A New York Times investigation of the Oct. 29, 2018, Lion Air Boeing 737 MAX crash suggests marketing considerations were at least partly behind Boeing’s and the FAA’s joint decision to not specifically train pilots in the maneuvering characteristics augmentation system (MCAS) that may have played a role in the crash. The Times story quotes various named sources as saying that Boeing wanted to maintain the cross compatibility between the new aircraft and earlier versions of the 737, thus simplifying conversion training and reducing costs for airlines buying the MAX.
The difficulty was that the physically larger engines that accomplish the plane’s main selling point—better fuel economy—had to be mounted higher and farther forward than on its predecessors and that significantly changed low-speed flight characteristics. MCAS was designed to compensate for the MAX’s increased tendency to stall in a low-speed turn by adjusting the angle of the horizonal stabilizer. The system takes data from one of two angle of attack indicators (there’s no redundancy or agreement requirement) and was designed to automatically push the nose down if an incipient stall was detected. Boeing convinced the FAA that because the system maintained the basic flight characteristics of earlier versions that pilots did not need specific training on MCAS even though its inclusion was considered necessary for certification of the aircraft.
The Times story also notes that other regulators at least initially determined that pilots should be made aware of MCAS. European regulators wanted pilots to be trained on it but eventually accepted the FAA’s and Boeing’s position. Brazil, however, stuck to its guns and required specific training for pilots on MCAS.
Boeing didn’t hide the addition of MCAS. It’s described in operation and maintenance manuals and was explained in technical briefings with prospective customers. It also included an emergency checklist covering disabling the system. But because they were not specifically trained in its use, most pilots didn’t know it was there and that it operated fundamentally differently from the speed trim system that operated the stabilizer setting on earlier 737s. Notably, pulling back on the yoke on older aircraft disables the automatic trim. Pulling back does not deactivate MCAS on the MAX.
Something the Times couldn’t determine was whether MCAS was tested in a failure mode, either in the simulator or on the aircraft itself. The predominant theory on the root cause of the crash was that faulty AOA data resulted in an erroneous and extreme reaction from the MCAS, pushing the aircraft into a high-speed dive that the pilots could not recover from. Boeing and the FAA are under investigation by the National Transportation Safety Board and Indonesian authorities to determine if the decision to skip pilot training in the new system played a role in what became the worst air crash of 2018.