FAA order tells how 737 pilots should arrest 'runaway
stabiliser'
The Federal Aviation Administration's emergency 737
Max order describes the confusing circumstances that could result from a faulty
angle-of-attack indicator.
The order, issued 7 November, also describes
steps pilots should take to recover in the event that faulty indicators cause
what the FAA calls "runaway stabiliser".
The FAA issued the order
following the 29 October crash of a Lion Air 737 Max 8, which killed 189 people.
Investigators have pointed a finger at faulty airspeed or angle-of-attack
indicators.
The order notes that the 737 Max's flight control computer
handles the aircraft's pitch so as "to improve longitudinal handling
characteristics".
But, an analysis by Boeing found that the flight
control computer, should it receive faulty readings from one of the
angle-of-attack sensors, can cause "repeated nose-down trim commands of the
horizontal stabiliser".
The aircraft might pitch down "in increments
lasting up to 10sec", says the order.
When that happens, the cockpit
might erupt with warnings.
Those could include continuous control column
shaking and low airspeed warnings - but only on one side of the aircraft, says
the order.
The pilots might also receive alerts warning that the computer
has detected conflicting airspeed, altitude and angle-of-attack readings. Also,
the autopilot might disengage, the FAA says.
Meanwhile, pilots facing
such circumstances might need to apply increasing force on the control column to
overcome the nose-down trim.
The FAA's directive orders airlines within
three days to update flight manuals to include specific steps pilots should take
to recover.
They should disengage the autopilot and start controlling the
aircraft's pitch using the control column and the "main electric trim", the FAA
say. Pilots should also flip the aircraft's stabiliser trim switches to
"cutout". Failing that, pilots should attempt to arrest downward pitch by
physically holding the stabilizer trim wheel, the FAA adds.
Boeing has
declined to comment following the FAA's order.
The company on 6 November
issued a service bulletin describing "existing flight crew procedures" for
dealing with faulty angle-of-attack inputs. Boeing says it is assisting with the
investigation.
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