Date: 202008
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: A321
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Parked
Aircraft component: Oxygen System/Crew
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural FAR
Detector: Person Flight Crew
When detected: Pre-flight; Aircraft In Service At Gate
Result: General Maintenance Action; General Flight Cancelled / Delayed
Contributing factors: Procedure; Aircraft
Primary problem: Procedure

Narrative: While performing the Flight Deck Preparation Oxygen Mask Test procedure; I noticed the oxygen pressure dropped excessively; by about 400 psi. I called Maintenance to have them check the oxygen bottle shutoff valve and they found it partially closed. This is a recurring problem and our procedures are not designed to detect it. I always check for pressure drop although it's not required and have caught this issue several times because of this. In my opinion; our current Airbus Normal SOP - Flight Deck Preparation - Oxygen Mask Test procedure could be improved to reduce the risk of operating with partially closed crew oxygen bottle valves by requiring the crew to check for excessive pressure drop.Unlike the Boeing SOP; Airbus pilots are not currently required to check for excessive pressure drop during flow testing. Although there are obviously design differences between the two fleets; both have rotary shutoff valves that could be left partially closed. The Airbus has a Regulator Low Pressure ECAM message that the crew is required to check after testing; but it only serves to warn the crew of a fully closed shutoff valve and is ineffective against a partially closed valve. Obviously; a partially closed valve may not provide enough flow for all the flight deck occupants.In my experience those valves are occasionally left partially closed after crew oxygen bottle replacements. When that happens; the aircraft may keep operating in that undesirable state for extensive periods of time. Typically; partial valve closures are only noticed when a crew dons the O2 masks for a restroom break and notices a lack of flow; even though they may have performed the published masks test procedure correctly.So far; on our Airbus fleet; I have personally experienced the issue three times and I am aware of at least one incident resulting in a diversion. After my last ASAP report on the subject a couple years ago I was told that changing our flight operations procedure was not necessary because maintenance procedures were changed to prevent the issue from happening again. However; it still is happening. The last time I witnessed Maintenance perform an Oxygen Mask Test after a bottle replacement; they didn't check for excessive pressure drop. So; it might be a good idea to look at their procedures as well.Given the recent change by the FAA regarding the use of oxygen mask during breaks; I'm concerned it is more likely that an issue with the O2 valve may remain unnoticed until the masks are needed in an emergency. That obviously could have very negative consequences.




Synopsis: A321 Captain reported finding an oxygen pressure drop while checking the crew O2 masks during preflight that was caused by a partially open shutoff valve on the crew oxygen cylinder.