Date: 201412
Local time of day: 0001-0600
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: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Technician
Reporter qualification: Maintenance Airframe; Maintenance Powerplant
Human factors: Confusion; Communication Breakdown
Second reporter function: Technician
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: Pre-flight
Result: General Maintenance Action
Contributing factors: Procedure; Company Policy; Human Factors
Primary problem: Human Factors

Narrative: I; [Technician X;] was called into the manager's office late morning January 2015 and was informed [that] in December 2014; on a B737-900 aircraft; I documented crew oxygen low in logbook and it was reported approximately ten days later in logbook that the crew oxy failed the operational (ops) check by [Flight] crew reading zero (0) psi. I performed [the Crew oxygen servicing] task according to MM 12-15-21 and the supplemental Job Card. Technician Y performed the ops check and installation of the Crew Oxy [bottle]. While accomplishing the task we verified that there were no abnormalities. This task was accomplished by the book and documented as such. Crew performed ops Checks numerous times per the Company flight manual Section (basically the same test we performed) and checked good. Between the time of servicing and the time of failing the test; approximately 46 flights [had occurred before] Oxygen Dropped to 0 psi. This verifies that I did this task according to Job Card; or it should have been detected the very next flight due to the fact that I did the 02 servicing on a Routine Overnight (RON) Maintenance Check and the first flight of the day O2 Mask test would have failed. I'm not sure why this event happened! I performed crew servicing per MM 12-15-21; Ops ck. and installation of crew oxygen verified not only by Technician Y; but also pressure test was performed by numerous flight crews every day for 11 days and checked good and then drops to 0 psi. I feel I did nothing wrong. Graveyard. [Found] on preflight 11 days after I performed servicing.

Callback: Reporter stated their steel Crew Oxygen bottles are removed when servicing is required and replaced with a fully serviced bottle. When reinstalling the lines and clamps for a serviced O2 bottle at the Forward Cargo pit forward bulkhead; they follow the Aircraft Maintenance Manual (AMM). They also use a Job Card (J/C) supplement to their AMM 12-15-21 procedures when they are ready to open the O2 bottle's valve handle and perform a Leak Check of the oxygen line connections and cockpit Crew Mask Pressure Check. Copper break-away wire is then used to safety the valve handle in the 'Open' position.Reporter stated his Air Carrier has had issues with loss of Crew O2 bottle pressures for more than 10 years; especially with Composite O2 bottles. Their O2 Servicing Job Card supplement has been revised three times. He has seen the Crew O2 steel bottles leak at the overpressure weep hole on the back side of the O2 bottle's shut-off valve housing for the Open/Close handle. He was a 24-year employee for the same Air Carrier with no problems until the Crew O2 incident came up. He was fired along with the other Mechanic he had performed the Crew O2 servicing procedures with. He recommends that all O2 bottles have serialized tags so that mechanics can sign-off the paperwork noting the tag number for the bottle they replaced. To date; no one can explain the cause of the sudden complete loss of the Crew O2 bottle pressure after 11 days and 46 flights and multiple flight crew mask checks.

Second narrative: January 2015; I was informed by Technician X that a B737-900 Aircraft had failed a crew oxygen check four days earlier that is routinely performed by the [Flight] crew. We were the last two [technicians] to document servicing of the crew oxygen December 2014 in logbook. As per task 12-15-21 Job Card supplement; I observed Technician X fully open the O2 cylinder valve [handle] then rotate clockwise a turn. I watched as Technician X safety wired the valve in that position and perform the leak check. I performed the operational checks with no drop in pressure as Technician X observed me doing so. I'm not sure why this event happened. I verified the oxygen cylinder was installed and opened correctly. The operational pressure checks were not only performed by myself while Technician X watched; they were also performed a number of times during the 46 some flights that followed us servicing the oxygen. Line Aircraft Technician.


Synopsis: Two Aircraft Maintenance Technicians (AMTs) describe their involvement with servicing crew oxygen on a B737-900 aircraft after a flight crew noted the cockpit O2 mask check had failed. Aircraft had been in service for 11 days and 46 flights before the O2 pressure had dropped to Zero (0) psi.