I found the following Bulletin in an
FCOM for a Classic. It details the different operation of the stabilizer trim main electric motor (turns in only one direction and drives the stabilizer trim actuator through two electro-magnetic clutches, one for NU, another for ND) and the autopilot trim system (turns in either direction and drives the stabilizer trim through a single clutch).
I'm assuming that MCAS commands the AP motor. I'm not an engineer but that arrangement would seem to create the potential for an interesting battle between one motor driving through a single clutch and the other motor driving through the repeated engagement of an electro-magnetic clutch. You might expect that the more efficient configuration will eventually win out over the less efficient in an extended duel, particular if the more efficient configuration is also driving at a faster rate.
The stuck clutch issue described in the bulletin might also be relevant.
Does anyone know if that described motor/clutch arrangement has been carried on through the NG/MAX line?
Would an engineer care to comment on how a duel between the two systems might play out?
PS. The bulletin tends to support one trim wheel rotation = 0.1 units of trim (...
the stabilizer trim wheel coasted up to 40 turns (four units of trim).)
Flight Crew Operations Manual Bulletin
Number: FLR-6(FR)
Issue Date: August 10, 2012
Subject: Runaway Stabilizer Procedure
Reason: This bulletin is issued to operators of an intermittent stabilizer trim system anomaly and provide additional guidance when accomplishing the Runaway Stabilizer procedure.
Background Information
Four operators have reported instances of excessive stabilizer trim system coasting (stabilizer trim wheel continues to rotate) after the control wheel stabilizer trim switches have been activated and released. The reports indicate that when the pilot released the trim switches, the stabilizer trim wheel coasted up to 40 turns (four units of trim). In some instances the trim wheel stopped moving in the commanded direction and then rotated up to 40 turns in the opposite direction.
The stabilizer trim main electric motor turns in only one direction. It drives the stabilizer trim actuator through two electro-magnetic clutches. One clutch is engaged for nose-up trim and the other is engaged for nose-down trim. Boeing examination of a suspect clutch showed that the reported coasting and/or reverse coasting of the stabilizer manual trim wheel was due to intermittent jamming of a clutch disc in one of the clutch assemblies. As a result, the electric motor will remain mechanically connected to the stabilizer trim mechanical actuator gear system after the control wheel stabilizer trim switches have been released.
With flaps down, the electric motor can continue to rotate up to 40 additional turns of the manual trim wheel after electrical power has been removed. With flaps up, manual trim wheel coasting is not significant because of the reduced trim motor speed. The autopilot trim system, which uses a motor that turns in either direction and drives the stabilizer trim through a single clutch, does not exhibit this problem.
Boeing Service Bulletin 737-27A1191, dated October 13, 1994, and revision dated November 3, 1994, provide instructions to replace the stabilizer trim electric actuator on the stabilizer trim control system.
Recommended Operating Procedures
The current Runaway Stabilizer procedure will effectively inhibit and limit an out of trim condition. Normal pilot reaction to a runaway stabilizer of opposing the runaway with main electric trim in addition to control column force will initially resolve a runaway. The Runaway Stabilizer Checklist recall action, “STABILIZER TRIM CUTOUT SWITCHES...CUTOUT” will isolate the malfunction if the runaway was caused by the main electric trim or autopilot trim systems. The stabilizer trim cutout switches only remove electrical power to the electric motors.
If the trim wheel continues to rotate after this action has been taken, the recall action “STBILIZER TRIM WHEEL....GRASP AND HOLD” will prevent further runaway or coasting. If the electric motor remains mechanically connected to the stabilizer trim mechanical actuator gear system because of a clutch malfunction, actuating the stabilizer trim cutout switches to cutout will not immediately stop the trim wheel rotation. Grasping the trim wheel will stop the rotation more quickly than allowing the trim wheel to coast to a stop, keeping the airplane more in trim.
In accordance with the procedure, trim the stabilizer manually for the remainder of the flight.