AN ACR CL65 CREW DENIED JUMPSEAT ACCESS TO AN ACI BECAUSE OF AN ACFT EQUIP PROB LOW OXYGEN PRESSURE. THEY COMPOUNDED THE PROB BY NOT FILLING THE OXYGEN AND NOT GETTING A MAINT DEFERRAL PRIOR TO DEP. ERROR ADMITTED.

Narrative

FAA FROM LOUISVILLE REQUESTED JUMPSEAT FOR FLT TO CINCINNATI. DURING PREFLT INSPECTION; THE OXYGEN PRESSURE REQUIREMENTS FOR 2 CREW MEMBERS AND 1 OBSERVER WERE FOUND TO BE TOO LOW; THEREFORE IT WAS NECESSARY TO DENY THE REQUEST FOR THE JUMPSEAT. THE FAA INSPECTOR TOOK A SEAT IN THE CABIN. ON ARR IN CINCINNATI; THE CAPT CONTACTED MAINT FOR OXYGEN SVCING. THE INSPECTOR WAS PRESENT FOR THIS REQUEST AND HEARD THE MAINT DISCUSSION ABOUT DEFERRING THE JUMPSEAT. THE FAA INSPECTOR INSISTED THAT THERE WAS NO PROVISION FOR DEFERRING THE JUMPSEAT; HOWEVER; THERE IS; LOCATED UNDER MISC EQUIP. THE DEFERRAL SHOULD HAVE BEEN MADE PRIOR TO DEPARTING LOUISVILLE. SUPPLEMENTAL INFO FROM ACN 341368: CURRENT CHKLIST PREFLT PROCS REQUIRES OXYGEN MASKS AND OXYGEN FLOW BEFORE EVERY FLT; CREATING SLOW AND STEADY CREW OXYGEN DEPLETION. INADEQUATE INFO IN CL65 MEL REGARDING OBSERVER'S SEAT DEFERRAL AND OXYGEN DEFERRALS. I AND FO WERE BOTH UNAWARE OF ANY DEFERRAL PROC REQUIREMENT PRIOR TO FLT. BUT I DID NOT CALL MAINT CTL. WE WERE VERY NEAR DEP TIME.