulltext/u2/102442.pdf $\endgroup$ Mar 17 '19 at 19:58
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    $\begingroup$ That's interesting! I wasn't aware of it $\endgroup$
    – Afe
    Mar 17 '19 at 20:22
  • $\begingroup$ The IRS can compute the flight path vector which, knowing the airspeed and ground-speed and thus wind speed could actually be used to calculate a pretty good estimation for the angle of attack. It is probably not fast enough through. $\endgroup$
    – Jan
    Mar 17 '19 at 20:27
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    $\begingroup$ @Jan It would not be very accurate, as the vertical motion of the air is an unknown, and the variation of vertical airmass speed seen in normal weather conditions is comparable to the range of the vertical component of the incident airflow at typical airspeeds and AofA. No doubt one could have a more sophisticated model that included the airplane's response to both gusts and control inputs, and so attempt to estimate the vertical motion of the air by the aircraft's response to it, but there is little point in doing so, as the value can much more easily be directly measured. $\endgroup$
    – sdenham
    Apr 5 '19 at 16:58
  • $\begingroup$ @NicolasHolthaus That is an interesting paper, but note that the pitot tubes described here are not simple ones like those used for airspeed measurements - they involve multiple ports, each with its own sensor, arranged around a conical or hemispherical body. Some of the other ideas here are quite wild, such as optically tracking smoke. $\endgroup$
    – sdenham
    Apr 5 '19 at 17:14

I think you are talking about the heavy MCAS "corrections" that led to the two crashes. The MCAS is part of the STS (Speed Trim System). Apart from the Angle of Attack sensor, MCAS also checks that the flaps are up (retracted) and the auto pilot is off before it triggers the heavy nose-down trim. Some sources add that MCAS also needs a high bank angle and the engine at high power before it activates.

BTW the MCAS does not check to see if the AoA sensor readings make sense. It accepts the sensor readings blindly.


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