Boeing anti-stall software on a doomed Ethiopian Airlines jet re-engaged and pushed the jet downwards after the pilots initially turned it off due to suspect data from an airflow sensor, two people familiar with the matter 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 but which is suspected of exacerbating a different scenario linked to two recent crashes.
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.