MCAS or Maneuvering Characteristics Augmentation System is a software method of helping prevent unwanted attitudes in the Boeing 737 Max series aircraft.
It works by applying nose down Horizontal Stabilizer trim imput when the aircraft’s Angle of Attack exceeds a certain point nearing a stall. Angle of Attack (AoA) is determined through two vanes that measure the difference between where the aircraft’s nose is pointing versus the relative wind or flight path (not necessarily where the horizon is), exceeding AoA will cause a stall (aircraft loses lift or “stops flying”), needless to say stalls are a bad thing.
MCAS is normally not operating if the normal flight envelope is respected, it should never engage or operate at all. It is a responsive system to deviations, not a normal flight control system.
Pilots can disable MCAS by switching the two electric Stabilizer Trim switches off. When the Stab Trim is off, manual trimming is required and possible. These switches are in between the two pilot seats and easily accessible to either pilot.
B737 Stabilizer Trim switches:
B737 Manual Stabilizer Trim wheel:
Trim wheels move either with the electric trim or using the handle while trimming manually.
.MCAS failures that result in un-commanded activation have very similar symptoms to another failure called Runaway Stabilizer Trim, the solution for both failures is the same, i.e. turn both electric and autopilot stab trim off (as described above).
The original stofware for MCAS was very poorly designed as it relied on just one of the two AoA vanes and allowed commanding excessive nose down trim imput up to full nose down, however, even in the original flawed configuration, MCAS could have been shut off by the pilots. The updated software now limits the maximum time and angle of nose down trim MCAS can command and now also needs both AoA vanes to agree.
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Author – Francisco Maldonado (Boeing 757/767 Captain and Instructor)