Two sources disclosed that the anti-stall software of Boeing on a doomed Ethiopian Airlines jet re-engaged and pushed the jet downwards after the pilots initially turned it off because of a suspected data from an airflow sensor.
It was unclear whether the crew re-deployed the MCAS system intentionally, which was designed to push the nose of the 737 MAX down in order to prevent a stall, however, it is suspected of exacerbating a different scenario that is linked to two recent crashes.
A person who has knowledge of the aircraft said that the system cannot re-activate itself unless it was prompted by the pilots. One possibility is that a crew may have re-activated the MCAS in an effort to reduce the forces on the control column, or since they were not able to adequately trim the plane manually.
Boeing released a warning against speculation ahead of the release of a preliminary report, while Ethiopian investigators could not be reached for comment regarding the matter.
The anti-stall software of Boeing is at the centre of investigations into the Ethiopian Airlines crash last month and a Lion Air accident in Indonesia that happened last October.
Ethiopia is scheduled to release its first report on Thursday.
A person who is familiar with the findings disclosed that no new significant technical issues have emerged in the Ethiopian investigation so far beyond those that are already being addressed by Boeing through updated software in the aftermath of the Lion Air crash.
People who are close to the Ethiopian investigation have said that the anti-stall software – which pushes the aircraft’s nose down automatically to guard against a loss of lift – was activated by erroneous ‘angle of attack’ data from a single sensor.
The probe has now turned towards how the MCAS was initially disabled by pilots, in line with part of a cockpit checklist procedure, however, the people said that it then appeared to start working again before the jet plunged to the ground.