Factual Information
On 18 January 2014, at approximately 1642 UTC, an Etihad Airways Airbus A319, registration A6-EID, arrived at Abu Dhabi International Airport from Minsk International Airport, operating flight ETD062. The aircraft was parked at Bay 402 and was received by the operator's line maintenance nightshift team. During a daily check, the maintenance engineer noticed the flight crew oxygen system pressure was 1450 psi, below the minimum dispatch pressure of 1500 psi. A decision was made to service the oxygen system.
While performing in-situ oxygen servicing with the assistance of a technician, the oxygen pressure regulator transmitter exploded, causing rapid bottle depressurization, a loud bang, and a flash of light. The rapid discharge of oxygen caused minor facial irritation to the maintenance engineer. The technician shut off the oxygen supply and attended to the engineer. The aircraft was de-powered, and emergency services were called. The engineer was transported to the airport medical center and then to a hospital, later discharged with minor skin and eye irritation.
The aircraft was moved to the maintenance facility and quarantined. All oxygen components related to the incident were removed and stored. The oxygen trolley was also quarantined. The aircraft returned to service on 21 January 2014 after installing a new flight crew oxygen bottle, plumbing, and insulation blankets, along with satisfactory leak and functional checks. The emergency transformer rectifier unit was replaced due to burn marks. The oxygen trolley was checked, re-calibrated, and returned to service.
Tests and Research
The oxygen cylinder, pressure regulating transmitter (PRT), and associated plumbing were sent to Zodiac Aerotechnics in France for investigation. Further investigation by WHA International indicated that the fire had probably been initiated at the filling valve of the PRT, with heat flow from the PRT to the cylinder valve, then a reverse flow maintained by the oxygen emptying. The discontinuity occurred at the threaded connection between the union and the PRT, though it could not be determined if a fracture had occurred previously.
The initial assessment revealed an internal component failure of the PRT. Maintenance procedures and precautions used during servicing were found to be in accordance with the aircraft maintenance manual. Airbus had no recommendations for an inspection plan.
Analysis
A computed tomography scan of the cylinder valve showed the stem partially melted and the outlet filter disappeared. Internal parts of the filling valves had also disappeared. The damage was mainly localized on the PRT at the CGA540 coupling level. The cylinder valve was damaged, with the stem and outlet pipe partially melted.
The investigation considered two scenarios: mechanical impact ignition from poppet chatter during filling, and adiabatic compression at the PRT fill valve seat due to improper operation of the ground support equipment filling trolley. The absence of a hissing noise and no pressure rise at the filling gage eliminated the first scenario. The second scenario was determined to be the probable cause, as the filling trolley does not prevent rapid compression depending on the quick-acting toggle valve opening time. It is believed the toggle valve was in the STOP position during filling and then placed in CHARGE, causing rapid pressurization and adiabatic compression, leading to ignition.