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Background Information
Flight control module failures have been reported on 737-600/-700/-700C/-800/-900/BBJ airplanes since June 2002. The majority of the failures have occurred during preflight checks prior to delivery acceptance test flights, but failures have also been reported on four in-service airplanes. Discussions with the supplier indicate the potential for this condition is most likely isolated to Flight Control Module Part Number 65-44891-7, Serial Numbers 8726 through 8936, installed on airplanes delivered after May 21, 2002. The FAA has issued Emergency Telegraphic AD 2002-19-51 to address required operator action.
There are two identical flight control modules on each airplane. Each module controls hydraulic fluid distribution from its respective hydraulic system to the aileron, elevator and rudder. The failure mode is a partial or complete blockage in the return passage of the module resulting in inadequate differential pressure available to flight control power control actuators (ailerons, elevators, rudder). Failure of one flight control module in flight may result in an increase in flight control forces. Failure of a second flight control module could result in serious degradation of airplane controllability, including high control forces.
Flight Crew Operations Manual Bulletin No. SHZ-30, dated September 16, 2002 (continued)
Because the blockage is in the return passage, the associated Flight Control LOW PRESSURE Light remains extinguished for the affected flight control module. The Hydraulic LOW PRESSURE Light also remains extinguished. The most likely flight deck indication is failure of both Autopilot A and B to engage. Other indications include possible increase in flight control forces (similar to manual reversion) and possible yaw damper disengagement.
Airplanes with two suspect modules are not to be flown until at least one non-suspect module has been installed.
In order to help identify a flight control module failure prior to flight, Boeing has developed Flight Control and Autopilot preflight checks to be performed on airplanes with a suspect flight control module installed. These checks can be performed anytime after the Electric Hydraulic Pump A and B Switches are positioned ON and prior to Engine Start. If MEL dispatch with one or both autopilot channels inoperative is planned, it is acceptable not to perform the Autopilot check on the inoperative channel(s).
If it is determined that a flight control module has failed, subsequent flights must not be made until the failed module has been removed and replaced.
Operating Instructions Preflight Check:
The following Flight Control and Autopilot checks must be performed prior to each flight on an airplane with a suspect flight control module installed. These checks can be performed anytime after the Electric Hydraulic Pump A and B Switches are positioned ON and prior to Engine Start. Ensure ground personnel are clear of all control surfaces.
Note: These checks are only designed to detect a failed flight control module.
Flight Control Check
1.
Ensure FLT CONTROL A & B Switches are ON
2.
FLT CONTROL A Switch . . . . OFF
. Verify Flight Control LOW PRESSURE Light illuminates within 2 seconds
3. FLT CONTROL A Switch . . . . ON
. Verify Flight Control LOW PRESSURE Light extinguishes
4. FLT CONTROL B Switch . . . . OFF
. Verify Flight Control LOW PRESSURE Light illuminates within 2 seconds
5. FLT CONTROL B Switch . . . . ON
Flight Crew Operations Manual Bulletin No. SHZ-30, dated September 16, 2002 (continued)
Autopilot Check If MEL dispatch with one or both autopilot channels inoperative is planned, it is acceptable not to perform the Autopilot check on the inoperative channel(s).
1.
Ensure both IRUs are in the NAV Mode
2.
A/P ENGAGE Switch . . . . CMD A
. Wait 10 seconds and verify autopilot CMD mode engages
3.
Disengage Autopilot A
4.
A/P ENGAGE Switch . . . . CMD B
. Wait 10 seconds and verify autopilot CMD mode engages
5.
Disengage Autopilot B
6.
To fail this test, one autopilot will fail to engage and the other will fail to remain engaged
Note: Failure of the autopilots to engage as described in Step 6 may indicate a failure of a flight control module.
Note: If either Flight Control Module Preflight Check fails, do not takeoff until the failed module has been replaced.
In-flight Failure
Failure of both Autopilot A and B to engage may indicate a failure of the module
in flight. Other indications include possible increase in flight control forces
(similar to manual reversion) and possible yaw damper disengagement.
Failure of a second flight control module in flight could result in serious
degradation of airplane controllability, including high control forces.
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