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.
Source: Cirium Dashboard
