Date: 201308
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Light: Night
Aircraft: B737-900
Operator: Air Carrier
Operating under: Part 121
Flight phase: Parked
Aircraft component: Oxygen System/Crew
Component problem: Design; Malfunctioning
Reporter organization: Air Carrier
Reporter function: Technician
Reporter qualification: Maintenance Airframe; Maintenance Powerplant
Human factors: Communication Breakdown
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: Aircraft In Service At Gate
Result: General Maintenance Action
Contributing factors: Human Factors; Aircraft; Manuals
Primary problem: Aircraft

Narrative: Accomplished required checks on Overnight Number 2 Service Check of a B737-900 Crew Oxygen (O2) System. Flow Check was accomplished as per Air Carrier Number 2 Service Work Card; Step-14c and [Flow] Check passed. Aircraft was moved to gate and flight crew reported no O2 flow to mask. Line Maintenance checked [02] system and found O2 bottle full; but not fully opened. Bottle was fully opened and returned flow to crew mask with no delay incurred. Safety concern. Maintenance Alerts were reviewed and further On-the Job Training (OJT) would enhance Maintenance.

Callback: Reporter stated the Crew Oxygen (O2) bottle on their B737-900 aircraft are a carbon fiber type bottle. He has thirty-five years maintenance experience as an Airframe/Powerplant Mechanic. His Air Carrier has alerted their mechanics in the recent past about problems with the valve handles on composite O2 bottles giving a false positive indication the valve handle is fully opened; when it's really not. What frustrated him was the day a company representative had informed him; approximately thirty days after he had accomplished the Number 2 Service Check; the flight crew had reported low O2 pressure on the same morning after he had completed the check.  He did not service or touch the Crew O2 bottle during that Check because the aircraft had arrived that evening with the O2 bottle previously serviced by an Out Station somewhere and still reading 1;750 LBS PSI; departure pressure reading was a minimum of 1;000 LBS PSI. Reporter stated when he performed the Crew O2 System and Mask Test; the O2 pressure did not have any noticeable 100 LBS PSI drop and had a quick recovery. Performing the O2 Mask Test requires some dexterity because a Mechanic has to hit the 'Press to Test' on the mask box; visually check the air flow ball in the mask and verify the overhead Oxygen pressure gage does not show a momentary pressure drop of 100 LBS PSI. He was also informed by their company representative that the flight crew had performed a more extensive flow test and that was when the low O2 flow and shut-off valve handle discrepancy was noted. That length of flow test is not part of the Work Card that Mechanics use to perform their Service Check. Another incident of Crew low O2 flow had occurred after his; on another B737-900 aircraft at a different Station; and that's what triggered the interview with him; since he was the last one to perform Test of the Crew O2 System. The company representative would not say who; or where; the last O2 Service had been accomplished.



Synopsis: A Line Aircraft Maintenance Technician (AMT) was informed that a flight crew noticed a pressure drop and low oxygen flow during an Oxygen (O2) Mask preflight check on a B737-900 aircraft. Technician noted the Crew O2 pressure did not require servicing during his Overnight Service Check. The carbon fiber composite crew O2 bottle valve handle was found not fully opened.