N855NW

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.

The No. 1 engine fire was caused by a fuel leak that originated from a high cycle fatigue crack on the fuel nozzle No. 1 fuel manifold supply line. Fuel nozzle No. 1 is located at the 12 o'clock position on the engine, so the fuel that leaked from the crack ran down the diffuser case and ignited on the engine case surface. The diffuser case reaches high enough temperatures during engine operation to auto-ignite Jet A fuel. The greatest concentration of soot/discoloration was observed from the 6 to 9 o'clock positions between the forward flange of the HPT case to the aft flange of the diffuser case, consistent with fuel collecting at the bottom of the engine and in the lower half of the thrust reverser cowls and creating a rich fuel to air mixture. Engine vibration testing was performed on multiple PW4000-94 and -100 inch series fan engines featuring a TALON II combustor. The testing identified a combustor tone that excites a natural (resonant) frequency in the fuel manifold assemblies. The amplitude of the combustor tone varied significantly between the four engines tested and the reason for this variation has not yet been determined. Pratt & Whitney released service bulletins for both the PW4000-94 inch and -100 inch fan engines recommending the installation of brackets and clamps on the fuel manifold assemblies. Engine vibration testing was performed on the same engine before and after installation of the brackets and clamps and the testing confirmed that the design was effective in dampening the elevated vibration levels driven by the combustor tone.

HISTORY OF FLIGHT On October 26, 2016, at about 1550 Pacific daylight time, a Delta Air Lines (DAL) Airbus A330-223, N855NW, equipped with two Pratt & Whitney (P&W) PW4168A-1D turbofan engines experienced a No. 1 (left) engine fire during initial climb from Seattle-Tacoma International Airport (SEA), Seattle, Washington. The flight crew declared an emergency, discharged both fire bottles, and returned to SEA for an uneventful overweight single engine landing. The electronic centralized aircraft monitor (ECAM) system fire indication warning remained illuminated on the cockpit display after both fire bottles were discharged. Airport rescue and firefighting (ARFF) crews met the airplane on the runway and did not observe fire or smoke coming from the engine. Water was sprayed on the airplane landing gear as a precautionary measure, but no water or fire retardants were sprayed on/into the No. 1 engine. There were no passenger or crew member injuries reported. The flight was a regularly scheduled flight from SEA to Hong Kong International Airport (HKG), Chep Lap Kok, Hong Kong and was conducted under the provisions of Title 14 Code of Federal Regulations Part 121. DAMAGE TO THE AIRPLANE There was no damage to the airplane structure. The left engine thrust reverser exhibited thermal damage and discoloration on all interior surfaces, most concentrated near the 6 o'clock position. The thrust reverser fire loops were thermally damaged and multiple fire loop rubber grommets were deformed or missing. TEST AND RESEARCH 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 fuelmanifold 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. Recorded Data 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." Figure 1- Fuel Nozzle No. 1 and Fuel Manifold Supply Line Section 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). ADDITIONAL INFORMATION Eagle Services Asia Engine Teardown The incident engine was shipped to ESA Singapore after it was released from the investigation for disassembly and parts evaluation as part of the normal repair process. The NTSB requested a copy of the engine teardown findings report. ESA did not identify any engine anomalies that may have contributed to the engine fire and the condition of all internal components, including the combustor and diffuser assemblies were consistent with similar service run engines. Engine Build Audit At the request of the NTSB, an announced build audit of a PW4000-100 inch fan engine diffuser and combustor assembly build was conducted at ESA in March 2018. During the assembly process, there were no deviations from the engine assembly manual procedures observed and both the part numbers and condition of all parts used met manual specifications. Corrective Action On November 8, 2017, P&W released SB PW4ENG-73-224 recommending the installation of fuel manifold brackets and clamps on PW4000-94 inch fan engines with TALON II combustors. The FAA issued AD 2019-06-02 on March 26, 2019 to mandate the SB. The AD has the same compliance requirements as those recommended in the SB and the AD effective date was April 30, 2019. Additional information about SB and AD scope and compliance is included in the NTSB powerplants group chairman's factual report in the docket. On March 14, 2018, P&W released service bulletin (SB) PW4G-A73-48 recommending the installation of brackets and clamps on the fuel manifold assemblies to dampen the resonant vibratory response to a combustor tone on PW4000-100 inch fan engines with TALON II combustors. The FAA mandated the installation of the fuel manifold brackets and clamps with airworthiness directive (AD)-2019-06-07 on March 28, 2019.

The failure of the flight crew to maintain adequate clearance while taxiing.

The Airbus was taxiing east via Taxiway B to runway 28L for departure. The Bombardier had landed, taxied to the gate area, and was stopped on the ramp located north of Taxiway B waiting to be marshaled to its parking spot. The Bombardier was sitting on a northerly heading, oriented perpendicular to Taxiway B with its tail towards the taxiway. The captain and augmenting captain of the Airbus visually checked for clearance between their left wingtip and the tail of the Bombardier and decided that there was adequate clearance. However, as the Airbus passed behind the Bombardier, the left wingtip of the Airbus struck the tail of the Bombardier. The left winglet of the Airbus was scraped, bent and cracked. The vertical stabilizer, horizontal stabilizer, both elevators, and the rudder of the Bombardier sustained structural damage.

