Date: 201603
Local time of day: 0001-0600
Location: LAS.Airport, NV
Flight conditions: VMC
Light: Daylight
Aircraft: Medium Large Transport; Low Wing; 2 Turbojet Eng
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
Aircraft component: Oxygen System/Crew
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Human factors: Communication Breakdown; Time Pressure
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: Aircraft In Service At Gate
Result: General None Reported / Taken
Contributing factors: Company Policy; Human Factors; Procedure
Primary problem: Procedure

Narrative: When I got to the airplane the previous Captain was still on the airplane. He briefed me on the logbook items. He told me that when he received the airplane in the morning he wrote up the Crew oxygen line pressure gauge as being below the green arc. Maintenance deferred the item per MEL. I asked the captain what line pressure they actually measured. He replied that no maintenance was performed and all that had been accomplished were the operational items (O-items) in the MEL itself. The O-items in the MEL require us to check the remaining components of the system (O2 masks for flow and bottle for pressure). It is my opinion that those O-items are not designed to verify whether the crew-line pressure gauge is inoperative; but merely to verify that the remaining components are operational once that it has been verified that the crew-oxygen line pressure gauge is indeed inoperative.Maintenance never verified if the gauge is inoperative. The mechanics that came on the airplane said if the gauge is below the green arc but the bottle pressure is good than it must be a bad crew oxygen line pressure gauge. I asked how he could be sure that the line-pressure is sufficient in case we need to don the masks if we had a depressurization. His reply was that the MEL does not require him to check the line pressure. I told him that the MEL assumes that the gauge is inoperative but that maintenance has done nothing to verify that this is indeed the case. I told him that first it has to be determined that the gauge is broken; than you can defer it using the MEL. It is improper procedure to use the O-items as verification as those are designed only to confirm that the rest of the system is operating normally. The O-items are NOT a troubleshooting guide to verify the operation of the crew oxygen line pressure gauge.It is my concern that the gauge actually performed correctly and that the pressure in the crew line is indeed below acceptable values. As support for my suspicion I would like to add that the crew oxygen pressure gauge was previously written up and it was replaced with a new gauge. What are the odds that 2 gauges are inoperative (one of them a mere 5 days after it was installed?). It is my believe that the reason for NOT checking the crew-oxygen line pressure in order to verify the gauge is inoperative is... because the gauges are operating normally and the problem is more complex.




Synopsis: The Captain questioned whether a maintenance procedure and MEL deferral was properly applied to an oxygen indicating system.