The Boeing 737 is the world’s most popular passenger plane in terms of numbers built since it first flew in 1967.
The latest version, with very large engines mounted on the front of the wings, was designed with special software known as the Maneuvering Characteristics Augmentation System (MCAS) – designed to prevent a stall. Boeing says it was installed “to provide consistent handling qualities in unusual flight conditions”.
The system is triggered by readings from the “angle of attack” sensor, which measures the angle between the direction that the nose is pointing and the direction of the oncoming wind.
In both Boeing 737 Max tragedies – Lion Air in 2018, which cost 189 lives, and Ethiopian Airlines in 2019, with 157 fatalities – MCAS was triggered by a single faulty sensor. It forced the nose of the aircraft down.
The pilots, unfamiliar with the software and its power, were unable to overcome the automatic commands that doomed both planes.
A US Congressional investigation found that the FAA had originally certified the Max as safe when “clearly it was not”.
The aircraft must be modified to remove the fatal flaw from its control system. Pilots must be re-trained. And crew and passengers must be convinced the Max is now safe to fly.
Returning the jet to service will be a massive challenge. Currently, 380 planes are with the airlines that bought them, with an additional seven technically delivered but still at Boeing’s facilities in the Seattle area.
A further 450 have been built but remain undelivered.
It demands new flight control computer (FCC) software, completing an angle of attack sensor system test, and performing an operational readiness flight.