N3176LBell Helicopter Textron 206L-1 · 1983

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.

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.

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.

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.

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.

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.