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It
was not immediately clear whether the crew had chosen to
re-deploy the system, which pushes 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 together
killed nearly 350 people.
People familiar with the 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 toward 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, 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 studying whether there are any conditions
under which MCAS could re-activate itself automatically, without
the pilots 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 two times
before hitting the stabilizer cut-out 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 hit the stabilizer cutout
switches to disable the MCAS system, the person added, making
the situation harder to manage.
After the pilots turned off 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 was available
for comment.
(Reporting by Eric M. Johnson in Seattle, Tim Hepher in Paris,
Jamie Freed in Singapore, Editing by Laurence Frost and Richard
Lough)
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