Show caption The Ethiopian Airlines crash site at Bishoftu. All 157 people onboard the plane were killed. Photograph: Mulugeta Ayene/AP Ethiopian Airlines crash Ethiopian Airlines crash: anti-stall system ‘engaged repeatedly’ Software may have redeployed without human input before plane went down, say sources Reuters Wed 3 Apr 2019 12.02 BST Share on Facebook

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Boeing’s anti-stall software on a doomed Ethiopian Airlines jet re-engaged up to four times after the crew initially turned it off due to suspect data from an airflow sensor, two people familiar with the matter have said.

It was not immediately clear whether the crew had chosen to redeploy the system, which pushes the nose of the Boeing 737 Max downwards, but one person with knowledge of the situation 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 centre of investigations into both the Ethiopian Airlines crash last month and a Lion Air crash in Indonesia in October, which together killed nearly 350 people.

People familiar with the investigation 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 MCAS was initially disabled by pilots following an emergency checklist procedure, but then appeared to repeatedly start working again before the jet plunged to the ground, they said.

A directive issued after the Indonesian crash instructed pilots to use cutout 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 studying whether there are any conditions under which MCAS could reactivate itself automatically, without the pilots reversing the cutout manoeuvre. Boeing is in the process of upgrading the software while adding extra training.

A preliminary report is expected within days.

The pilots manoeuvred the plane back upwards at least two times before pressing the stabiliser cutout switches to disable the system, the other person familiar with the matter said.

However, initial flight data indicates the aircraft was not in a “neutral” attitude when pilots used the stabiliser cutout switches to disable the MCAS system, the person added, making the situation harder to manage.

After the pilots turned off MCAS, the plane gained roughly 2,000ft over the next few minutes, but dived to the ground after the renewed succession of nose-down inputs from MCAS.