Dallas, TX, USA
B18701
BOEING 747
On February 25, 2013, a Boeing B747-409F freighter, B-18701, powered by four General Electric CF6-80C2B1F turbofan engines, experienced a No. 2 (left inboard) engine undercowl fire while taxiing after landing at Dallas/Fort Worth International Airport, Dallas Texas. After receiving the fire warning, the pilots discharged a fire suppression bottle and the fire ceased. There were no injuries to the three crewmembers on board and no airplane damage was reported. The incident flight was a regularly scheduled cargo flight, operated by China Airlines under provisions of 14 Code of Federal Regulations Part 129, from Taipei, Taiwan, to Dallas with stops in Anchorage, Alaska, and Atlanta, Georgia. Initial visual examination of the engine found that at the No. 2 fuel nozzle position, the right fuel manifold feeder tube aft o-ring support was displaced rearward against the snap ring. When the No. 2 fuel nozzle feeder tube shroud was removed, the feeder tube was found fractured and separated between the feeder tube-to-ferrule weld joint and the aft o-ring, which is one of the features that prevents a fuel leak when the feeder tube fractures. Metallurgical examination revealed that the feeder tube fractured due to lower alternating stress high-cycle fatigue cracking that transitioned to increasing alternating stress fatigue cracking before ultimately failing in overload. Further, the o-ring had sustained a spiral-like separation. Additional examination confirmed that no inclusions or any material anomalies at the feeder tube crack initiation sites and that the material properties of the feeder tube and o-ring and the weld quality met their required specifications. The fracture mode of the feeder tube is well-documented and GE issued Service Bulletin (SB) 73-0371 to introduce a redesigned fuel manifold and bracket to address this issue. Service Bulletin 73-0371 also introduced changes to the manifold feeder tube configuration and orientation (to avoid resonance frequencies of the manifold that were within the engine operating range) to prevent fuel manifold fractures and wear-through of the tube thickness that had resulted in previous fuel leaks and undercowl fires. The SB was issued just 7 days before the installation of the incident fuel manifold on the incident engine; therefore the operator was unable to install the most current fuel manifold design.
HISTORY OF FLIGHT On February 25, 2013, a China Airlines Boeing B747-409F freighter, registration B-18701, flight number CI-5254, powered by four General Electric CF6-80C2B1F turbofan engines experienced a No. 2 (left inboard) engine undercowl fire while taxiing after landing at Dallas/Fort Worth International Airport, Dallas Texas. After receiving the fire warning, the flight crew performed the Quick Reference Handbook (QRH) fire emergency check list and discharged a fire suppression bottle. The fire warning was reported to have ceased after the fire suppression bottle was discharged. There were no injuries to the three crew members on board, and no airplane damage was reported. The incident flight was a regularly scheduled cargo flight, operated under provisions of Title 14 CFR Part 129, from Taipei, Taiwan to Dallas with stops in Anchorage, Alaska and Atlanta, Georgia. ENGINE DAMAGE Initial Visual Examination Post event inspection of the No. 2 engine by China Airlines maintenance personnel while the engine was still installed in the airplane, confirmed sooting and thermal distress to the right-hand side of the engine. China Airlines maintenance personnel performed a wet-motor of the engine to find the source of the flammable fluid leak and reported that fuel was seen spraying in the vicinity of the No. 3 fuel nozzle fuel manifold shroud position. Prior to the arrival of the investigative team, China Airlines was requested to remove the engine and perform an internal borescope inspection of the engine. According to China Airlines maintenance personnel, the borescope inspection found no anomalies or internal damage. The No. 2 engine exhibited localized soot circumferentially from the 12:00 – 3:00 o’clock position and lengthwise from the fuel manifold aft to the high pressure turbine active clearance control manifold with the heaviest sooting observed on the compressor rear frame just aft of the right-hand fuel manifold, between the No. 2 and 5 fuel nozzle positions. Some electric bundles and cushion clamps in the vicinity of the fire/soot area exhibited thermal distress. The fire detection loops located aft of the right-hand fuel manifold were intact; the loop isolators from the 1:00 to 2:00 o’clock position behind the Nos. 3 – 5 fuel nozzle positions were present but exhibited thermal distress and oxidation. Initial visual examination of the right-hand fuel manifold before any disassembly was performed revealed that all the associated hardware - clamps, brackets, bolts, lockwire, shrouds, snap rings, etc. - was present and intact and none of the feeder tubes that connect the fuel manifold to the fuel nozzle were deformed. The only visual anomaly noted was at the No. 2 fuel nozzle position, where the feeder tube aft o-ring support was displaced rearward against the snap