Commercial aircraft designers need to rethink fundamental assumptions that pilots have sufficient knowledge, training and skill to cope with failures, the inquiry into the Lion Air Boeing 737 Max accident has concluded.
The investigation into the fatal October 2018 accident has revealed that the crew did not respond as Boeing had expected when the aircraft's Manoeuvring Characteristics Augmentation System repeatedly pushed the aircraft into a nose-down attitude, as a result of false angle-of-attack sensor data.
While the inquiry has criticised short-sightedness in Boeing's thought processes and analyses during the development of the 737 Max, and its MCAS in particular, it has also highlighted a discrepancy between the presumed and actual abilities of pilots.
Boeing had used flight-test pilots to demonstrate regulatory compliance during the certification of the 737 Max.
But Indonesian investigation authority KNKT says such pilots "normally have exceptional skill and experience", and more knowledge of design characteristics than regular line pilots.
"This level of competence usually cannot be translated to most pilots," it adds.
Test pilots are trained to replicate average crews, says the inquiry, and line pilots participate in standardisation processes to help ensure that requirements are operationally representative.
But the investigators believe a rethink by commercial aircraft designers, as well as regulators, is necessary to revise suppositions on the likely competence of airline customers' crews.
KNKT says the US FAA and manufacturers should "re-evaluate their assumptions" as to what constitutes an "average flight crew's basic skill" as well as the presumed level of knowledge that a "properly-trained average flight crew" possesses when confronted with system failures.
The inquiry heard that Boeing engineers and test pilots informally discussed the possibility of erroneous angle-of-attack data repeatedly triggering MCAS, but assumed that pilots would take immediate action to correct the attitude and trim out the resulting control forces.
This led to the conclusion that "no redesign was necessary", says KNKT.
But following its analysis of the 737 Max and MCAS development process, the inquiry states that Boeing should include a "larger tolerance" in designs in order to "allow operability by a larger population" of pilots.