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Old 12th Apr 2019, 13:32
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737 Driver
Join Date: Apr 2019
Location: USA
Posts: 217
Question 737 AOA failure - System implications

Trying to noodle through all the system effects from an AOA failure that generates a false stall signal from the SMYD computer to the active FCC (some of this could also result from a bad airspeed input/blocked pitot, but want to focus on AOA for now). A stall signal from an SMYD will activate certain system response. Good in an actual stall, but unhelpful if result of bad AOA input. From what I have read so far, there is no "compare" function between AOA's that would inhibit these system responses.

Here's a list of things that may happen:
  • Stick shaker activation (failed side).
  • Elevator Feel Shift Module (EFSM) activation. Independent of active FCC. Heavier elevator control forces.
  • Stall ID function of Speed Trim System (STS) trims nose down at .27 units/sec. Can be stopped by aft column cutout switch
  • Autoslat activation. Independent of active FCC. Full extend slats for any flap setting except 0. Full extend placard limit 230 knots, and will effect LE slat position lights on forward console. Amber "LE FLAPS TRANSIT" inhibited. Green "LE FLAPS EXT" would only illuminate for TE position that would command full extend (varies by specific tail number). Not a huge problem except that it potentially creates another distraction.
This is in addition to the other system implications of unreliable airspeed and altitude (no A/T, no FPV, possibly no A/P).

Informed comments from those with 737 experience appreciated.
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