The pilots of the doomed Ethiopian Airlines jet turned off an anti-stall software but it re-engaged and pushed the plane downwards, new information on the tragedy reveals.
It comes as Ethiopia was on Thursday due to release its first report on last month’s fatal crash of the flight from Addis Ababa, while US aviation regulators announced a new safety review of the airline’s 737 Max planes, which were grounded in the wake of the disaster.
Boeing this week pleaded for more time to produce a software fix for the planes, saying it was still weeks away.
Boeing’s MCAS system, which was designed to push the nose of the 737 Max plane down to prevent a stall, has been linked to two recent crashes.
It was not immediately clear whether the crew intentionally re-deployed the system.
A person with knowledge of the aircraft said the system cannot re-activate itself unless prompted by pilots.
One possibility is that the crew may have re-activated MCAS in an attempt to reduce the forces on the control column, or because they were unable to adequately trim the plane manually.
Boeing warned against speculation ahead of a preliminary report, while Ethiopian investigators could not be reached for comment.
Boeing’s anti-stall software is at the centre of investigations into last month’s Ethiopian Airlines crash and a Lion Air accident in Indonesia in October 2018.
No significant new technical issues have so far emerged in the Ethiopian investigation beyond those already being addressed by Boeing through updated software in the aftermath of the Lion Air crash, a person familiar with the findings said.
People close to the Ethiopian investigation have said the anti-stall software – which automatically pushes the aircraft’s nose down to guard against a loss of lift – was activated by erroneous ‘angle of attack’ data from a single sensor.
The investigation has turned towards how 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, the people said.
Officials briefed on the matter said a key question is when did the pilots at the helm of the Ethiopian Airlines 737 Max disengage the MCAS system, and did they do it too late to regain control of the plane.
Two people briefed on the matter said the system was not designed to resume operations unless the crew acted. The crew might have unintentionally re-engaged the system as they desperately tried to pull the plane out of its nose-down descent.
They cautioned 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.
The FAA said it was establishing a technical review “to ensure the safety of the Boeing 737 Max”. It will be chaired by former NTSB chairman Christopher Hart.
More than 300 Boeing 737 Max jets have been grounded worldwide after two crashes – in Indonesia in October and in Ethiopia last month – killed nearly 350 people.