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THE FOLLOWING PROCEDURE AND/OR INFORMATION IS EFFECTIVE UPON RECEIPT
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.
Page 1 of 4
Flight Crew Operations Manual Bulletin No. ELF-17 R1,dated September 20, 1995 (continued)
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.
Administrative Information
Insert this bulletin behind the FCOM Bulletin Record Page in Volume 1 of your FCOM. Amend the FCOM Bulletin Record Page to show Bulletin ELF-17 R1 "IN EFFECT" (IE).
This Operations Manual Bulletin will be cancelled after Boeing is notified that all affected airplanes in the operator’s fleet have been modified by Boeing Service Bulletin 737-27A1191. If the operator does not plan to modify all the airplanes and would like to have the contents of this Bulletin incorporated in the Operations Manual, please advise Boeing accordingly.
Page 2 of 4
Flight Crew Operations Manual Bulletin No. ELF-17 R1,dated September 20, 1995 (continued)
Please send all correspondence regarding FCOM bulletin status to one of the following addresses:
Mailing Address: Manager 737, Flight Technical Integration & Data
Boeing Commercial Airplane Group
P. O. Box 3707 MS 20-89
Seattle, WA 98124-2207
USA
Fax Number: (206) 662-7812
Telex: 329430 Station 627
SITA: LKEBO7X
E-MAIL: FTID@Boeing.com
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Flight Crew Operations Manual Bulletin No. ELF-17 R1,dated September 20, 1995 (continued)
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Flight Crew Operations Manual Bulletin
for
The Boeing Company Seattle, Washington 98124-2207
737
Number: ELF-22 Date: March 12, 1999 Document Effectivity:
Subject: Control Wheel Microphone/Interphone Switch Anomaly
Reason: To inform flight crews the position of the Control Microphone/ Interphone switch can prevent PA announcementhe flight deck Wheel ts from
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