Boeing fix will prevent repeated activation of anti-stall
system -sources
By Eric M. Johnson and Tim Hepher
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)
Back to Top | 
