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Old 13th Nov 2018, 23:52
  #1138 (permalink)  
Join Date: Jul 2014
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original #978 (permalink)
Originally Posted by Halfnut
MCAS (Maneuvering Characteristics Augmentation System) is implemented on the 737 MAX to enhance pitch characteristics with flaps UP and at elevated angles of attack. The MCAS function commands nose down stabilizer to enhance pitch characteristics during steep turns with elevated load factors and during flaps up flight at airspeeds approaching stall. MCAS is activated without pilot input and only operates in manual, flaps up flight. The system is designed to allow the flight crew to use column trim switch or stabilizer aislestand cutout switches to override MCAS input. The function is commanded by the Flight Control computer using input data from sensors and other airplane systems.

The MCAS function becomes active when the airplane Angle of Attack exceeds a threshold based on airspeed and altitude. Stabilizer incremental commands are limited to 2.5 degrees and are provided at a rate of 0.27 degrees per second. The magnitude of the stabilizer input is lower at high Mach number and greater at low Mach numbers. The function is reset once angle of attack falls below the Angle of Attack threshold or if manual stabilizer commands are provided by the flight crew. If the original elevated AOA condition persists, the MCAS function commands another incremental stabilizer nose down command according to current aircraft Mach number at actuation.
Just to clarify & simplify the above statement. The MCAS is active when:
  • Flaps UP
  • High Angle of Attack signal (real or spurious)
MCAS inactivation
  • Flaps extended
  • Reduced AoA
  • Pilot Electric trim
  • Stabilizer trim cutout switches to cutout - Does this prevent the MCAS operating.
When active, its rate of change is dependant upon mach number. At low level, low mach number, highest rate of MCAS trim is commanded. At 5000 feet, we could expect the trim to run at or close to the full rate.

From the extremely brief description of the MCAS, we can possibly conclude:
  • JT 610 retracted flaps to the clean configuration (MCAS becomes active)
  • Low mach number, therefore maximum rate of horizontal stabilizer of 0.27 degrees per second, and a forward deflection to the limit with a spurious AoA input.
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