Delta A332 at Seattle on Oct 26th 2016, engine fire indication
Last Update: October 1, 2019 / 21:02:08 GMT/Zulu time
A replacement Airbus A330-200 registration N859NW reached Hong Kong with a delay of 6.5 hours.
On Nov 4th 2016 the NTSB reported the crew received a #1 engine fire indication, discharged both fire bottles and returned to Seattle for an uneventful landing. The occurrence was rated a serious incident and is being investigated.
On Nov 9th 2016 the NTSB reported that following the discharge of both fire bottles the fire indication remained active. Following landing emergency services did not observe any smoke or fire from the engine or the aircraft, however, cooled the brakes of the aircraft spraying water onto the landing gear. A visual inspection of the engine at Seattle showed the outboard side of the engine and thrust reverser cowl were thermally damaged and discoloured. The occurrence is now rated an incident and is being investigated.
On Sep 19th 2019 the NTSB described tests and research done on the engine:
Engine Examination and Disassembly
The No. 1 engine was removed from the airplane by maintenance crews at SEA and prepared for shipment to Delta TechOps in Atlanta, Georgia for examination and partial disassembly. The engine was sooted/discolored from the 5 o'clock to 12:30 positions, between the fan case and exhaust case. The soot/discoloration was most concentrated from the 6 to 9 o'clock positions between the forward flange of the high pressure turbine (HPT) case and the aft flange of the diffuser case. Clamps, wiring harness cables, and the electrical power cables (feeder cables) exhibited thermal damage. Localized soot/discoloration was observed at the 12 o'clock position on the diffuser case, adjacent to fuel nozzle No. 1. Immediately above the area of localized sooting, the middle of three pressure regulating valve pneumatic lines exhibited burn through.
Fuel System Pneumatic Leak Check
Each fuel nozzle was individually numbered and photographed. All engine case openings that were exposed during removal of external engine components were covered with tape. The fuel flow transmitter was removed and pressurized shop air (~120 pounds per square inch (psi)) was ported into the supply side of the fuel distribution valve in accordance with the engine maintenance manual. An air leak was detected coming from the fuel manifold supply line that feeds fuel nozzle No. 1, located at the 12 o'clock position. Fuel nozzle No. 1 and the attached fuel manifold supply line were cleaned, and a fine crack was observed on the manifold supply line near the nozzle braze joint. A second pressure check was completed after each fuel manifold and nozzle assembly was cleaned with isopropyl alcohol with no additional findings.
The digital flight data recorder (DFDR) and electronic engine control (EEC) files were downloaded and reviewed. The only anomaly observed during the incident flight was an increase in the exhaust gas temperature (EGT) reading shortly after the No. 1 engine fire warning indication. According to P&W Flight Safety, there are several historical cases where EGT readings have increased due to an undercowl fire (external to the engine) in proximity to the EGT probes.
Materials Laboratory Analysis
The fuel manifold supply line that supplies fuel nozzle No. 1 was cut several inches away from the supply line-to-nozzle elbow braze joint to facilitate removal of the nozzle from the engine case. The nozzle and manifold supply line were shipped to the P&W materials laboratory in East Hartford, Connecticut as an assembly with the fuel nozzle b-nut torqued in the same condition as when it was removed from the engine. The remaining section of the fuel manifold triplet assembly was also shipped with fuel nozzles No. 2 and No. 3 removed.
A visual and binocular examination identified a crack that originated from the run out of the braze fillet around the toe of a ball tack weld (Figure 1). Scanning electron microscopy was used to examine the crack surface features and they were consistent with high cycle fatigue (HCF). There were no defects or material damage on the manifold supply line near the crack origin. A test of the manifold supply line material confirmed the material met drawing specification. The braze joint where the manifold supply line joins the elbow fitting had sufficient coverage and no voids in the braze material were observed. A complete copy of the materials laboratory report is available in the docket under the title "Pratt & Whitney Materials & Processes Engineering Metallurgical Investigation of Fuel Triplet and Fuel Nozzle from PW4168 Engine P733619."
Engine Vibration Testing
Between July 2017 and November 2017, P&W performed vibration testing on two PW4000-100 inch fan engines and two PW4000-94 inch fan engines with TALON (Technology for Advanced Low NOx) II combustors at P&W's Eagle Services Asia (ESA) facility in Singapore. One of the PW4000-94 inch fan engines had experienced three prior fuel leak events due to fuel manifold cracks. The other three engines tested had no history of fuel leaks. The testing identified an acoustic tone created by the combustor during engine operation that excites a dynamic (resonant) response in the fuel manifold assembly. The amplitude of the vibratory response at the fuel manifolds varied between engines but was present at the same frequency on all engines tested. One engine was tested to correlate the combustor tone/resonant frequency to an engine speed range. The combustor tone peaked near the end of a 5 second snap acceleration (rapid increase in throttle position from idle to a designated high power setting) and then decreased when the engine was allowed to dwell at the higher speed. According to the tests, the tone that excites the resonant response in the fuel manifold assemblies occurs between 9,000 revolution per minute (RPM) (86% low pressure rotor speed (N2)) and 10,000 RPM (95.6% N2).
On Oct 1st 2019 the NTSB released their final report concluding the probable cause of the incident was:
A No. 1 engine fire caused by a fuel manifold supply line high cycle fatigue crack which led to a fuel leak that subsequently ignited on hot engine case surfaces. The fatigue crack originated and progressed due to elevated fuel manifold assembly vibration levels. Engine vibration testing identified a combustor tone that excites a natural (resonant) frequency of the fuel manifolds at specific engine speeds on Pratt & Whitney PW4000-94 and -100 inch fan series engines that feature a TALON II combustor.
Aircraft Registration Data
This article is published under license from Avherald.com. © of text by Avherald.com.
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