Wednesday, August 21, 2019

Connecting the Dots: From Command to Action

The 737 stabilizer trim system evolved from its 707 roots with the introduction of the NG, when separate Main and Autopilot Actuators were combined into a single stabilizer actuator, retaining two electrical interfaces. The basic tenants were: 
  1. Manual trim (by trim wheel) has highest priority
  2. Electric trim has second priority
  3. Autopilot trim has lowest priority
  4. Significant column aft movement shall inhibit airplane nose down trim commands
  5. Significant column fwd movement shall inhibit electric airplane nose up trim commands
  6. Two cutout switches are used together to remove both Electric and Autopilot trim commands
  7. Stabilizer travel shall be limited to the same point during airplane nose up motion
  8. Stabilizer travel shall be limited using to an intermediate point while flaps up, using electric trim 
    1. to limit the nose down runaway mistrim
  9. Stabilizer travel shall be limited using autopilot airplane trim nose down to the flaps down limit
    1. autopilot "in command" stab trim objective is to maximize elevator authority
      1. Stab Trim fail indicator provided if trim ineffective
    2. NG FCC speed trim schedule was extended to stall speeds, to facilitate stall recovery
      1. Speed Trim fail indicator if command does not match monitor command
  10. Fast Electric trim speed is selected when flaps-not-up
  11. FCC fast trim speed is commanded by FCC based on flaps down
The 737 MAX revised the pitch trim system to accommodate MCAS. MCAS intruded upon the long-held stabilizer control logic because it was designed to command with nose down stab trim even while the column is significantly pulled back (aft column cutout). MCAS design includes an override feature to allow FCC airplane nose down stab trim to continue in the presence of opposing column movement (tenant 4, above).  MCAS also tripled the trim rate, increasing the mistrim in any runaway scenario.