History of Flight
On February 15, 2015, at about 1500 eastern standard time, a Bell 206 L-1 helicopter, registration N3176L, was substantially damaged during takeoff from the Sixty Six Heliport (6TN3) in Sevierville, Tennessee. The aircraft experienced an uncontained engine failure and subsequent fire. The commercial pilot and five passengers were not injured. Visual meteorological conditions prevailed, and no flight plan was filed. The helicopter was operated by Great Smoky Mountains Helicopter, Inc., for a local aerial sightseeing flight under 14 CFR Part 91.
According to the pilot, the helicopter had been operating since about 0830, and the engine was running during passenger boarding. After the passengers were seated and had fastened their seat belts, he increased engine power to 100% and raised the collective, feeling a shudder through the airframe. He lowered the collective, verified passenger safety, and again raised the collective. Another shudder occurred, followed by a loud bang and a whine; smoke appeared. The pilot attempted to roll off the throttle, but it would not move. He then turned off the fuel to shut down the engine. Ground personnel assisted in evacuating the passengers and extinguishing the fire.
Postaccident photographs showed substantial thermal and impact damage to the upper deck and engine. The main rotor blades had dents, score marks, and punctures on their bottom surfaces. Multiple fragments of turbine section components were found in the engine bay, on the ground, 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, and a second-class medical certificate issued January 26, 2015, with no limitations. He 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. The most recent 100-hour inspection was completed on February 2, 2015, at an airframe total time of 8,586.1 hours and engine time since major overhaul of 854.9 hours. At the time of the accident, the engine had accumulated 12.5 hours since the last inspection, 867.4 hours since major overhaul, 8,204.4 hours since new, and 6,633 cycles since new.
Meteorological Information
The 1455 weather observation at Gatlinburg-Pigeon Forge Airport, about 3 miles from the heliport, reported 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 had no control tower. The turf helipad was 200 ft long by 200 ft wide and about 1,010 ft above mean sea level.
Test and Research
Postaccident examination of the engine revealed that most of the exhaust collector support was missing along the top half, with punctures and tears in remaining sections. The compressor air discharge tubes had multiple penetrations and material loss. The power turbine 3rd and 4th stage disks and nozzles, along with the power turbine shaft and coupling, had separated from the engine. All 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, with all blades separated at the roots; the rest was not located. The 3rd stage nozzle was fractured into two pieces. The power turbine coupling splines showed thermal damage and 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. Multiple components had carbon buildup, including the oil supply nozzle filter screen to the Nos. 4 and 5 bearings, the No. 8 bearing sump, and the power turbine shafting.
Examination of turbine components by the Rolls Royce Materials Lab under NTSB supervision indicated that the separated turbine components failed due to overload, with no evidence of fatigue.