Date: 202002
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: A321
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise; Parked
Airspace: Class B ZZZ
Aircraft component: Oxygen System/Crew
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown; Troubleshooting
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Maintenance
Detector: Person Flight Crew
When detected: Pre-flight; In-flight
Result: General Maintenance Action
Contributing factors: Aircraft; Procedure; MEL
Primary problem: Procedure

Narrative: According to COM 1 3.1.62; we were clearly legal to initiate the flight.  Our confusion comes from what we should do if the crew oxygen drops below the minimum required crew oxygen psi with the in flight reference temperature calculation because it also talks about that the minimum bottle pressure is to compensate for preflight checks; normal crew use while a crewmember is away from his/her station; normal system leakage; and use during an abnormal/emergency situation.  This insinuates that there is an understanding that one can take off with the exact minimum O2 psi and lose some under normal system leakage and crew use while a crewmember is away from their station.  But to my knowledge; there isn't a procedure in place to determine what the critical level is to initiate a diversion.  I suggest that the company addresses this and creates a procedure that can give us better guidance in this kind of situation.  It's important to note that this situation ultimately began in ZZZ during preflight of XXXX for [flight] to ZZZ1 when I noticed that the crew oxygen pressure was roughly 870 psi.  I pointed out that the pressure was fairly low given the current reference temperature was calculated to be 25 degrees C; but we were still legal to dispatch for only 2 crew members and no jumpseaters per COM 1 3.1.62.  The decision was made to not notify maintenance and proceed to ZZZ1.  During cruise we received a message for the crew oxygen dropping below 800 psi.  COM 2 ECAM Advisory Conditions 3.80.25 list the parameters of what triggers this message and pointed us to AOM VOL 1 to make sure the oxygen masks are stowed correctly; which we determined they were.  We continued to ZZZ1.  The captain sent maintenance and dispatch ACARS messages requesting Maintenance to meet us in ZZZ1 and service the crew oxygen before our flight to ZZZ2.Once on the ground in ZZZ1; we were advised that oxygen was unavailable.  The captain had already written a discrepancy in the logbook requesting that Maintenance service the crew oxygen.  The crew oxygen was at 780 psi; and with a given reference temperature of 28 degrees C; we were still above the required amount of approximately 750 psi.  Maintenance opted to defer the cockpit jumpseat being that there was only enough crew oxygen for the Captain and First Officer and clear the discrepancy.  We received our amendment from Dispatch and initiated the flight to ZZZ2.  Once in cruise; the crew oxygen psi continued to drop.  At the rate the crew oxygen psi was dropping; I estimated we would land in ZZZ2 with approximately 600 psi.  We looked in COM 2 for a procedure that could help us determine when we should consider making a precautionary diversion; but no such procedure exists.  Instead; we determined that the flashing green crew oxygen psi on the DOOR ECAM page would turn amber at 400 psi according to COM 1 ECAM Advisory Conditions 3.80.25 and we would use that figure as our diversion point since we were now over land and had ample airports to divert to with acceptable facilities and weather.  We also determined that we had the greatest rate of crew oxygen psi loss at higher altitudes; so we had Dispatch send us new performance numbers for completing the rest of the flight at FL250.  During this time; we also asked dispatch for guidance from a Chief Pilot and Maintenance Control of how to arrest the dissipation of crew oxygen; and if a diversion was required for the current amount on board; approximately 670 psi.  Maintenance told us via ACARS that we could check the masks to see if they were at 100% O2 and turn off the crew oxygen system if not using it.  We opted to not turn the system off being that it is required to be on.  Once we received new numbers for the lower flight level; be descended to FL250 roughly 400 nautical miles from ZZZ2 which seemed to slow the dissipation of crew oxygen.  We landed uneventfully and arrived at the gate with 620 psi of crew oxygen remaining.




Synopsis: First Officer reported concerns over crew oxygen pressure dropping below limits and then MEL compliance.