Experts believe the MCAS system on the doomed Ethiopian Airlines 737 Max then re-engaged and pushed the jet downwards after the pilots turned it off due to suspect data from an airflow sensor. The crash claimed the lives of 149 passengers and eight crew members and sent shockwaves through the aviation industry. Nine Britons were among the passengers from more than 30 nations aboard Flight 302 when it plunged to the ground six minutes after take-off on March 10.
Sources close to the accident inquiry said the anti-stall software – which automatically pushes the aircraft’s nose down to prevent loss of lift – was activated by erroneous “angle of attack” data from a single sensor which could have hit by a bird or other foreign object.
The software is at the centre of investigations into last month’s Ethiopian Airlines crash and a Lion Air accident in Indonesia last October.
The investigation has now turned towards how the MCAS was initially disabled by pilots, in line with part of a cockpit checklist procedure, but then appeared to start working again before the jet plunged to the ground.
A directive issued after the Indonesian crash instructed pilots to use two cut-out switches to disengage the system in the event of problems and then leave them switched off.
Officials briefed on the matter said a key question was when did the pilots disengage the MCAS system and did they do it too late to regain control of the airplane.
They said the system is not designed to resume operations unless the crew acts and the crew may have unintentionally re-engaged it as they desperately tried to pull the plane out of its nose-down descent.
Sources said the pilots manoeuvred the plane back upwards at least twice before hitting the stabiliser cut-out switches to disable the system.
But initial flight data indicates the aircraft was flying nose-heavy when pilots hit the stabiliser cut-out switches to disable the MCAS system.
The plane gained 2,000 feet after the pilots turned off MCAS but then plunged into the ground when the system re-engaged.
Officials warned the data was still being reviewed and that the findings were preliminary.
The US Federal Aviation Administration and National Transportation Safety Board declined to comment.
Boeing’s fastest-selling 737 MAX jet, with orders worth more than £380billion at list prices, has been grounded globally by the Federal Aviation Administration (FAA).
The UK Civil Aviation Authority announced that it had banned the jet from UK airspace and the European Aviation Safety Agency has suspended flights involving Boeing 737 Max 8 and 9.