B737 Next Generation Undifferentiated
Flight Crew Air Transport Pilot (ATP)
Flight Crew Type 4243.
Confusion
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural Published Material / Policy
Aircraft; Human Factors
Flight planning and preflight were uneventful except for a deferral for a single channel on the right pack. As push time approach; we transitioned to APU air and the right pack ran away full hot. We were not able to control the temperature in auto or manual. Contract Maintenance was called to the field and along with discussion with dispatch and [company maintenance]; it was agreed to dispatch with the right pack inoperable. After an hour plus delay; a new release with appropriate deferral and extra fuel was received. We had reviewed the MEL during the maintenance process and were prepared for flight with a single pack inoperable; remaining below 25K ft and a bleeds off takeoff and landing.With the [maintenance release] verified we pushed. During engine start and the before takeoff checklists we discussed the mechanics of the bleeds off takeoff using the 'C' and 'reverse C' pattern as taught during training with the additional step of leaving the deferred right pack with uncontrollable temperature in the off position rather than 'auto'.Takeoff and climb out was uneventful. During cruise; we pulled out the [maintenance release] to further analyze what was written and discovered a couple of errors. First was that the MEL printed was actually for the left pack rather than the right. The release and flight plan were correct; but the verified [maintenance release] had the incorrect number reference. We believe the error was simply a clerical one and had no negative consequences due to the fact that both packs are discussed in one long MEL.Second issue was concerning actual steps of the printed MEL on the [maintenance release]. After running through the verbiage multiple times; we determined that we had incorrectly accomplished the steps that actually called for the 'affected pack' (deferred right pack) switch to be placed in the 'high' position. A discussion and review of the air conditioning system amongst the crew confirmed our first assumption that the deferred pack with uncontrollable temperature should not be moved from the 'off' position. We discussed with dispatch our concerns and [maintenance control] was consulted. Time constraints prevented a complete discussion with maintenance. With pressurization stable and fully controlled; we chose to forgo the discussion and prepare for descent and landing. At 10K ft we configured the aircraft for a bleeds off landing. Approach and landing were uneventful.Upon landing; [company maintenance] was called to clarify the confusion. It was determined that we incorrectly left the deferred pack switch in the 'off' position when the printed MEL called for it to be in the 'high' position. The purpose was to effect logic on the operating pack but was counter to flight crew system understanding.During crew debriefing; the root causes of the flight crew error were discussed and listed below:Familiarity with the supplemental procedure for bleeds off takeoff and landing combined with the verbiage in the MEL that stated a 'bleeds off takeoff and landing shall be accomplished'. When I looked at the MEL; I did not realize that the steps listed were not the same as the supplementary procedure. I noted two differences. One was for the 'affected' pack (incorrectly interpreted as 'operating') to remain in high; and the second was for the right recirculation fan to remain off. We have been trained to do a bleeds off takeoff/landing in a particular manner and I did not realize the MEL differed from the supp procedure.A very confusing written MEL for one pack inoperable that combines information for both the right and left packs being inoperable. It took substantial time and effort for both dispatch; maintenance; and the crew to decipher what needed to be done.Contract maintenance that may have failed to accomplish the step to place the pack switch in the 'high' position.Not taking the time to go line by line and step by step throw the entire written MEL to be certain nothing was different from the supplementary procedure for bleeds off takeoff and landing.Flight manual text that states that a single pack in high can fulfill pressurization requirements. During our discussion during cruise; this made it even more confusing as to why the deferred pack should be in high and not remain in off.Fortunately the left pack did a great job with pressurization and there were no real consequences from our error; but hopefully identifying the issues with the MEL confusion and perhaps adding some notes to flight crew for clarification will prevent any future crews from a similar mistake.
B737 NG First Officer reported an incorrect setup on a departure with a single pack deferral due to a misunderstanding of the MEL procedure.