There is a problem related to:

• Motor Drive Unit (MDU) fault bit status failure
• SPOST failures caused by incorrect velocity-feedback or brake-hold test-current feedback failure
• Horizontal Stab Trim (HST) Motor-Brake coil failure

• Pitch Trim Actuator Motor Drive Unit (MDU) (A184)
• Pitch-Trim Actuator (B44)
• Flight Control Unit (FCU) 1 (A65)
• Associated Wiring

Advisory Wire/Service Bulletin: None

Full Throttle Blog/Forum Articles/Infoservice/Newsletter: None

Flight Operation Notifications Manual (FONM): None

NOTES:

• This fault may sometime be cleared by performing the flight control system reset procedure. If the message comes back more then once within 10 flight legs, troubleshooting is required.
• The status of the of the pitch trim system can verify in CAIMS:

1. SYSTEM DIAG
2. 27-00 FLIGHT CONTROLS
3. FLIGHT CONTROL UNIT #1
4. LRU TEST
5. DISPLAY ADVANCED EFCS DATA
6. Go to page 5 of 8

1. Interchange FCU 1 with FCU 2.

   1. If system checks are good, replace FCU 1. Reset FCUs latched faults and do close out.
   2. If fault remains, continue with next step.

2. Interchange MDU Channel, if the use of the Ground Support Equipment 27X-41-09 "MDU INVERTING HARNESS" can be done to isolate the fault to the MDU or pitch trim Actuator motor.

   1. If system checks are good, replace defective MDU Channel and do close out.
   2. If fault remains, remove ground support equipment and continue with next step.

3. Perform wiring checks MDU (A184) and FCU 1 (A65).

   From	To	Result
   A184P3-37	A65DP1-3H
   A184P3-23	A65DP1-3J
   A184P3-6	A65DP1-6G
   A184P3-7	A65DP1-5K
   A184P3-8	A65DP1-5J
   A184P3-24	A65DP1-3G

   1. If wiring checks are not good, repair defective wiring as required and do close out.
   2. If wiring checks are good, continue with next step.

4. Interchange pitch trim actuator motors. Is the fault still present?

   1. If NO, replace pitch trim actuator motor 1 and do close out.
   2. If YES, continue with next step

5. Perform wiring checks between MDU (A184) and HSTA (B44).

   From	To	Result
   A184P2-A	B44P1-A
   A184P2-B	B44P1-B
   A184P2-C	B44P1-C
   A184P3-1	B44P2-34
   A184P3-2	B44P2-26
   A184P3-3	B44P2-12
   A184P3-4	B44P2-11
   A184P3-10	B44P2-32
   A184P3-25	B44P2-33
   A184P3-19	B44P2-14
   A184P3-20	B44P2-13

   1. If wiring checks are not good, repair defective wiring as required and do close out.
   2. If wiring checks are good, continue with next step.

6. Perform wiring checks between FCU 1(A65) and HSTA (B44)

   From	To	Result
   A65EP1-14J	B44P2-8
   A65EP1-15J	B44P2-9
   A65BP1-14C	B44P2-6
   A65BP1-15C	B44P2-5

   NOTE: Make sure wiring checked for short circuit to ground or shield. If high resistance is measured, check all disconnects along the link for bent, damaged or unlocked pins.
   

   1. If wiring checks are not good, repair defective wiring as required and do close out.
   2. If wiring checks are good, continue with next step.

7. Replace MDU and do close out.