History of Flight
On March 5, 2013, at about 1120 Pacific standard time, a Bombardier Inc. DHC-8-402 airplane, registration N417QX, sustained minor damage following an in-flight engine fire during initial climb near Danville, California. The airplane was registered to Horizon Air Industries of Seatac, Washington, and operated under 14 Code of Federal Regulations Part 121. Visual meteorological conditions prevailed, and an instrument flight rules flight plan had been filed and activated. The flight departed Norman Y. Mineta San Jose International Airport (SJC) at about 1108, destined for Boise Air Terminal (BOI), Boise, Idaho.
The pilot-in-command reported that while climbing through 18,000 feet mean sea level, the flight crew heard a muffled "boom" in the flight deck. Immediately after, the number 2 engine's internal turbine temperature (ITT) exceeded limits, and the crew observed yaw indicating power loss on that engine. An engine fire warning illuminated, and the crew performed the emergency Engine Fire Checklist. After discharging the first fire bottle, the warning continued; a second bottle was discharged but the warning remained. The crew requested and received ATC clearance to return to SJC. They advised ATC and requested fire trucks on landing. After touchdown at SJC, the aircraft stopped on the runway. Emergency personnel reported no visible smoke or fire. The crew evacuated passengers via the main cabin door, and buses transported everyone to the terminal.
Engine Examination
The engine was removed and shipped to Pratt & Whitney Canada (PWC) in St. Hubert, Quebec, for disassembly. A section of the small exit duct (SED) within the combustion section had separated and been ingested into the gas path. Internal hardware aft of the combustion section showed thermal damage and sooting consistent with airflow disruption. The intact SED section was examined, revealing improper weld penetration between the SED outer dome and the combustion housing inner duct, per drawing specifications.
The advanced pneumatic detection (APD) fire loop exhibited thermal distress with deformed internal diaphragm switches, causing a latched fire warning. Kidde Aerospace made design improvements, and Bombardier released service bulletins (SBs) 84-26-08A, 84-26-09A, and 84-26-12A for replacement with updated models. Airworthiness directives from Transport Canada and the FAA mandated the replacement.
A quality investigation into the improper weld penetration identified an inadvertent adjustment to the electron beam (EB) current level during manufacturing as the probable cause. A lot of five SED assemblies were potentially affected; two, including the one in this engine, experienced in-flight failures, and the remaining three were removed for repair.
Engine serial number PCE-FA0184 had accumulated 18,764 hours time since new, 19,351 cycles since new, 4,244 hours time since repair, and 4,391 cycles since repair. The PWC PW150A engine is a three-spool free-turbine turboprop with a three-stage axial compressor, centrifugal compressor, reverse flow annular combustor, and two-stage power turbine driving an offset reduction gearbox.
Disassembly revealed sooting on external case surfaces, concentrated aft. Flexible lines showed thermal distress. Borescope examination confirmed SED separation. The 1st stage turbine nozzle vanes exhibited hard particle damage. Upon receipt at PWC, the engine was intact without case breaches. Soot concentration was heaviest aft, lighter forward. The ITT harness showed charring and blistering 360 degrees. The oil pressure supply line to No. 7 bearing was thermally damaged. Magnetic chip detector plugs showed only normal debris. The reduction gearbox and accessory gearbox were not disassembled due to no evidence of internal damage.
The high pressure turbine vane assembly had pieces of the SED outer dome separated and ingested. The inner duct exhibited thermal distress and burn through at approximately the 11 o'clock position. The HPT disk assembly had impact damage and metal splatter. Low pressure and power turbine components showed sooting and metal spray. Engine bearings were intact and rotated freely.
Laboratory Results
Metallurgical examination of the intact SED outer dome at three locations showed insufficient weld penetration; welds were contained within the outer dome and did not penetrate to the inner duct as required. Chemical analysis of white residue on external surfaces identified silicon dioxide, a byproduct of high-temperature silicone exposure to hot gases or fire.
Additional Information
Engine Monitoring Unit and Quick Access Recorder data confirmed a surge event and subsequent flameout. In December 2012, a similar SED failure occurred on an All Nippon Airways PW150A engine. PWC's internal investigation initially concluded low EB current was the cause. A third failure on a Philippines Airlines Express engine in September 2013 prompted a re-investigation, which identified variation in EB cathode filament tip radius and filament installation position as possible factors; the exact cause remained unknown. The manufacturer later implemented x-ray computed tomography scans on all SED assemblies and replaced the EB weld machine.
The APD fire detection system on the incident aircraft suffered diaphragm switch deformation from prolonged high heat, preventing reset. Improved APD systems were developed and mandated by airworthiness directives.