I have a little question concerning MCAS from Boeing and Alpha Protection from Airbus.

I think these two systems are the same, if I'm wrong please tell me.

If the two 737 MAX crashed because of the pilots not knowing how to behave when the MCAS behaves wrong, isn't it possible that the same could happen to A321 pilots?

I remember a Lufthansa flight (Airbus) in which the sensors froze and the pilots also didn't know how to behave. They turned the sensor off after talking with the technicians.

  • $\begingroup$ Could it happen to Airbus Pilots? Yes! Could it happen to Bombardier Pilots? Yes! Pilots of other manufacturers? Yes! While during the takeoff, pilots have a huge workload and little time to evaluate any problems, this appears to me to be more of a training problem rather then piloting skills. $\endgroup$ Commented Mar 14, 2019 at 7:53

1 Answer 1


No, the two systems are completely different.

The MCAS of the new Boeing 737 MAX was introduced to help the pilots lower the nose in high angle of attack situations by applying down stabilator trim. This was necessary because the engines are mounted further forwards compared to the previous NG models.

Note: We do not know whether or not MCAS caused the recent crash in Ethiopia yet, it is a bit premature to jump to conclusions now!

The Alpha Protection on Airbus aircraft is part of the fly-by-wire control (What is the Alpha protection?) and will limit control inputs from the pilots instead of applying trim. This is very different from how MCAS works.

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    $\begingroup$ Also, and kind of importantly, based on what I read so far, the design features regarding e.g. redundancy of input sensors are very different (Boeing: one (or two?) sensors operative at a time, meaning pilot must identify whether system works correctly; Airbus: three sensors operative at a time, meaning the system can determine whether it receives correct data (by comparing all three parameters) and can declare itself inoperative in case it doesn’t). $\endgroup$ Commented Mar 13, 2019 at 18:36
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    $\begingroup$ Also, on Airbus, it is not overriding control inputs, it limits control inputs. For example, on MMO/VMO protection if you push down your nose, and you reach VMo or MMO, you will still be able to pushdown but the pitch angle you will be able to reach will be limited by the EFCS. During a stall approach it is quite different since you also have the Alpha floor protection from the engine. $\endgroup$
    – Chris Lau
    Commented Mar 13, 2019 at 20:31
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    $\begingroup$ @ChrisLau you're right, override was a wrong choice of words, I fixed it to limit. $\endgroup$
    – Bianfable
    Commented Mar 14, 2019 at 7:22
  • $\begingroup$ re "We do not know whether or not MCAS caused the recent crash in Ethiopia". What about Lion Air? If I understood correctly, you can handle "nose down" by MCAS as a runaways stabilizers problem, even without knowing the existence of MCAS; the stab trim cutoff avoids the MCAS to be able to do anything, but in the same time makes it necessary to trim manually (if you are not at high altitude and realized the problem too late, the stabilizers could be all "nose down" and maybe there will be no time to trim those back up). My point: put in certain ways, MCAS alone never will be the cause. $\endgroup$ Commented Mar 31, 2019 at 13:18
  • $\begingroup$ @ChrisLau this answer says somewhat differently, stating the Alpha protection will act to lower the nose of the aircraft. Although I think its a somewhat academic distinction... if flight conditions change and suddenly alpha prot is limiting your control input to a lot less than it was a second ago, I think most people would call that being "overridden". $\endgroup$
    – mbrig
    Commented Apr 2, 2019 at 18:58

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