MD-11
Flight Crew Air Transport Pilot (ATP)
Troubleshooting
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Returned To Gate; General Maintenance Action
Aircraft; Human Factors; Procedure
Aircraft was deferred with MEL 36-10-XX-XX (Pneumatic Supply Systems #3). The MEL does not specify whether to operate the air system in auto or manual--only that isolation valves are functioning. On pushback and starting #3 engine; the auto controller did not open the appropriate isolation valves to pressurize the #3 manifold for engine start and there was consequently no N2 rotation. At this point; while FO was starting the engine and stated that there was no rotation; I switched the controller to manual and opened the 1-2 and 1-3 isolation valves as is done to pressurize the manifold for start; thinking this was an MEL issue. I directed the FO to start all three engines in this configuration with the thought that after engine start the auto controller would operate normally. When switching back to Auto; after all engines running; the 1-3 isolation valve closed and the #3 manifold depressurized - same problem.At this point; we realized that to operate in manual; we should have explicit direction in the MEL and wanted to check with maintenance about the need to defer the auto mode or some other procedure to pressurize the #3 manifold. Ramp control directed us to park at maintenance area and maintenance boarded the aircraft through the avionics door. Subsequent maintenance efforts to correct the problem showed that a manifold fail indication (part of their process to do an air system decay check) had not been cleared. By the time this was discovered as the problem; and another manifold decay check was executed; we had burned enough fuel to require return to a gate and refuel.What led to problem: rather than applying systems knowledge and operations with an MEL of uncertain effects; in hindsight; when the #3 system did not pressurize; I should have gotten pulled back into the blocks; written it up and handed the problem to maintenance. It is unclear whether an earlier block turn back would have led to an earlier discovery of the problem.Even though the MEL didn't direct operation in the auto mode; there was nothing that directed use of manual mode. Automatic mode is the default on an MD-11. If there was an explicit note in the deferral about operating in auto; I may not have been inclined to attempt a start in manual. Having flown the first leg of the evening in a different aircraft type may have given me some propensity on my part to do so in this case on an MD-11. From a CRM standpoint; as a minimum; I should have stopped the start and discussed any actions with the FO so we were on the same page. Also; regarding the source of the problem (Maintenance procedures for the deferral results in a manifold fail detected); Maintenance procedures never direct this fault to be cleared and consequently; the auto controller will not try to pressurize a manifold for which it has an indication  of failure. Maintenance procedures should explicitly direct that this fault is reset after the mandated manifold decay check. Such a note would have prevented the issue altogether.
An underlying cause of this issue was the omitted resetting of the intentional manifold failure directed by the decay test. This led to other than expected indications of the air system. The system did in fact work as intended in AUTO when it 'thought' the manifold was failed and did not allow the system #3 to be pressurized. Thus; is not the aircraft system as a causal factor but the maintenance publication which does not direct the malfunction to be reset after testing is completed. Maintenance experience with this particular omission is not common place so they did not know to look for this particular factor.On the human side: Fast hands in my humble opinion led to what could be deemed a prolonged resolution. The action of setting the system in Manual before stopping for a moment to think about it set in motion actions that were resolved through combined effort. But I wonder if issue would have been approached in quite the same way had we stopped the push immediately and pulled back to allow maintenance to troubleshoot on their own after a debrief by the CA.1) Hindsight: I could/should have been more verbal in taking the more conservative route and recommending that we stop the start attempt and figure/think things out before proceeding further. 2) The MEL did not specify to operate the air system in AUTO or MANUAL. Correct it to reflect expected mode in such a condition and perhaps specify expected indications. For example: Instead of stating '...provided: a. Both isolation valves are operative...' in the remarks or exceptions box of the MEL; perhaps it could state '...provided system is in AUTO and: a. Both isolation valves are confirmed operational (green) with APU air ON; and b. Pneumatic pressure is verified in the affected system manifold when supplied from an alternate source.'3) Correct the Maintenance publication to ensure it specifically directs the resetting of the intentional manifold failure.
MD-11 flight crew reported that instructions were unclear for an MEL item for the pneumatic system.