Date: 201201
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Aircraft: EMB ERJ 145 ER/LR
Operator: Air Carrier
Operating under: Part 121
Flight phase: Parked
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Maintenance
When detected: Routine Inspection
Result: General None Reported / Taken
Contributing factors: Aircraft; Human Factors
Primary problem: Aircraft

Narrative: After performing successfully all first flight of the day checks (which included O2 pressure level and O2 Mask); I proceeded with the logbook review. Upon review of it; I noticed that MEL 25-10 (observer seat and equipment) was still open despite the oxygen pressure level being at optimum range. The entry referring to Low Level O2 had been previously balanced and didn't mention anything related to a leak; it actually only mentioned the cockpit O2 to be below the required level; and the balancing entry was 'O2 serviced to 1;800 PSI'. After calling Maintenance to ask about the reason for MEL 25-10 being still open despite the O2 Level found to be within required range; he replied they were not required to clear that MEL until the next PS2. Upon arrival we were informed by our Chief Pilot of a Maintenance discrepancy found relating a possible O2 leak on our aircraft. During preflight; in cruise and after landing 02 pressure level was always found to be above 1;750 PSI.


Second narrative: On the previous day's logbook page; there was an entry showing that the cockpit O2 system was serviced; but with no indication that the MEL had been restored. Maintenance was called to verify whether the MEL should be open or closed. When the mechanic arrived at our aircraft; we questioned him on the matter. His response was that even though the O2 system was fully serviced; the MEL was to remain open; and that the next PS2 inspection would resolve the observer seat MEL issue. Several minutes prior to gate departure; an ACARS request was sent to our Dispatcher to keep the aircraft for our subsequent RIC turn. His response came a few moments before takeoff; which was approximately 20 minutes after gate departure. To the best of my recollection; the message suggested that the aircraft could not fly with a suspected O2 leak; and that we should expect to deplane. I assumed he was referring to our planned aircraft swap as a response to my original inquiry. The flight took off moments thereafter. O2 level was 1;700 plus PSI at departure; and 1;800 PSI upon arrival.Since there were no logbook entries suggesting anything about O2 leakage; we had no reason to suspect that prior to gate departure. Our Dispatcher mentioned O2 leakage in his ACARS message. Perhaps that should have served as a warning; but it did not trigger the proper response from us as a crew.I believe it is fair to argue that inadequate logbook procedures greatly contributed to the event. Our preflight AML review was thorough and complete. The detail in our review even prompted us to question a mechanic in person for clarification. That individual asserted that the aircraft was airworthy; when the Dispatcher mentioned the words 'O2 leakage' perhaps that should have prompted more scrutiny of the matter. We did not regard his message as notification of an unairworthy condition. If the Dispatcher was trying to warn us of a situation; then more specific verbiage or instructions might have prompted a better response from us as flight crew.


Synopsis: EMB145 flight crew is dispatched with the observer seat MEL'ed apparently due to low oxygen level; yet the oxygen had been serviced to full. After landing the crew is informed that an oxygen leak was suspected and the aircraft should not have been flown.