SEATTLE / PARIS (Reuters) – Boeing anti-stall software on a doomed Ethiopian Airlines was re-engaged as many as four times after the crew was initially turned off due to suspicious data from an airflow sensor, two people familiar with matter said.
FILE PHOTO: Airplane engine parts are seen at the scene of the Ethiopian Airlines Flight ET 302 plane crash, near the town of Bishoftu, southeast of Addis Ababa, Ethiopia March 11, 2019. REUTERS / Tiksa Negeri / File Photo
] It was not clear whether the crew had chosen to re-deploy the system, which pushed the nose of the Boeing 737 MAX downwards, but one person with knowledge of the matter said investigators were studying the possibility that the software had kicked in again without human intervention.
A Boeing spokeswoman declined to comment. Ethiopian investigators were not immediately available for comment.
Boeing's anti-stall software known as MCAS is at the center of investigations into both the Ethiopian Airlines crash last month and a Lion Air accident in Indonesia in October that killed nearly 350 people.
People familiar with the investigation have said the anti-stall software – which automatically pushed the aircraft's nose down to a guard against a loss of lift – was activated by erroneous 'angle of attack' data from a single sensor.
The investigation has now turned to how MCAS was initially disabled by pilots following an emergency checklist procedure but then it appeared to have repeatedly started working again before the jet plunged to the ground, the people said.
A directive issued after the Indonesian crash instructed pilots to use cut-out switches to disengage the system in the event of problems and leave it switched off.
Doing so does not shut down the MCAS system completely but severs an electrical link between the software and aircraft systems, a person familiar with the technology said.
Investigators are investigating whether there are any conditions under which MCAS could re-activate itself automatically, without the pilot reversing the cut-out maneuver. Boeing is in the midst of upgrading the software while adding extra training.
A preliminary report is expected within days.
The pilots maneuvered the plane back upwards at least twice before hitting the stabilizer cut-out switches to disable the system, the other person familiar with the matter said.
However, the initial flight data indicates that the aircraft was not in a "neutral" attitude when pilots hit the stabilizer cutout switches to disable the MCAS system, the person added, making the situation harder to manage.
After the pilots turned off the MCAS, the airplane over the next few minutes gained roughly 2,000 feet, but dived into the ground after the renewed succession of nose-down inputs from MCAS.
None of the parties involved in the investigation were available for comment.