Date: 201211
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737-300
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Parked
Aircraft component: Mach Trim
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer
Human factors: Confusion; Communication Breakdown; Situational Awareness
Second reporter function: Captain
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: Other After Pushback
Result: General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft; Human Factors; MEL; Procedure
Primary problem: MEL

Narrative: We originated the flight which had MEL 22-5-01 for a single channel Mach Trim System failure. During the Before Taxi Checklist when Recall was pressed; the Mach Trim Fail and Master Caution lights did not illuminate. We discussed the MEL and it was incorrectly concluded that the indications were correct for our condition. We continued the flight and landed. During the Before Taxi Checklist; we began to second guess our initial interpretation and decided that we should clarify with Maintenance before continuing. The Captain called Maintenance Control and from what I understood of the conversation; Maintenance and Dispatch agreed that the indications were correct for our condition. At one point the call was disconnected and when the Captain called back; a second Maintenance Controller told us that the indications were correct and that we were good to go. We accepted this and did not return to the gate but continued the flight. During this flight; after re-reading the MEL; we again began to second guess the interpretation and decided to call Maintenance again once on the ground to confirm. In this conversation; Maintenance agreed that we should be observing [seeing] the Mach Trim Fail; Flight Control; and Master Caution lights during the recall test. Contract Maintenance was called; we incurred a significant delay; and the MEL was changed to 22-5-02 for both channels of the Mach Trim System. Part B of the MEL 22-5-01 calls for pressing and releasing the Recall annunciator on the glare shield and observing the Mach Trim Fail light; Flight Control annunciator and the Master Caution lights illuminating. After pushback during the Before Taxi Checklist; these lights did not illuminate. Initially; it was wrongly concluded that the Recall Test and illumination of these lights is performed only during the initial MEL procedure to verify that the remaining channel is operating and that after deactivating the failed channel; the illumination of these lights is inhibited. The call to Maintenance Control reinforced this misinterpretation. Additionally; we had both seen the wording of other MELs that specifically state that the system will be tested before each takeoff. MEL 22-5-01 does not specifically state that the Recall Test will be performed before every takeoff which contributed to our misinterpretation. However; Part 'B' of MEL 22-5-01 is an (O) Procedure which should have been understood to mean the Recall Test must be done for each flight. At the least; as soon as there was a question about the MEL on the originating flight; a call should have been placed to Maintenance at that time while at a Base with Company Maintenance and more options available to us. As a pilot; we are the last line of defense in preventing mistakes such as this and we failed at that in this case. We are conditioned to want to believe that Maintenance wouldn't make a mistake and tell us wrong. But in this case; we should have questioned Maintenance more as to why there was an (O) Procedure from the start; and returned to the gate to have someone confirm that the indications were correct for our condition before departing for flight. Additionally; the wording that the Recall Test will be performed before each flight could be added to the MEL which would leave less chance for misinterpretation.


Second narrative: I misread and misunderstood MEL 22-5-01. This resulted in me not complying with the MEL procedures on [several] flights. The MEL states; 'All (M) Procedures listed in section must be accomplished by a qualified Mechanic.' I mistakenly understood this to mean that the procedure was accomplished once by the Mechanic at the time of deactivation and deferral of the Mach Trim channel. I mistakenly believed the light for that channel to be inhibited. We did not get a Mach Trim Fail light on Recalls. In the first leg; I believed it prudent to confirm my interpretation of the MEL. We talked to Maintenance Control; explained our indications; and asked if we should be getting a Mach Trim Fail light. He believed our indications were correct and we were good to go. In the next leg; I reread the MEL and concluded I may have misread/misinterpreted the procedure; and again contacted Maintenance Control to seek clarification. A different Maintenance Controller informed me we should get the Mach Trim light on Recall. I had misunderstood the procedure. The light not illuminating also indicated MEL 22-5-02 (both channels failed) was the correct MEL for the condition. Maintenance then applied the correct MELMy knowledge of the symbols; terminology; and procedures written in MELs was very weak. I did not really even perceive or comprehend the significance of the (M)/(O) symbology. I had previously just read each step and ensured it was accomplished (in this case by the Mechanic). I have now read the explanation of MEL item information section in the Performance computer to ensure my level of knowledge is good and I fully understand why I made this mistake. I see now that I began to doubt myself; however; I padlocked onto my original thought process and could not escape that train of thought. Some MELs indicate; 'The crew will before each flight [perform test]' Perhaps such language could have helped make this more obvious; however; sound knowledge on my part would have prevented this mistake. I also now recognize that I should have stopped at the first sign of doubt.


Synopsis: A Captain and First Officer report about the misinterpretation and confusion they and Maintenance Control had with a previous MEL 22-5-01 deferral for a Mach Trim System failure on their B737-300 aircraft. The aircraft had flown with a dual channel Mach Trim failure but had been deferred as a single channel failure.