Boeing’s new MCAS software fix will compare the inputs from both AOA sensors. Specifically, the AoA signals coming from the sensor to the ADIRU (Air Data Inertia Reference Unit) on each side of the aircraft will be compared before sending it on to the Flight Control Computer (FCC) to trigger the MCAS function.
• If the AoAs disagree by more than 5.5° when the flaps are retracted, MCAS will be disabled for the flight.
• if the AoAs disagree by more than 10° for over 10 seconds, MCAS will also be disabled for the flight.
• if MCAS is activated, it will only provide one nose down stabilizer command for each elevated AOA event.
This MCAS fix will not stop the stick shaker from being activated on the side where an AoA signal exceeds the threshold for a stall. Therefore, if one of the AoA signals exceeds the threshold for a stall during takeoff, the stick shaker’s vibration and annoying sound will remain active until the aircraft has landed, making it more difficult for the pilot to control the aircraft. The MCAS fix will also not prevent the display of different air speeds and altitudes on the displays of the pilot and F/O as a result of the correction of these values by the AoA signals. This may also cause pilot confusion in control of the aircraft. Finally, if the MCAS function is triggered as a result of one of the above conditions, then MCAS is disabled, leaving control of the aircraft’s pitch attitude unprotected. This can increase the possibility of a stall that MCAS was intended to prevent.
It seems to me that a better fix would be the following. If the stick shaker activates on either side, then the pilot or F/O should immediately de-activate the ADIRU on the side that the stick shaker is active, causing the ADIRU on the non-activated side to be used for both displays, the Flight Control Computer (FCC), and for the MCAS function. The MCAS function should then operate according to the new software fix. This change will assure that the stick shaker is turned off, that the pilot and F/O both receive the same known good display information, and that the MCAS function remains active until it issues its first nose down command. This switching of ADIRU functions should be possible because the ADIRU’s send their data to the displays and to the Flight Control Computer (FCC) to trigger the MCAS function. It may even be possible to automate the switching of the ADIRU’s on the condition that one of the AoA signals exceeds some value indicative of an impending stall. Finally, if both AoA signals exceed some value indicative of an impending stall, then both ADIRU’s and MCAS will be switched off, allowing the pilot to take manual control of the aircraft.
Now, I am not a pilot. But I would like to hear the reaction of B737 MAX pilots to this enhanced MCAS fix. Perhaps it can’t be implemented as soon as Boeing’s latest MCAS fix, which is intended to get the B737 MAX fleet back in the air as quickly as possible. But perhaps it can be implemented in a later software fix which improves upon Boeing’s latest MCAS fix to further help the pilot maintain control of the aircraft.