ring. In all the other locations, the feeder tube aft o-ring support was in the normally installed forward position. The snap rings from the fuel shrouds at the Nos. 1 to 5 fuel nozzle positions were removed and the shrouds were pulled back exposing the feeder tube-to-fuel nozzle joint. When the No. 2 fuel shroud was slid aft, the aft o-ring was found separated and the feeder tube was fractured at the feeder tube-to-ferrule weld joint located just forward of the aft o-ring support location. Closer examination of the aft o-ring found it to be round and somewhat pliable with no compression set or thermal distress observed. The forward o-ring, the feeder tube-to-fuel nozzle attachment nut, and the attachment lock wire, were all present, intact, and in good condition. None of the other feeder tubes were found fractured and the only other anomalies observed were: 1) the aft o-ring for the No. 4 fuel shroud was separated but in good condition, and 2) the aft o-ring for the No. 5 fuel shroud exhibited thermal distress, was flatted, and it appeared that some of the o-ring material was missing. TEST AND RESEARCH Metallurgical Examination Examination of the separated ferrule portion of the No. 2 feeder tube found that it had fractured 360° circumferentially adjacent to the rear weld toe feature. Multiple crack origins were found located on the outer diameter of the tube at one localized area with the cracks propagating inwards and in both circumferential directions around the tube up to the tensile fracture region. No evidence of inclusions or any material anomalies were noted at the crack initiation sites. According to GE, the features of the cracks indicated initial propagation consistent with lower alternating stress high cycle fatigue (HCF) that transitioned to features consistent with increasing alternating stress fatigue. Piecing together the No. 2 feeder tube aft o-ring confirmed that the entire o-ring was present and that it had sustained a spiral-like separation. Additional evaluation was performed to determine the quality of the ferrule weld and the material composition of the right-hand fuel manifold assembly, the o-rings, and fuel shroud. No anomalies were found and the results confirmed that all met their required specifications. The No. 2 fuel shroud was sectioned lengthwise to expose the inner diameter for evaluation. The shroud inner diameter exhibited two sets of circumferential wear marks, one located at the normal feeder tube location (forward) and the other fully aft at the snap ring location. Neither of the sets of wear marks were 360° circumferential around the inner diameter; instead the wear marks were localized. GE concluded that this aft wear/fretting pattern on the fuel shroud was created after the feeder tube was fractured when the fuel pressure loads forced the fractured feeder tube against the snap ring. Furthermore, GE concluded that the presence of the fuel shroud aft wear/fretting mark indicated that the o-ring held the fuel within the fuel shroud after the initial feeder tube fracture which allowed the fractured tube to operate for some time at that aft position before the o-ring eventually failed. ADDITIONAL INFORMATION Previous Events of Undercowl Fires Resulting from Fuel Manifold Fuel Leaks Since GE introduced the drainless fuel manifold back in August 1996, there have been four undercowl fires resulting from fuel manifold leaks including this most recent event. The NTSB has investigated or participated in the investigation of each of these four events. The first event occurred on October 14, 2002. An Atlas Air Boeing B747-400F, N497MC, experienced an engine under cowl during the takeoff climb from the Chiang Kai Shek International Airport in Taipei, Taiwan. During the initial climb, the flight crew reported hearing a loud bang, the airplane immediately yawed, and a fire warning activated. The engine was shutdown, both fire suppression bottles were discharged, and the airplane returned for an uneventful landing. Examination of the engine revealed that two fuel manifold feeder lines were distorted and displaced aft, each retaining ring (nomenclature of the old configuration retention ring) had become dislodged from their fuel shroud retaining groove and found loose on their respective feeder lines, and both feeder lines were fractured under the fuel shroud. Metallurgical examination found that the feeder line fractures were consistent with high amplitude fatigue (HAF) which had been attributed to a high pressure turbine rotor out of balance caused by a blade separation event. Prior to the Atlas Air event, there had been two previous drainless fuel manifold feeder line fractures where the fuel was not captured and held within the fuel shroud but instead leaked out past the o-rings. No under cowl fires were reported in either of the first two cases. All three events showed that the then current retaining ring design did not provide adequate retention of a fractured fuel manifold feeder line and allowed fuel to leak resulting in one undercowl fire. GE issued SB 73-0337 to introduce an more robust C-shaped snap ring (new nomenclature) configuration to retain the fractured feeder line within the fuel shroud and prevent a fuel