Sevierville, TN, USA
N3176L
BELL 206
The commercial pilot was departing in the helicopter for a local aerial sightseeing flight with five passengers. The engine was operating during the boarding process. After the passengers were seated and had fastened their seat belts, the pilot increased the engine power to 100%, raised the collective, and felt the helicopter shudder. He lowered the collective and looked to make sure all passengers were seated. He raised the collective a second time, again felt the shudder, lowered the collective, and then heard "a loud bang." He subsequently turned off the fuel to shut down the engine. A fire began in the engine compartment, and ground personnel helped evacuate the passengers and extinguish the fire. Multiple fragments of engine turbine section components were found resting in the engine bay, on the ground around the helicopter, and embedded in the bottom surfaces of the main rotor blades. Postaccident examination of the engine revealed that the power turbine shaft had decoupled due to a No. 4 bearing failure. Carbon buildup was found on the filter screen of the No. 4 bearing's oil supply nozzle that partially obstructed oil flow to the bearing and likely resulted in insufficient lubrication of the bearing. The No. 4 bearing inner race spun on the bearing journal instead of remaining stationary as designed. The turbine rotates at extremely high speeds, and the resistance from the failed bearing caused the power turbine pinion splines to decouple, leading to an instantaneous power turbine overspeed. The overspeed led to a third stage turbine disk burst and radial uncontainment of fragmented power turbine components through the exhaust support and airframe. All turbine component fractures were due to overload failure, and there was no evidence of fatigue. Therefore, it is likely that the uncontained engine failure was the result of insufficient lubrication of the No. 4 bearing due to carbon buildup on the oil supply nozzle's filter screen.
HISTORY OF FLIGHT On February 15, 2015, about 1500 eastern standard time, a Bell 206 L-1, N3176L, was substantially damaged by an uncontained engine failure and fire during takeoff from the Sixty Six Heliport (6TN3), Sevierville, Tennessee. The commercial pilot and five passengers were not injured. Visual meteorological conditions prevailed, and no flight plan was filed. The helicopter was registered to a private individual and was operated by Great Smoky Mountains Helicopter, Inc., for the local aerial sightseeing flight, which was conducted under the provisions of 14 Code of Federal Regulations Part 91. According to the pilot, the helicopter had been operating since about 0830, and the engine was operating during the boarding of the five passengers. After the passengers were seated and had fastened their seat belts, he increased the engine power to 100%, raised the collective, and felt a shudder through the airframe. He immediately lowered the collective, verified that the passengers were in their seats, and again raised the collective. Immediately, he again felt a shudder, lowered the collective, subsequently heard "a loud bang followed by a loud whine" and saw smoke. He attempted to roll off the throttle, but it would not move. He subsequently turned off the fuel to shutdown the engine. Ground personnel helped evacuate the passengers and extinguish the fire. According to photographs provided by a Federal Aviation Administration inspector who examined the helicopter, the upper deck and engine had substantial thermal and impact damage. The main rotor blades exhibited dents, score marks, and punctures on their bottom surfaces. Multiple fragments of engine turbine section components were found resting in the engine bay, on the ground around the helicopter, and embedded in the rotor blades. PERSONNEL INFORMATION The pilot, age 34, held a commercial pilot certificate with ratings for rotorcraft-helicopter and instrument helicopter; he also held a second-class medical certificate issued January 26, 2015, with no limitations. The pilot reported 1,385 total flight hours with 859 hours in the accident helicopter make and model. AIRCRAFT INFORMATION The seven-seat helicopter, serial number 45648, was manufactured in 1983. It was powered by a 500-shaft-horsepower Allison 250-C28B engine. According to maintenance records and pilot-provided information, the most recent 100-hour inspection was completed on February 2, 2015, at a recorded airframe total time of 8,586.1 hours and an engine time since major overhaul of 854.9 hours. At the time of the accident, the engine had accumulated 12.5 hours since the most recent inspection, 867.4 hours since major overhaul, 8,204.4 hours since new, and 6,633 cycles since new. METEOROLOGICAL INFORMATION The 1455 recorded weather observation at Gatlinburg-Pigeon Forge Airport, Sevierville, Tennessee, located about 3 miles from 6TN3, included wind from 020° at 7 knots, 10 miles visibility, clear skies, temperature minus 4°C, dew point minus 22°C, and an altimeter setting of 30.32 inches of mercury. AIRPORT INFORMATION The private helipad was owned by the operator and did not have an operating control tower. The turf helipad was 200 ft long by 200 ft wide and was about 1,010 ft above mean sea level. TEST AND RESEARCH Postaccident examination of the engine revealed that a majority of the exhaust collector support was missing along the top half of the engine, and the remaining sections exhibited punctures and tears. The compressor air discharge tubes had multiple penetrations and were missing material. The power turbine 3rd and 4th stage disks (wheels) and nozzles as well as the power turbine shaft and coupling had separated from the engine. All the 4th stage turbine blades were separated at the blade roots, and the disk hub had substantial impact damage. The 4th stage nozzle was fractured into three 120° sections. One 120° section of the 3rd stage disk was recovered near the helicopter, and all blades were separated at the blade roots. The remainder of the 3rd stage disk was not located. The 3rd stage nozzle was fractured into two pieces. The power turbine coupling splines exhibited thermal damage and spline deformation. The No. 4 bearing race surfaces and rollers were plastically deformed and thermally damaged. The aft end of the power turbine pinion gear exhibited rub wear 360° around, consistent with the No. 4 bearing inner race spinning on the pinion surface that functions as a bearing journal. Multiple components exhibited carbon buildup including the filter screen of the oil supply nozzle that supplied lubricating oil to the Nos. 4 and 5 bearings, the No. 8 bearing sump in the gas producer support, and the power turbine shafting. The turbine components were examined by the Rolls Royce Materials Lab under NTSBsupervision. The examination indicated that the turbine components that separated from the engine during the failure sequence failed due to overload, and there was no evidence found of fatigue.
An uncontained engine failure, which resulted from insufficient lubrication of the No. 4 bearing due to carbon buildup on the filter screen of the bearing's oil supply nozzle.
Source: NTSB Aviation Accident Database
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