What was the problem with the Boeing 737 MAX?

The 737 MAX was born as a quick response to the Airbus A320neo. Boeing chose to re-engine the 737 with the larger-diameter CFM LEAP-1B engines, and given the aircraft's low landing gear height, had to move them forward and raise them above the wing. At high angles of attack, that position generated additional upward thrust that tended to raise the nose. To make the aircraft behave like its predecessors and avoid requiring pilots to undergo a new type rating, Boeing added MCAS (Maneuvering Characteristics Augmentation System), which acted on the horizontal stabilizer to push the nose down.

MCAS relied on a single angle-of-attack sensor, without cross-checking a second one. A single erroneous reading was enough to trigger it repeatedly, with far greater authority than described in the initial certification. Moreover, pilots were not informed of its existence in the manuals or in transition training. In Lion Air flight 610 (October 2018) and Ethiopian Airlines flight 302 (March 2019), a damaged sensor triggered the system after takeoff and the crews were unable to counter it in time.

In March 2019 the worldwide fleet was grounded. The FAA and EASA required MCAS to use both sensors, activate only once per event, and be unable to overpower the pilot's input on the controls, along with changes to the wiring and simulator training. The aircraft returned to service in the United States at the end of 2020 and in Europe in 2021. Investigations by the US Congress and the NTSB also pointed to shortcomings in the delegated oversight of certification, which led to a reform of the FAA's designee system.