Narrative:

The flight crew was on our regular scheduled trip (first day of 3 day sequence and sixth leg of the first day). On arrival at our last departure city company operations informed us of an aircraft change. The aircraft change, preflight, cockpit checks, startup and all phases of flight were normal. On arrival at destination city a crew change was effected. As the captain was zeroing the flight recorder numbers he noticed the crew oxygen gauge indicating 0. We both remembered specifically checking the crew and passenger O2 quantity gauges during our preflight checks as well as checking our crew O2 masks functioning properly because there had been a previous discrepancy written up re a reported passenger O2 leak. The maintenance department had corrected the discrepancy, signed it off in the logbook and released the aircraft for flight. We then briefed the oncoming crew of the situation and proceeded to the hotel. Later we checked with company operations and they informed us that contract maintenance had found the main crew O2 bottle shutoff valve in the off position. This valve is located in the forward cargo bin and is not readily accessible or required to be checked by flight crew. We were sure that we definitely checked the crew and passenger O2 quantities as well as the O2 flow at the masks during preflight (as stated earlier) because of the previous write up, so we decided to investigate the system further. On checking the crew O2 schematic and verifying with maintenance, it was confirmed that if the O2 system is fully serviced and the main crew O2 bottle supply valve is shutoff, the pressure will remain in the line to the cockpit gauge and will continue to show pressure until bled off. Evidently what happened during preflight checks was that both the captain and myself checked the gauge reading the approximately pressure, performed the O2 mask checks and then at sometime during the remainder of the flight, the residual pressure bld off to indicate 0 on arrival. No crew O2 would have been available during the flight. Recommend gauge pressure be read directly from tank (after shutoff valve) or procedures be established to allow a specified length of time for mask check to bleed off residual pressure and then verify quantity at gauge. Supplemental information from acn 84721: the crew O2 pressure gauge reads pressure upstream of the bottle shutoff valve, and I was reading normal pressure which actually was the O2 pressure trapped in the line between the bottle and our cockpit regulators. We bled the pressure in the line below minimum acceptable during our mask preflight check. Our procedures require both pilots to check system pressure at the overhead panel and then to check mask pressure flow. There is no requirement to look back at the gauge again after the mask check as I interpret the procedures. (Our before start checklist has an 'O2 mask quantity' response, but everyone responds west/O checking the gauge a second time as there should be no need to do so.)

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Original NASA ASRS Text

Title: ACFT DISPATCHED WITH THE FLT CREW OXYGEN SYSTEM TURNED OFF.

Narrative: THE FLT CREW WAS ON OUR REGULAR SCHEDULED TRIP (FIRST DAY OF 3 DAY SEQUENCE AND SIXTH LEG OF THE FIRST DAY). ON ARR AT OUR LAST DEP CITY COMPANY OPS INFORMED US OF AN ACFT CHANGE. THE ACFT CHANGE, PREFLT, COCKPIT CHKS, STARTUP AND ALL PHASES OF FLT WERE NORMAL. ON ARR AT DEST CITY A CREW CHANGE WAS EFFECTED. AS THE CAPT WAS ZEROING THE FLT RECORDER NUMBERS HE NOTICED THE CREW OXYGEN GAUGE INDICATING 0. WE BOTH REMEMBERED SPECIFICALLY CHKING THE CREW AND PAX O2 QUANTITY GAUGES DURING OUR PREFLT CHKS AS WELL AS CHKING OUR CREW O2 MASKS FUNCTIONING PROPERLY BECAUSE THERE HAD BEEN A PREVIOUS DISCREPANCY WRITTEN UP RE A RPTED PAX O2 LEAK. THE MAINT DEPT HAD CORRECTED THE DISCREPANCY, SIGNED IT OFF IN THE LOGBOOK AND RELEASED THE ACFT FOR FLT. WE THEN BRIEFED THE ONCOMING CREW OF THE SITUATION AND PROCEEDED TO THE HOTEL. LATER WE CHKED WITH COMPANY OPS AND THEY INFORMED US THAT CONTRACT MAINT HAD FOUND THE MAIN CREW O2 BOTTLE SHUTOFF VALVE IN THE OFF POS. THIS VALVE IS LOCATED IN THE FORWARD CARGO BIN AND IS NOT READILY ACCESSIBLE OR REQUIRED TO BE CHKED BY FLT CREW. WE WERE SURE THAT WE DEFINITELY CHKED THE CREW AND PAX O2 QUANTITIES AS WELL AS THE O2 FLOW AT THE MASKS DURING PREFLT (AS STATED EARLIER) BECAUSE OF THE PREVIOUS WRITE UP, SO WE DECIDED TO INVESTIGATE THE SYS FURTHER. ON CHKING THE CREW O2 SCHEMATIC AND VERIFYING WITH MAINT, IT WAS CONFIRMED THAT IF THE O2 SYS IS FULLY SERVICED AND THE MAIN CREW O2 BOTTLE SUPPLY VALVE IS SHUTOFF, THE PRESSURE WILL REMAIN IN THE LINE TO THE COCKPIT GAUGE AND WILL CONTINUE TO SHOW PRESSURE UNTIL BLED OFF. EVIDENTLY WHAT HAPPENED DURING PREFLT CHKS WAS THAT BOTH THE CAPT AND MYSELF CHKED THE GAUGE READING THE APPROX PRESSURE, PERFORMED THE O2 MASK CHKS AND THEN AT SOMETIME DURING THE REMAINDER OF THE FLT, THE RESIDUAL PRESSURE BLD OFF TO INDICATE 0 ON ARR. NO CREW O2 WOULD HAVE BEEN AVAILABLE DURING THE FLT. RECOMMEND GAUGE PRESSURE BE READ DIRECTLY FROM TANK (AFTER SHUTOFF VALVE) OR PROCS BE ESTABLISHED TO ALLOW A SPECIFIED LENGTH OF TIME FOR MASK CHK TO BLEED OFF RESIDUAL PRESSURE AND THEN VERIFY QUANTITY AT GAUGE. SUPPLEMENTAL INFO FROM ACN 84721: THE CREW O2 PRESSURE GAUGE READS PRESSURE UPSTREAM OF THE BOTTLE SHUTOFF VALVE, AND I WAS READING NORMAL PRESSURE WHICH ACTUALLY WAS THE O2 PRESSURE TRAPPED IN THE LINE BTWN THE BOTTLE AND OUR COCKPIT REGULATORS. WE BLED THE PRESSURE IN THE LINE BELOW MINIMUM ACCEPTABLE DURING OUR MASK PREFLT CHK. OUR PROCS REQUIRE BOTH PLTS TO CHK SYS PRESSURE AT THE OVERHEAD PANEL AND THEN TO CHK MASK PRESSURE FLOW. THERE IS NO REQUIREMENT TO LOOK BACK AT THE GAUGE AGAIN AFTER THE MASK CHK AS I INTERPRET THE PROCS. (OUR BEFORE START CHKLIST HAS AN 'O2 MASK QUANTITY' RESPONSE, BUT EVERYONE RESPONDS W/O CHKING THE GAUGE A SECOND TIME AS THERE SHOULD BE NO NEED TO DO SO.)

Data retrieved from NASA's ASRS site as of August 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.