leak. On December 5, 2005, a Skymark Boeing 767, registration number JA767B, experienced high engine vibration and an undercowl fire during cruise. The flight crew shutdown the engine, pulled the fire handle, and the fire warning went out. No fire bottles were reported to have discharged. The Japanese Aircraft and Railway Accidents Investigation Commission (ARAIC) investigated this incident and the National Transportation Safety Board (NTSB) assigned a U.S. accredited representative per Annex 13 to the Convention on International Civil Aviation Organization (ICAO) as the state of manufacturer of the engines and the airplane. Similar to the Atlas Air event, examination of the Skymark engine revealed that a fuel manifold feeder line was displaced aft, the retaining ring had become dislodged from the fuel shroud retaining groove and found loose on their respective feeder lines, and the feeder lines were fractured under the fuel shroud. The last shop visit for the engine was prior to the release of the new snap ring configuration SB 73-0377; therefore, the retaining ring was the same configuration as the Atlas Air and in both cases failed to hold the fuel within the fuel shroud and an undercowl fire was the result. After the Skymark event, the FAA mandated the incorporation of the new snap ring configuration by AD 2007-11-20. On December 30, 2006, an Air New Zealand Boeing 767 experienced an engine under cowl fire after landing at Auckland International Airport while stowing the thrust reverser. The flight crew pulled the fire handle, discharging a single fire suppression bottle that extinguished the fire. Inspection of the engine revealed chaffing and a pin hole in the fuel manifold with its corresponding support clamp exhibiting extensive wear of the rubber cushion material allowing metal-to-metal contact. The New Zealand Transport Accident Investigation Commission investigated this incident and NTSB assigned a U.S. accredited representative per ICAO Annex 13 as the state of manufacturer of the engines and the airplane. To address this problem, GE issued SB 73-0365 that called for the installation of the Polytetrafluoroethylene (PTFE) tape under the clamps to provide a protective layer of material between the clamps and the fuel manifolds. The FAA issued AD 2009-05-02 to mandate a repetitive inspection for fuel manifold wear and replacement of P-clamps; however, the AD did not apply to the China Air event fuel manifold configuration. However, the repetitive inspection was also included into Chapter 05-21-73 of the CF6-80C2 engine manual so the event fuel manifolds were subject to this same repetitive inspection as was mandated by the AD. The configuration of the fuel manifolds installed on the event engine at the last shop visit was the drainless shroud configuration with all the required Service Bulletins incorporated except for one, Service Bulletin 73-0371. Service Bulletin 73-0371 introduced a new fuel manifold configuration with brazed on clamps, new brackets, and changes to the manifold feeder tube configuration and orientation (to avoid resonance frequencies of the manifold that were within the engine operation range) to prevent fuel manifold fractures and wear through the tube thickness that had resulted in previous fuel leaks and undercowl fires. GE issued Service Bulletin 73-0371 (basic version) just 7 days prior to the installation of the event fuel manifold on the event engine; therefore, the operator was unable to install the most current fuel manifold configuration. Service Bulletin 73-0371 introduced a new fuel manifold configuration with brazed on clamps, new brackets, and changes to the manifold feeder tube configuration and orientation to prevent fuel manifold fractures and wear through the tube thickness that had resulted in previous fuel leaks and undercowl fires. Corrective Actions When SB 73-0371 was first released, GE established a Category 5 compliance for the incorporation of the SB and accomplishment was to be performed at a maintenance shop visit. For Category 5 compliance, GE recommends that the SB be incorporated as soon as the fuel manifold system is removed from the engine. Based on this most recent event, GE released an accelerated incorporation schedule and revised SB 73-0371 to change the compliance Category from a 5 to a 3 and allowed incorporation to be done on-wing as well. Category 3 is compliance with the SB at the next shop visit of the engine or module regardless of the reason for the shop visit. The revised SB, SB 73-0371 R02, was issued on May 20, 2013.
Fuel leaking from a fractured fuel manifold feeder line that ignited on contact with the engine’s hot compressor rear frame, which resulted in an engine undercowl fire. The fuel manifold feeder line failed due to high amplitude fatigue because the fuel manifold resonance frequencies were within the engine operation range. Contributing to the incident was that the engine was overhauled about the same time as the most current fuel manifold configuration that addressed the high amplitude fatigue failure mode was introduced, thus it was not installed on the engine.
Source: NTSB Aviation Accident Database
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