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In Jan. 1984, the Royal Australian Air Force (RAAF) experienced a ground fire incident that destroyed a six million dollar P3B Orion Aircraft (A9-300). The incident occurred during removal of an onboard oxygen cylinder, one of three that supply the Orion flight crew. Examination of the aircraft's oxygen system revealed that the fire had initiated in an oxygen manifold check valve (MCV) assembly. The primary cause of the incident was a leaking poppet valve, which allowed oxygen stored at 12 MPa (1800 psi) to escape to atmosphere. Deterioration of the silicone rubber seal and galvanic corrosion are believed responsible for the valve failure. Contributory causes to the fire were system contamination and failure to bleed the oxygen system before cylinder disconnection. A thermite reaction involving the aluminum MCV assembly, metal particles, and metal oxide particles was the most likely cause of ignition. RAAF investigations revealed similar contamination throughout the entire fleet of P3B/C Orion aircraft; iron oxide dust was the major contaminant found. Oxygen system filters were also examined, and trials aimed at improving particulate matter filtration are currently being conducted. Extensive cleaning of both aircraft oxygen systems and ground support equipment was also undertaken.
RAAF findings, supported more recently by the U.S. Navy investigations, indicate the need to consider using materials other than silicone rubber in oxygen systems. Further research into the ignition of aluminum and other candidate materials through metal particle impingement in the presence of metal oxides in a high-pressure oxygen environment is also required.
ground fire, Orion aircraft, oxygen, silicone, galvanic corrosion, contamination, thermite reaction, metal oxide, filtration
Squadron leader, Staff officer aircraft equipment engineering, Royal Australian Air Force, Embassy of Australia, Washington, DC, NW