Date: 201304
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight phase: Parked
Aircraft component: Mach Trim
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Technician
Reporter qualification: Maintenance Airframe; Maintenance Powerplant
Human factors: Communication Breakdown; Confusion; Situational Awareness; Troubleshooting
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: Taxi
Result: Flight Crew Returned To Gate; General Maintenance Action
Contributing factors: Chart Or Publication; Human Factors; Manuals; Procedure; Aircraft
Primary problem: Procedure

Narrative: B737-700 aircraft came in on MEL for Dual Channel Mach Trim failure. I pulled Maintenance Manual (M/M) and performed Digital Flight Control System (DFCS) Built-in Test Equipment (Bite) Check. DFCS passed all tests; verified by Lead Mechanic; and on the next day [verified] by the Line Maintenance Manager. I have cleared write-ups like this before; but did not know there were [Mach Trim] Circuit Breakers (C/Bs) pulled; the DFCS passed all tests with [Mach Trim] Circuit Breakers out (pulled). When I got home; I thought about the situation and called Dayshift Line Lead on shift and he verified before aircraft left that the [Mach Trim] C/Bs were pulled. Unfortunately; the aircraft returned to the gate during this time. Maintenance went out and found that there were circuit breakers pulled and collared. The only circuit breakers I knew about were Stab Cutout [Switches] to perform this test. MEL write-ups should have Circuit Breakers (C/Bs) listed that are pulled for any MEL. Graveyard [shift].

Callback: Reporter stated he had signed-off the Mach Trim MEL deferral based on the results of the BITE Test of the Digital Flight Control System (DFCS). He has worked Line Maintenance for thirty years and that was the first time he has ever experienced a BITE Test passing for a Mach Trim with all four Mach Trim Circuit Breakers (C/Bs) open (pulled). Part of his check procedure prior to performing a Mach Trim Test is to position the two guarded Stab Trim Cutout Switches on the Center Pedestal from 'Normal' to 'Cut-Out.' He never noticed the Mach Trim C/Bs behind the Captain and First Officer's seats were pulled (open) as part of the original MEL deferral for the earlier Dual Channel Mach Trim failure. Mostly due to the fact his Air Carrier uses codes on the MEL to indicate which C/Bs are pulled instead of actually identifying the C/B location and system. His expectation was the C/Bs would be closed. That's why he has suggested his Air Carrier specifically identify pulled Circuit Breakers on any MEL deferral. Reporter stated what still bothers him; is how could the DFCS BITE Test for the Mach Trim still pass; no one that he has talked with has given any explanation for that situation. The aircraft returned to the gate because the flight crew noticed the C/Bs for the Mach trim were still pulled and collared; even though the Mach Trim MEL had been signed-off.



Synopsis: A Line Aircraft Maintenance Technician (AMT) questions how a Digital Flight Control System (DFCS) Test for a Dual Channel Mach Trim System could have passed with all four Mach Trim Circuit Breakers (C/Bs) 'Open' (pulled). The B737-700 flight crew returned to the gate after noticing the C/Bs were still pulled; even though a Mach Trim MEL had been signed-off.