N434AT
ATR ATR72
During takeoff the No. 1 (left) engine fire warning illuminated in the cockpit. The pilot declared an emergency, shut down the No. 1 engine and discharged one fire bottle. The fire warning continued and the pilot discharged the second bottle; but the fire warning did not extinguish. The pilot performed an air turnback and landed the airplane uneventfully approximately 11 minutes later. Engine testing at the manufacturer’s facility revealed a large fuel leak emanating from the No. 2 fuel nozzle manifold adapter-to-transfer tube O-ring connection. Examination revealed two manufacturing defects in the No. 2 fuel nozzle manifold adapter. The first defect, poor surface finish, caused damage to the transfer tube O-ring during the installation process, initiating an internal fuel leak. The second defect, blockage of the internal safety tell-tale drain due to an improper machining and subsequent cleaning procedure, prevented the leaking fuel from flowing into the tell-tale drain and being detected by maintenance personnel. Examination of the transfer tubes revealed an undersized O-ring retention zone, which allowed the O-ring to fit loosely such that it was not capable of withstanding continuous maximum fuel pressure load. Eventually the O-ring failed to seal properly, fuel leaked and was ignited by the hot combustor case. The undersized O-ring retention zone was created during the cleaning and overhaul process of the transfer tube.
History of Flight: On January 11, 2010, about 2000 Atlantic Standard Time, an Aerospatiale ATR72, registration number 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 deplaned normally in St. Croix and no injuries were reported. The flight was registered and operated by Executive Airlines under the provisions of 14 Code of Federal Regulations (CFR) Part 121 as flight 4835. Visual flight rules (VFR) conditions prevailed for the flight, which operated on an instrument flight rules (IFR) flight plan. The flight destination was San Juan, Puerto Rico. Damage to Airplane and Engine: An on-scene inspection of the No. 1 engine nacelle confirmed the presence of a large fire within the nacelle cavity. A search for the flammable fluid leak confirmed that the engine oil tank and the hydraulic system were still intact. The only evidence of a possible fuel leak was a small wetted area on one fuel nozzle and a coked fuel nozzle heat shield. For detailed information of the airplane damage during the on scene examination, please refer to Docket document ‘On-site Airplane Fire Documentation’. The engine was then removed from the airplane and shipped to the Pratt & Whitney Canada facility in Montreal, Canada where testing revealed a large spray emanating from the No. 2 fuel manifold adapter-to-transfer tube O-ring connection, confirming the field observations. Accident Engine Fuel Manifold Service/Maintenance History: Executive Airlines (EA) reported that they had been having leaking problems with the PW127 fleet engine fuel manifolds during the last few years. On the event engine, there had been persistent fuel leak problems since it was overhauled by PWC in September 2009 and installed on the accident aircraft on October 12, 2009. During normal fuel nozzle maintenance, the fuel manifold assembly is replaced entirely with new or overhauled hardware. A review of maintenance records revealed that on October 20, 2009, EA maintenance staff repaired a fuel leak on the No. 1 fuel nozzle. Because of the fuel manifold assembling geometry, a leak of a single nozzle requires the disassembly of the entire half ring of nozzles. Based on the repair procedures, the No. 2 fuel manifold adapter was removed and re-assembled at this time. On November 15, 2009, EA maintenance staff, responding to a continuing fuel leak, again replaced all the O-rings of the fuel manifold. System Description The fuel nozzle manifold adapter and the fuel transfer tubes work in conjunction with each other to form the complete sealed fuel manifold around the engine combustor. Please refer to figures 4 and 5 in the powerplant group chairman’s group factual report for images and cross-sections of the parts. The fuel transfer tubes have two O-rings (a primary and a secondary) on either end which are inserted into the fuel nozzle manifold adapter. In normal operation, the primary O-ring seals the main fuel cavity, directing all the metered fuel to the nozzles. If it should fail, the fuel leaking past the primary O-ring is sealed by the secondary O-ring and then flows through the cross-drilled holes inside the fuel nozzle manifold adapter, into the telltale drain. The secondary O-ring prevents fuel from leaking directly outside the fuel nozzle manifold adapter where it could contact the high temperature combustor housing and ignite. Examination and Findings of Fuel Nozzle Manifold Adapter and Fuel Transfer Tubes: The leaking fuel nozzle manifold adapter and its connecting fuel transfer tube hardware were examined in detail and the following findings were discovered in each component: a) Fuel Nozzle Manifold Adapter: Using tomography and boroscope equipment, two significant anomalies were discovered in the internal passageways of the fuel nozzle manifold adapter: The first anomaly found on the fuel nozzle manifold adapter consisted of a rough patch inside one of the six transfer tube insertion bores. This occurred during a service bulletin rework at the vendor and was due to an incorrect or missed machining process that was intended to clean a welding seam. The vendors’ quality control system failed to capture this quality escape. This rough surface can cause the primary O-ring of the transfer tube to twist and tear when being inserted into the fuel nozzle manifold adapter. This may have initiated the internal fuel leak. The second anomaly found on the fuel nozzle manifold adapter was a blocked cross-drilled hole. This blockage was probably due to an incorrect welding process which allowed excess brazing material to flow into the passage. The vendors’ quality control system failed to capture this quality escape as well. This passage directs fuel from any internal leak to the telltale-drain, thereby alerting maintenance of the leak. Because of this blockage, fuel was prevented from flowing into the telltale drain thus giving no indication to the maintenance crew that the primary O-ring had failed. With no indication of a leak, no preventative maintenance was performed on the fuel manifold to correct the problem. b) Fuel Transfer Tubes: Dimensional and visual inspection 16 of the 28 fuel transfer tubes were found to have a loss of coating and parent material below allowable limits at the critical locations of the O-ring grooves and shoulders. These tubes had previously been overhauled by the vendor. The altering of the O-ring retention and sealing geometry features beyond allowable tolerances caused a loose O-ring fit in the fuel nozzle manifold adapter bore and allowed the fuel pressure to force the loose-fitting O-ring up along the sloped shoulder causing the extrusion and ultimate external fuel leak. PWC was EAs’ sole supplier of overhauled "Fuel Nozzle Kits" so it is most likely that the damage to the fuel transfer tubes was done in the PWC facilities during the repair & overhaul of the fuel nozzle kit. PWCs’ quality control system failed to capture this quality escape. Failure Sequence: The primary O-ring was initially damaged when it was inserted into the fuel nozzle manifold adapter which had the rough internal bore. The damaged primary O-ring started leaking sometime after assembly. The blockage of the cross-drilled hole in the fuel nozzle manifold adapter prevented fuel from passing to the telltale drain where a mechanic would detect the primary O-ring leak. The secondary O-ring now acting as the primary O-ring prevented the fuel from leaking out of the manifold assembly and onto the hot combustor case. The secondary O-ring was unable to withstand the increased fuel pressure in the manifold because the tolerances of the transfer tubes were beyond limits causing a loose fit between the O-ring and the transfer tube. This loose fit caused the secondary O-ring to become extruded and eventually fail causing the external leak and undercowl fire.
The sealing failure between the fuel nozzle adapter assembly and fuel transfer tubes that allowed fuel to leak into the nacelle fire zone where it was subsequently ignited by the hot combustor case. Contributing to the failure was a combination of manufacturing defects of the fuel nozzle adapter assembly by the manufacturer and incorrectly overhauled fuel transfer tubes by the engine manufacturer. Contributing to the incident was a second defect of the fuel nozzle adapter assembly by the manufacturer which prevented maintenance personnel from detecting the internal fuel leak.
Source: NTSB Aviation Accident Database
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