On August 29, 2005, approximately 1409 Pacific daylight time, an Airbus A330-223, N855NW, was taxiing for takeoff when it collided with a Bombardier DHC-8-202, N363PH, that was standing with engines operating waiting to be marshaled to its parking spot on the Portland International Airport, Portland, Oregon. There were no injuries to the 3 flight crewmembers, 9 flight attendants, and 235 passengers aboard the Airbus or to the 2 flight crewmembers, 1 flight attendant, and 20 passengers aboard the Bombardier. There was minor damage to the left winglet of the Airbus and substantial damage to the tail of the Bombardier. The Airbus was operated by Northwest Airlines as flight number 5, a 14 CFR Part 121 scheduled international passenger flight from Portland to Narita Airport, Japan. The Bombardier was operated by Horizon Air as flight number 2066, a 14 CFR Part 121 scheduled domestic passenger flight from Medford, Oregon to Portland. Visual meteorological conditions prevailed, and instrument flight rules flight plans were filed for both flights. According to information provided by FAA inspectors who responded to the scene of the accident, the Airbus was taxiing east via Taxiway B to runway 28L for departure. The Bombardier had landed, taxied to the gate area, and was stopped on the ramp located north of Taxiway B waiting to be marshaled to its parking spot. The Bombardier was sitting on a northerly heading, oriented perpendicular to Taxiway B with its tail towards the taxiway. As the Airbus passed behind the Bombardier, the left wingtip of the Airbus struck the tail of the Bombardier. The left winglet of the Airbus was scraped, bent and cracked. The vertical stabilizer, horizontal stabilizer, both elevators, and the rudder on the Bombardier sustained structural damage. Portland Air Traffic Control Tower prepared a chronological summary of the event and a transcript of the communications on the ground control frequency. The chronology indicated that at 1403, the Bombardier contacted ground control stating they had exited the runway at Taxiway B-5 for spot 14, and the ground controller cleared them to taxi to spot 14. At 1405, the Airbus advised they were ready to taxi, and the ground controller instructed them to taxi to runway 28L. The transcript indicated that at 1406:40, the ground controller advised the Airbus to "use caution for a dash eight [Bombardier] pulled up ah on taxiway tango by bravo ahead and to your left should have wing tip clearance there." The Airbus acknowledged the transmission. At 1408:58, the unidentified transmission, "hey ah northwest are you (unintelligible)" was made. At 1412:45, the Airbus advised the ground controller that they had hit the Bombardier. During an interview conducted by FAA inspectors, the captain of the Bombardier reported that the flight's landing and taxi in were normal. He further reported that the flight was instructed to park in spot 14, and he held on the ramp short of the spot awaiting a marshaler todirect him into the parking spot. While waiting to be marshaled, the captain heard the ground controller direct the Airbus to taxi behind his airplane. He then taxied his airplane "as far forward as possible but did not cross the access road due to the missing marshaler." The captain reported that the Bombardier was stopped when the Airbus passed behind it. Just prior to the impact, the captain attempted to contact the ground controller, but the impact pulled his hand away from the radio transmit switch. Following the event, the flight crew shut down the engines, and the passengers exited the Bombardier and walked to the terminal. There was no emergency evacuation. According to a written statement submitted by the captain of the Airbus, the flight was cleared to taxi to the runway, and the ground controller commented that the flight should "not have a problem" getting past the Bombardier that was clear of the taxiway on the ramp. The captain reported that the tail of the Bombardier "appeared to be 20-30 feet outside the taxi line" and that he was "not comfortable" with the Bombardier's position. As the Airbus approached the Bombardier, the Bombardier "moved forward about another 20-30 feet perpendicularly away from the yellow taxiway edge line." The captain asked the augmenting captain seated in the jump seat if it looked like they would clear the Bombardier, and the augmenting captain got out of his seat and looked out the left window. The augmenting captain told the captain that it looked like he would have 10-20 feet of clearance. The captain reported that he also looked at the left wingtip and thought there was adequate clearance. According to the captain, the nose wheel of the Airbus was slightly right of the taxiway centerline when he passed behind the Bombardier. The captain felt a "shudder" and stopped the airplane. Following the event, the Airbus taxied back to the gate, and the passengers de-planed. Written statements submitted by the first officer and the augmenting captain of the Airbus collaborated the captain's statement. When FAA inspectors reached the scene, the Airbus was parked at the gate, and the Bombardier had not been moved since the collision. FAA inspectors measured the distance between the Bombardier's nose wheel and the centerline of Taxiway B as approximately 150 feet. According to information provided by Horizon Air, the distance from the nose wheel to the tip of the Bombardier's tail was approximately 67 feet. This placed the tip of the Bombardier's tail approximately 83 feet from the centerline of Taxiway B. According to information provided by Northwest Airlines, the wingspan of the Airbus was 197 feet 10 inches. With the nose wheel of the Airbus on the centerline of the taxiway, the tip of the left wing would have been located 98 feet 10 inches left of the centerline.