This reportedly triggered erroneous data and the activation of an anti-stall system that sent the plane down on March 10 shortly after take off from Addis Ababa.
Boeing anti-stall software on the doomed jet re-engaged and pushed the jet downwards after the pilots initially turned it off due to suspect data from an airflow sensor, two sources said.
It was not immediately clear whether the crew intentionally re-deployed the MCAS system, which was designed to push the nose of the 737 MAX down to prevent a stall.
The pilots were not able to get control of the plane back, sources told ABC News.
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.
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 told Reuters.
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 now turned towards how the MCAS system 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, sources said.
Speaking to CNN, pilot and aviation analyst Miles O’Brien gave his take on what could have happened in the cockpit once pilots reportedly turned off the automatic flight control system.
“They were left with a manual wheel to try to get the nose in the proper orientation and evidently what we are hearing is, that maybe the wheel didn’t have enough authority, that it was too hard to move, or could not move fast enough, given the amount of altitude they had in order to recover. So that put them in a situation where they followed the book, and the recovery procedure was not good enough.
“The idea that they would have this troubleshooting system, they followed the book and it wan’t good enough, is just horrifying,” said pilot and CNN aviation analyst Miles O’Brien.
149 passengers and eight crew members were killed when the plane went down just six minutes after take-off.
Boeing is working to submit an upgrade of the software to the US regulators in a couple of weeks and adding extra training.
Boeing’s fastest-selling 737 MAX jet, with orders worth more than $500 billion at list prices, has been grounded globally by the Federal Aviation Administration (FAA).