ACFT EQUIP PROB. SHORTLY AFTER TKOF THIS ACR CREW NOTED THAT THEY HAD LOST THEIR CREW OXYGEN.

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THIS INCIDENT INVOLVED THE LOSS OF CREW OXYGEN PRESSURE AFTER DEP. THE FLT NUMBER WAS XXX DEPARTING FROM ORD TO SLC ON AUG/FRI/95. SCHEDULED DEP WAS XA30. THE ACFT HAD OVERNIGHTED IN ORD AND WAS TOWED TO THE DEP GATE FROM THE HANGAR. IT HAD RECEIVED A MAINT LAYOVER CHK THAT NIGHT. I BOARDED THE ACFT WITH THE REST OF THE CREW AT XA00. I PERFORMED MY PREFLT DUTIES AND CHKED ALL REQUIRED DISPATCH QUANTITIES. THE CREW OXYGEN INDICATED APPROX 1400 PSI; WELL ABOVE THE 1000 PSI MINIMUM REQUIRED. I TESTED MY OXYGEN MASK IN THE EMER POS; NOTING POSITIVE AIRFLOW BEFORE I PLACED THE SWITCH TO OFF AND STOWED THE MASK BESIDE ME. AFTER OBTAINING THE CLRNC; MY CREW ACCOMPLISHED THE BEFORE START CHKLIST AT XA15. THE CREW OXYGEN PRESSURE AGAIN CHKED NORMAL. THE FLT XXX PUSHED BACK FROM THE GATE AT XA37 AND TAXIED TO RWY 32L. WE WAITED 29 MINS FOR TKOF. I WAS FLYING THE ACFT. SHORTLY AFTER TKOF AND GEAR AND FLAP RETRACTION THE SO NOTICED THAT THE CREW OXYGEN PRESSURE WAS READING '0' PSI. OUR FIRST REACTION WAS THAT THERE WAS A GAUGE MALFUNCTION SO I MOVED MY REGULATOR SWITCH TO THE EMER POS TO LISTEN FOR AIRFLOW AND THEN TO OFF. I DID NOT NOTICE ANY FLOW WHICH INDICATED THAT THE OXYGEN SUPPLY WAS DEPLETED. WE STOPPED OUR CLB AT 7000 FT MSL AND THE CAPT REQUESTED A RETURN TO ORD. WE FLEW AN APCH TO RWY 9R AND MADE AN UNEVENTFUL LNDG. UPON ARR AT THE GATE; MAINT ASKED US TO CHK THAT ALL REGULATOR EMER SWITCHES OFF; WHICH THEY APPEARED TO BE. A MECH ALSO CAME TO THE COCKPIT TO VERIFY THE POS OF THE SWITCHES. THE CREW OXYGEN BOTTLE WAS REPLACED; LINE CHKED; AND ALL MASKS CHKED AGAIN PRIOR TO DEP. THE FLT TO SLC WAS UNEVENTFUL AND NO FURTHER OXYGEN LOSS WAS NOTED. THE CREW AND I DISCUSSED WHAT MIGHT HAVE CAUSED THE OXYGEN LOSS/THERMAL DISCHARGE; AN EMPTY BOTTLE WITH TRAPPED LINE PRESSURE OR A FAULTY REGULATOR. I TEND TO BELIEVE THAT THERE MAY HAVE BEEN A PROB WITH THE REGULATORS. I HAVE NOTICED SINCE THE INCIDENT THAT THE EMER SWITCH ON MANY REGULATORS WILL STICK A FRACTION OUT OF THE OFF POS RESULTING IN SUBSTANTIAL AIR FLOW FROM THE MASK. I DISCUSSED THIS MATTER WITH MAINT AND THEY CONFIRMED THAT THIS SIT COULD OCCUR AND DEPLETE THE OXYGEN SYS IN 30-40 MINS. ALTHOUGH THE EMER SWITCH HAS A PLASTIC GUARD ON EITHER SIDE OF IT; IT IS POSSIBLE TO ACCIDENTALLY BUMP IT WHILE USING THE MIKE BUTTON ON THE COM PANEL DIRECTLY BELOW THE SWITCH. I HAVE ALSO BEEN INFORMED OF ANOTHER SIMILAR INCIDENT WHERE A MIKE CORD CAUGHT AND RAISED THE SWITCH RESULTING IN AN UNCOMMANDED LOSS OF OXYGEN INFLT. AFTER REVIEWING THE B-727 FLT MANUAL; I NOTICED THAT THERE IS NO CAUTION THAT THIS TYPE OF INCIDENT MAY OCCUR. I BELIEVE THAT THE EMER OXYGEN SWITCH SHOULD BE REPLACED WITH A TRUE ON/OFF GUARDED SWITCH. AN OXYGEN LOW PRESSURE LIGHT (ACTIVATED AT LESS THAN MINIMUM REQUIRED PRESSURE) WOULD ALSO BE HELPFUL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THIS MODEL OF OXYGEN PANEL HAS AN EMER SELECTOR SWITCH THAT IS NOT LOCKED IN THE 'OFF' POS. INSTEAD; THE SWITCH HAS 2 'HALF-MOON' SHAPED BARRIERS ON EACH SIDE AND A METAL WIRE BAR THAT CROSSES ABOVE THE SWITCH AND THESE DEVICES ARE TO PREVENT INADVERTENT ACTUATION. OCCASIONALLY THESE GUARDS FAIL TO DO THAT. ANOTHER PROB NOTED BY THE RPTR IS THAT THE SWITCHES DO NOT SNAP ALL THE WAY TO THE 'OFF' POS WHEN THEY ARE MOVED BEYOND THE CTR POS TOWARDS THE 'OFF' POS. WHETHER THIS IS A DESIGN; A WEAR OR A CLEANING AND MAINT PROB IS PRESENTLY UNKNOWN. DURING ANOTHER DISCUSSION THE RPTR HAD WITH A FELLOW PLT HE WAS TOLD OF AN INCIDENT IN WHICH A BROKEN 'HALF-MOON' SHAPED BARRIER HAD ALLOWED A MIKE CORD TO MOVE THE EMER OXYGEN SELECTOR OUT OF THE 'OFF' POS RESULTING IN OXYGEN LOSS UNTIL IT WAS NOTICED AND CORRECTED. THE RPTR IS ABSOLUTELY SURE THAT THE OXYGEN QUANTITY WAS MORE THAN SUFFICIENT FOR FLT DURING THE PREFLT. HE IS EQUALLY UNSURE OF HOW THE LOSS WAS INITIATED.