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Can someone with flight experience help me with this question? So, MCAS is watching the AOA sensor and dipping the nose to prevent aerodynamic stall. When act

Can someone with flight experience help me with this question? So, MCAS is watching the AOA sensor and dipping the nose to prevent aerodynamic stall. When activated (and altering the pilots loudly) what would happen to airspeed in the valid-AOA and borked-AOA sensor states? I would have thought in the bogus-AOA case, airspeed would be increasing MUCH faster? Is this factored into regarding validation of the AOA input? Or are airspeed indicators unreliable near aerodynamic stall? Or ..? It just feels like MCAS should be able to detect the contradiction state from other variables without requiring the pilot to act.

bitwalker 8y ago
Airspeed and angle-of-attack are different measurements, whether the AoA is borked or not would make no difference to airspeed measurements, and vice versa. Without AoA though, airspeed alone is not enough to determine a stall. If one or both sets of probes are incorrect or inoperable, detection of stall conditions would be unreliable at best, either reporting a stall when there isn't one, or failing to report one that is valid. As for MCAS, it would likely be able to see that two AoA probes disagree, but would be unable to choose the "correct" probe. A third AoA probe _might_ address that, but I never saw that kind of configuration on aircraft I worked on, and it isn't on the MAX. Regardless though, from what I've read, MCAS only applies gradual nose down trim, so ignoring whether or not MCAS even knows enough to disable itself in abnormal conditions, the pilot/first officer should have more plenty of time to react to it (I believe it was something like 2.5deg of trim every 10s). Either the pilots in these situations are not aware that it is a trim issue, which seems very unlikely, or they are overloaded and don't remember in time to set the stab trim to cutoff which would disable those inputs. Very much still on Boeing in my opinion, but it isn't like the aircraft is just diving into the ground the second MCAS starts nosing down.

bitwalker 8y ago

Airspeed and angle-of-attack are different measurements, whether the AoA is borked or not would make no difference to airspeed measurements, and vice versa. Without AoA though, airspeed alone is not enough to determine a stall. If one or both sets of probes are incorrect or inoperable, detection of stall conditions would be unreliable at best, either reporting a stall when there isn't one, or failing to report one that is valid. As for MCAS, it would likely be able to see that two AoA probes disagree, but would be unable to choose the "correct" probe. A third AoA probe _might_ address that, but I never saw that kind of configuration on aircraft I worked on, and it isn't on the MAX. Regardless though, from what I've read, MCAS only applies gradual nose down trim, so ignoring whether or not MCAS even knows enough to disable itself in abnormal conditions, the pilot/first officer should have more plenty of time to react to it (I believe it was something like 2.5deg of trim every 10s). Either the pilots in these situations are not aware that it is a trim issue, which seems very unlikely, or they are overloaded and don't remember in time to set the stab trim to cutoff which would disable those inputs. Very much still on Boeing in my opinion, but it isn't like the aircraft is just diving into the ground the second MCAS starts nosing down.