History of Flight
On January 11, 2010, at about 2000 Atlantic Standard Time, an Aerospatiale ATR72, registration N434AT, experienced a No. 1 (left) engine fire during takeoff from Henry E Rohlsen Airport, St. Croix, United States Virgin Islands. The pilot declared an emergency, shut down the No. 1 engine, and discharged one fire bottle. When the fire warning continued, the pilot discharged the second bottle but the fire warning persisted. The pilot performed an air turn back and landed approximately 11 minutes later. It was reported that the fire warning went out just before landing. The 44 passengers and 4 crew deplaned normally in St. Croix and no injuries were reported. The flight was operated by Executive Airlines under 14 CFR Part 121 as flight 4835. Visual flight rules conditions prevailed, and the flight operated on an instrument flight rules flight plan. The destination was San Juan, Puerto Rico.
Damage and Examination
An on-scene inspection of the No. 1 engine nacelle confirmed a large fire within the nacelle cavity. The engine oil tank and hydraulic system were intact. A small wetted area on one fuel nozzle and a coked fuel nozzle heat shield suggested a possible fuel leak. The engine was removed and shipped to Pratt & Whitney Canada in Montreal, where testing revealed a large spray from the No. 2 fuel manifold adapter-to-transfer tube O-ring connection, confirming field observations.
Maintenance History
Executive Airlines reported persistent fuel leak problems with PW127 fleet engine fuel manifolds. On the accident engine, fuel leaks had been noted since its overhaul by PWC in September 2009 and installation on October 12, 2009. On October 20, 2009, maintenance staff repaired a fuel leak on the No. 1 fuel nozzle, requiring disassembly of the entire half ring of nozzles. On November 15, 2009, all O-rings of the fuel manifold were replaced due to continuing leaks.
System Description and Findings
The fuel nozzle manifold adapter and fuel transfer tubes form the sealed fuel manifold around the engine combustor. Transfer tubes have primary and secondary O-rings. The primary O-ring seals the main fuel cavity; if it fails, the secondary O-ring seals the leak and routes fuel through cross-drilled holes to a telltale drain. Examination of the leaking manifold adapter and transfer tubes revealed two anomalies in the adapter: a rough patch inside one transfer tube insertion bore due to incorrect machining after a service bulletin rework, and a blocked cross-drilled hole due to excess brazing material from incorrect welding. The vendor's quality control system failed to capture these escapes. Additionally, 16 of 28 fuel transfer tubes had loss of coating and parent material below allowable limits at O-ring grooves and shoulders, causing loose O-ring fit. This damage likely occurred during overhaul at PWC facilities, whose quality control also failed.
Failure Sequence
The primary O-ring was initially damaged when inserted into the rough bore. It began leaking after assembly. The blocked cross-drilled hole prevented fuel from reaching the telltale drain, so the leak went undetected. The secondary O-ring then acted as the primary seal but was unable to withstand increased fuel pressure due to loose fit from the damaged transfer tubes. The secondary O-ring extruded and failed, causing an external fuel leak and undercowl fire.