Frederick, MD, USA
N208JW
CESSNA 208
During the landing roll, with the power lever at idle, the pilot heard a “pop,” followed by indications of the engine “spooling down.” He exited the runway and observed smoke coming from the engine compartment. Thinking this was residual fuel, he “spooled the engine.” Realizing there was a fire, he called for assistance and shut off the fuel selectors and firewall shutoff valve. After the accident, a FAA inspector examined the airplane and reported extensive thermal damage to the fuselage sub-frame and evidence of an engine fire. Numerous engine turbine blades were found on the pavement adjacent to where the fire occurred. A disassembly and inspection of the engine revealed that one or more of the compressor turbine (CT) blades failed. The engine was overhauled about 6 hours prior to the accident. The CT blades were manufactured under the FAA Parts Manufacture Approval (PMA) program. Extreme heat damage obliterated the fracture surfaces of the blades, therefore the reason for the blade failure(s) could not be determined. A functional test of the fuel control unit (FCU) revealed the Sg (specific gravity) dome, high idle stop screw, and minimum flow stop were set above the recommended range, which would result in higher than normal fuel flow. The FCU adjustments appeared to be made after the overhaul, however it could not be determined when the adjustments occurred. Although the higher than normal fuel flow setting probably did not result in the CT blade failures, it could have exacerbated the thermal damage and fire.
On April 21, 2009, about 1355 eastern daylight time, a Cessna 208, N208JW, powered by a single Pratt & Whitney Canada (P&WC) PT6A-114 turbo-propeller engine, was substantially damaged following an engine fire after landing at Frederick Municipal Airport (FDK), Frederick, Maryland. The airplane was registered to and operated by Greystoke Engineering, Inc. The certificated airline transport pilot was not injured. Visual meteorological conditions prevailed at the time, and no flight plan was filed for the 14 Code of Federal Regulations Part 91 ferry flight. The flight originated at Winchester Regional Airport (OKV), Winchester, Virginia, at 1330. The pilot reported that he landed on runway 23 at FDK following a normal flight. During the landing roll, with the power at idle, he heard a “pop” and the engine “started spooling down.” He exited the runway and observed smoke coming from the engine compartment. Thinking this was residual fuel, he advanced the power lever. He did not observe a master caution or engine fire light. When the smoke became worse, he realized that there was a fire. He called for assistance and shut off the fuel selectors and firewall shutoff valve. The airport line crew arrived and extinguished the fire. An inspector from the Federal Aviation Administration (FAA) visited the accident site and examined the airplane. He reported that there was extensive thermal damage to the fuselage sub-frame. A preliminary inspection of the engine revealed heat damage in the turbine section. Numerous turbine blade fragments were found on the runway where the fire occurred. The airplane owner provided a photograph of the interior of the exhaust duct showing a crescent-shaped witness mark approximately 3/16 inches in length. According to the owner, the witness mark was consistent in size and shape with a compressor turbine (CT) blade tip sample at his facility. At the request of the airplane owner, the engine was sent to Mint Turbines, Stroud, Oklahoma, for disassembly and inspection. The inspection was conducted by the NTSB investigator-in-charge (IIC), assisted by a powerplants investigator from the NTSB Office of Aviation Safety Aviation Engineering Division. Representatives from Cessna, Pratt and Whitney Canada (P&WC), Greystoke Engineering, and Mint Turbines LLC also participated. An external examination of the engine revealed that the accessory gearbox and accessories were covered with soot, but no thermal damage was noted. The turbine exhaust duct exhibited heat discoloration (case bluing) and metal splatter. Examination of the combustion chamber and large exit duct did not exhibit a burn-through or any hot streaks. No cracks were noted. The fuel nozzle shrouds were coated in black soot. The fuel nozzle orifice appeared unblocked and clean. The igniter plugs were intact and exhibited slight sooting on the tips. No wear around the tips was noted. Examination of the compressor turbine (CT) revealed that all CT blades were melted (thermally damaged) down to the blade platform and the blade roots remained secured to the disk. The CT disk was intact. Slight circumferential scoring was noted on the aft side of the disk and the outer diameter of the bore (spigot) consistent with contact with the power turbine (PT) guide vane inner drum baffle cup. All of the CT guide vanes were present and exhibited trailing edge thermal distress and missing material while the leading edges were undamaged. The outer shroud inner air flow surface of the CT guide vane was heavily coated with metal spray. The CT rotor was turned by hand and no grinding sounds were noted. The P&WC representative reported that the CT blades were manufactured under FAA Parts Manufacture Approval (PMA) and were not P&WC-manufactured parts. Examination of the power turbine (PT) revealed that all of the PT blades were melted (thermally distressed) with the outer half of the airfoil missing, the blade roots still secured to the disk, and molten material on and in between the blades. The remaining part of the PT blade airfoils were bent in the direction opposite of rotation. The PT disk was intact and exhibited circumferential rubbing on the outer diameter of the aft bore (spigot). All PT guide vanes were melted and missing their airfoil with only the outer shroud remaining. All of the T5 temperature probes were thermally damaged and missing their tips. The inner diameter of the exhaust duct was heavily coated with metal spray. The exhaust duct exhibited no bulging or other evidence of uncontainment. The containment ring appeared intact with no outward bulging evident. The PT was turned by hand and the propeller shaft moved along with it confirming continuity between the PT and the propeller shaft. The reduction gearbox chip detector remained safety-wired. When removed, the detector was clean. The fuel control unit (FCU) was removed and shipped to PWC, Longueuil, Quebec for examination and testing. An inspector with Transport Canada provided government oversight of the examination. There were no defects or damage evident that would have prevented normal operation of the FCU prior to the event. High fuel flow was noted during the testing of the FCU. The Sg (specific gravity) dome was found to be adjusted to a setting beyond the recommended range, which would result in higher than normal fuel flow. Adjustment of the high-idle stop screw and the minimum flow stop was also detected. Separation of the anti-tamper paint was noted on the high idle stop and the anti-tamper lock-wire seal was missing on the minimum flow stop. The CT disk and recovered CT blades were shipped to the NTSB Materials Laboratory for examination. Visual examination of the CT disk revealed that all blades fractured near the root. Bench binocular microscope examination of the blades revealed that the fracture faces were obliterated by severe heat damage. Scanning electron microscope examination of one of the fractured blades verified the severe heat damage. Due to the severe heat damage, the determination of which, if any, blades had a pre-existing crack was not possible. Maintenance records revealed that at the time of the accident, the engine had accumulated approximately 6 hours since overhaul. The overhaul was accomplished at Northstar Aerospace, Stroud, Oklahoma on September 18, 2008 and the engine was installed on N208JW at Air Services, Inc., Carrollton, Georgia on January 23, 2009. Subsequent to the accident, Northstar Aerospace was purchased by Mint Turbines LLC. The airplane owner is the Managing Director of Air Services, Inc. An examination of the airplane maintenance records, overhaul facility test cell records, and inquiries to the airplane owner and overhaul facility personnel did not reveal the source of the FCU adjustments. The 1338 weather observation for FDK included the following: scattered clouds at 4,300 feet, surface winds calm, 10 statute miles visibility, temperature 20 degrees Celsius, dew point 10 degrees Celsius, and an altimeter setting of 29.70 inches of mercury.
A loss of engine power due to the failure of one or more compressor turbine blades for an undetermined reason.
Source: NTSB Aviation Accident Database
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