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SEATTLE/LONDON, March 25 (Reuters) - A Boeing Co software fix for the grounded 737 MAX will prevent repeated operation of an anti-stall system at the centre of safety concerns and deactivate it altogether if two sensors disagree widely, two people familiar with pilot briefings said.

The anti-stall system - known as MCAS, or Maneuvering Characteristics Augmentation System - has been pinpointed by investigators probing October's fatal Lion Air crash and faces new scrutiny in the wake of another fatal accident in Ethiopia.

Those accidents, which killed nearly 350 people, triggered the worldwide grounding of Boeing's flagship 737 MAX aircraft and ignited a debate over the proper balance between man and machine in piloting the latest version of the 50-year-old 737.

The MAX has bigger engines, mounted further forward, which can force the plane's nose higher, threatening a stall. MCAS was designed to counter this but some experts say it overcompensated and the latest changes give some authority back to the pilot.

Airline briefings on the software upgrade, which is designed to address the situation faced by pilots of the doomed Lion Air jet last October, started on Saturday.

Pilots have been told that the MCAS system - which forces the nose downwards to avoid a stall, or loss of lift - will only operate one time for each event rather than impose repeated corrections like those believed to have pushed the Lion Air jet into a dive, the two people familiar with the briefings said.

Additionally, MCAS will be disabled whenever two sensors that measure the 'angle of attack' - a parameter that determines how close a plane is to an aerodynamic stall - differ too much.

"Otherwise it would be garbage in, garbage out," a third person familiar with the briefings said.

This is a change from the previous set-up which only linked MCAS to one sensor at a time, ignoring the other, and which may have resulted in a single point of failure on Lion Air 610.

The pilot will be able to deduce that MCAS is no longer working in the background because the system will show a warning message labelled "AOA disagree", indicating the two sensors are producing values that differ by an excessive margin.
Previously the "AOA disagree" warning would not have halted the MCAS software because the system was designed to focus on either the left or right sensor, alternating between flights. It was oblivious to whether readings from the sensors were aligned.
Boeing said on Monday its software patch would incorporate more than one angle of attack input, limit trim commands and limit authority but gave few details.
"We've been working diligently and in close cooperation with the FAA on the software update. We are taking a comprehensive and careful approach to design, develop and test the software that will ultimately lead to certification," a statement said.

FAA APPROVAL NEEDED
The change sheds light on Boeing's previously reported decision to make the warning light a standard feature, since the change in flight control laws now makes it indispensable.

The third person said Boeing would need to give pilots in their training a full explanation of what the fix is and why it is being implemented. Both the software fix and the training have to be approved by the Federal Aviation Administration.

Other methods for holding the nose of the aircraft in the right position, known as manual or electric trim, are unchanged as is the ability to cut out the automated trim system altogether using a standard step-by-step checklist.

Boeing has previously said that existing crew procedures, which include using a pair of cut-out switches, would have addressed a condition known as a stabilizer trim runaway and by doing so, automatically deal with any problem with MCAS.

But it has faced criticism for designing a system that potentially out-runs the ability of pilots to recover by repeatedly forcing the nose down using hefty forces, as the pilots in the doomed Lion Air flight experienced. (

(Reporting by Eric M. Johnson in Seattle, Tim Hepher in London, Allison Lampert in Montreal; Editing by Lisa Shumaker