UPS B748 at Hong Kong on Jul 20th 2021, engine fire
Last Update: December 7, 2022 / 19:31:54 GMT/Zulu time
The aircraft remained on the ground until Aug 11th 2021 before returning to service about 3 weeks after the occurrence.
On Aug 20th 2021 Hong Kong's CAD released their preliminary report rating the occurrence a serious incident.
The CAD briefly summarized the sequence of events: "The aircraft returned to Hong Kong due to shutdown of the Number One Engine by the flight crew per operating procedures about four minutes after take-off. After landing on Runway 07L, the Number One Engine caught fire. There was no other damage to the aircraft apart from the engine damage. No one was injured."
On Dec 7th 2022 the CAD released their final report (backup due to obvious connectivity trouble) concluding the probable cause of the serious incident was:
This serious incident was caused by improper installation of the P1 bypass valve port fitting on the fuel metering unit, resulting in a fuel leak that rendered the engine fire.
The CAD analysed with respect to aircraft maintenance: "The investigation team did not identify any maintenance-related issue, nor inherent aircraft defect that may lead to the serious incident. Aircraft maintenance was not a factor."
The CAD analysed the fuel leakage:
- The fuel system leak checks at the GE facility identified multiple fuel leaks from the FMU. The FMU was removed from the engine and sent to the manufacturer for further examination.
- The further examination of the FMU at the Woodward facility found that the fuel leaked from the P1 bypass valve port fitting. The fitting was hand tightened only and found to have a 0.018-inch gap observed between fitting and FMU housing. If the fitting was properly installed, there should be no gap between the fitting and the FMU housing.
- Since the safety issue on the FMU was identified, the AAIA immediately worked with the NTSB and the technical advisors to identify any safety actions in order to prevent future occurrences. On 24 September 2021, the AAIA issued Safety Recommendations 10-2021 and 11-2021 to the FAA.
Based on the FDR data the CAD analysed:
- The FMU leak started at idle.
- During the take-off roll, the N1 exceedance warning was annunciated.
- The engine did not respond to throttle reduction. The engine control system became unstable and LOTC occurred.
- Fuel was shut off and the fire started 12 seconds after engine shutdown due to a reduction in under-cowl airflow and ignition on the hot surfaces.
- The flight crew pulled the fire handle to isolate the engine from the aircraft and discharged fire extinguisher bottle A.
- The fire warning remained and the flight crew immediately discharged the fire extinguisher bottle B.
- The fire lasted about 9.5 minutes.
- The fire reignited on the ground approximately 22 minutes after the aircraft stopped and lasted about 40 seconds.
The CAD analysed the various phases of the occurrence:
Phase 1: Engine Overspeed
- Referring to the FMU examination in Section 1.16.2, it was found that the P1 bypass valve port fitting (a threaded fitting) on the FMU was finger tightened at production and safety cabled in place without final torque being applied. The O-ring between the fitting and the FMU housing was not properly supported, resulting in the failure of the O-ring and a fuel leak from this location on the event flight.
- Referring to the study of N1 reaching the overspeed limit, as the throttle was commanded to take-off power, the fuel leak rate increased and the disagreement between calculated and actual fuel flow increased.
- The calculated fuel flow became low enough that the FADEC demanded additional fuel flow to prevent flame-out of the combustor. The additional fuel flow to the combustor was sufficient to result in N1 reaching the FADEC overspeed limit.
- The FADEC then experienced a control system oscillation as it competed between reducing fuel flow (closing the FMV) to protect against N1 overspeed, which functioned as intended, and increasing fuel flow (opening the FMV) to prevent flame-out of the combustor.
- In response to the overspeed warning, the flight crew commanded the engine to idle. However, the control system oscillation persisted, and the engine did not follow the N1 command.
To sum up, the fuel leakage from the P1 bypass valve port fitting of the FMU caused the N1 to reach the overspeed limit.
Phase 2: In-flight Fire
- Referring to the study of the fuel system leakage after engine shutdown, the fuel leak at this point was a high-pressure spray/mist/vapour combination but was carried out of the vent areas by the under-cowl air flow before it had time to ignite.
- The flight crew commanded the shutdown of the engine due to the loss of N1 control. The fuel shut-off command closed both the HPSOV in the FMU, and the aircraft spar valve, with fuel trapped in the circuit between the two valves.
- The engine windmilling speed continued to drive the main fuel pump (MFP) which continued to recirculate fuel upstream of the HPSOV. The fuel leak also affected the bypass valve position after shutdown and allowed it to close, rather than stay open as intended. This resulted in high pressure in the fuel circuit upstream of the HPSOV instead of low pressure.
- With the engine at windmill speed, the under-cowl air flow was greatly reduced. The fuel leak remained at high pressure in form of a spray/mist/vapour combination, and had sufficient time to ignite adjacent to the hot engine surfaces.
- The engine fire warning alerted, and the flight crew pulled the fire handle and discharged both fire extinguisher bottles. The high-pressure fuel leak persisted after the fire extinguisher bottles were deployed and fuel continued to reignite on the hot engine surfaces.
- Over the course of about 9.5 minutes, the pressure/flow of the trapped volume of fuel feeding the fuel leak decreased until it could no longer sustain the fire. The fire warning was turned off approximately 20 seconds before landing.
To sum up, the fuel leakage from the P1 bypass valve port fitting of the FMU in the engine fire zone caused the in-flight fire.
Phase 3: Ground Fire
- After landing, the AFC attended the aircraft and noticed no indication of fire. Approximately 22 minutes after landing, the AFC on the ground noticed white smoke and then a fire in the No. 1 engine as when water was being applied to the engine surrounding to maintain a cooling effect. The AFC extinguished the ground fire after about 40 seconds using fire suppressants.
- Referring to the powerplant examination, as a result of the in-flight fire, there was likely secondary damage to some of the fuel-carrying components in the under-cowl area.
- Referring to the study of the fuel system leakage after engine shutdown, while on the ground, a small amount of residual fuel continued to leak/drip out of those fuel-carrying components where seals or lines had been compromised by the in-flight fire.
- Based on the flight test result for post-shutdown surface temperatures, some surface temperatures were above the auto-ignition threshold for fuel vapour (450 °F) for more than 22 minutes after landing.
- Referring to the study of the fuel system leakage after engine shutdown, the 3.3 gallons of remaining fuel at landing was available for the 30-40 second ground fire.
- There was sufficient fuel vapour accumulated to potentially create a deflagration, which is subsonic combustion propagating through heat transfer where hot burning material heats the next layer of cold material and ignites it.
To sum up, the ground fire was likely due to secondary damage to the fuel-carrying components in the under-cowl area.
Aircraft Registration Data
This article is published under license from Avherald.com. © of text by Avherald.com.
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