Boeing sold the Boeing 737 MAX 8 and 9 aircraft with
MCAS “flight control law” software to allow a similar “feel” as flying other 737 models to pilots and reduce stall possibilities with this new weight and balance presentation of the engine-forward MAX. MCAS did this by using angle of attack information from one of two angle of attack sensor instruments mounted on either side of the aircraft’s nose under the assumption data from just one AoA Indicator was sufficient and there was a minimal chance an AoA Indicator would fail - but this appears to be exactly what occurred in the fatal Lion Air and Ethiopian Airlines 737 MAX 8 losses.
Boeing sold as additional and optional safety features related to MCAS two options:
1) The option to display the AoA data within the flight display presented to the pilots.
2) An Angle of Attack disagree warning that would activate and warn the pilots if the two AoA instruments didn’t report the same AoA.
But: even on aircraft with the AoA disagree warning, they were inoperable because the AoA indicators in the flight display had to be installed and operating. Remember, both were sold as separate “optional extras” rather than included as safety measures.
American Airlines purchased both systems for their MAX 8s. Southwest Airlines purchased the AoA disagree warning alone, but they were not told for over three years the purchased optional extra was inoperable without the other option offered, the AoA flight display. (United, Lion, Ethiopian purchased neither.) Boeing failed to disclose this to Southwest, apparently lowballed the MCAS related issues, and possibly the airlines didn’t perform due diligence and / or chose to save a few dollars.
Boeing failed to disclose to Southwest they had paid for a safety system that did not work. ☹️
https://www.upi.com/Top_News/US/2019...2021556535826/