Date: 201303
Local time of day: 1801-2400
Location: ZZZ.ARTCC, US
Light: Dusk
Aircraft: Cessna Citation Sovereign (C680)
Operator: Fractional
Operating under: Part 91
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Oxygen System/Crew
Component problem: Design; Malfunctioning
Reporter organization: Fractional
Reporter function: Other / Unknown
Reporter qualification: Maintenance Airframe; Maintenance Powerplant
Human factors: Situational Awareness; Communication Breakdown
Second reporter function: Inspector
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown
Detector: Person Flight Crew
When detected: In-flight
Result: General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Equipment / Tooling; Human Factors; Procedure; Manuals
Primary problem: Equipment / Tooling

Narrative: CREW REPORTED DISCREPANCY - At FL430; while on oxygen (O2); the PIC's O2 flow suddenly stopped in mid breath. Bottle pressure was 1;500 PSI at the time. Investigation revealed no switch positions out of place. First Officer's mask proved to have the same issue. Descended aircraft to FL240 whereupon masks were frequently tested and both were found to deliver about one good breath after a several minute wait; after which no O2 could be obtained from the mask without another several minute wait. On ground; mask appears to function properly for several breaths. Bottle pressure on ground is at top of green arc. Discrepancy corrective action - upon inspection found O2 bottle regulator; Shut-Off valve not in the fully open position. Removed faulty safety wire and installed new safety wire securing valve in the correct position. O2 System operations check is good. Recommend Maintenance personnel involved be familiarized with ensuring the regulator valve in seated fully in the 'ON' position. Maintenance Supervisor.

Callback: Reporter stated he is a Maintenance Supervisor with twenty-four years maintenance experience. The flight crew's oxygen bottle is a composite type located in the belly under the fuselage at an Oxygen panel; forward of the wings. The bottle is manufactured by AVOX and serviced on-board; not removed from the aircraft. The AVOX bottles are an OEM for the Cessna CE-680 and similar Cessna aircraft. The crew O2 bottle regulator shut-off valve handle is not moved or rotated to 'shut-off' position during on-board servicing. He believes the bottle is removed during a C-Check or Maintenance Visit. The only other source of oxygen for the crew is a therapeutic type bottle towards the rear of the cabin.

Second narrative: I was informed by the Director of Inspection that in March 2013; a Cessna CE-680 aircraft was en route when the pilot reported that oxygen flow to his mask stopped suddenly in mid-breath. Further troubleshooting [by the flight crew] revealed the co-pilot's mask was the same. The aircraft diverted where Technicians from Cessna Citation discovered that the Oxygen Bottle Regulator valve had not been fully open. Seven days earlier; the same aircraft was at [our] station for a Scheduled Maintenance visit.  During that visit I inspected the installation of the pilot's O2 mask box. The last two steps in the installation procedure of the Crew Storage Mask box are to 'Turn' the Cylinder-Regulator to 'ON' and install safety wire on the cylinder-regulator in the 'ON' position.  I visually verified that the Cylinder-Regulator was turned to the 'On' position and safetied. [Recommend] physically verifying that the bottle is to the complete 'On' position. Maintenance Inspector.

Second callback: Reporter stated his company is a Contract Maintenance provider performing maintenance on Cessna CE-680 aircraft for Air Carrier X.  The handle on the Avox Crew Oxygen composite bottle only needs to be rotated  approximately 180 degrees to the 'On' position stamped on the O2 bottle. The lower fuselage belly panel had been removed for access to the O2 bottle for maintenance during the Maintenance Visit At his Station. He personally watched the Mechanic position the bottle lever to the 'On' position and safety wired the handle. To this day; he cannot understand how the handle could have moved to a halfway position from what he had visually observed. Reporter stated that a similar situation had occurred previously with the same type of bottle. The bottle Manufacturer recently notified his group about a newer designed Crew O2 Shut-Off handle/lever that will 'spring' to the 'Full' open position; whenever the handle moves halfway towards the 'Open' position. He does not know if the Crew bottle he observed was the newer design. The Cessna Maintenance Manual (M/M) does not have any specific diagrams for safetying the valve handle.

Synopsis: A Maintenance Supervisor and a contract Maintenance Inspector report that a Cessna CE680 flight crew at FL430 experienced a sudden stoppage of oxygen (O2) flow in mid-breath; even though the crew O2 bottle pressure was 1500 PSI. Maintenance found the composite type Crew Oxygen Bottle shut-off valve not fully opened.