History of Flight
On April 4, 2016, at approximately 1610 eastern daylight time, a Bell 206L helicopter, registration N16760, was destroyed when it impacted terrain following a loss of engine power while maneuvering near Pigeon Forge, Tennessee. The helicopter, registered to a private individual and operated by Great Smoky Mountain Helicopters, Inc., doing business as Smoky Mountain Helicopters, was conducting a local air tour flight under Title 14 Code of Federal Regulations Part 91. The commercial pilot and four passengers sustained fatal injuries. Visual meteorological conditions prevailed, and no flight plan was filed. The flight had departed Sixty Six Heliport (6TN3) in Sevierville, Tennessee, about 1600.
According to the operator, the helicopter had been purchased in 1986 for air tour purposes. The operator owned two helicopters at the time: a Bell 206B based in Cherokee, North Carolina, and the accident helicopter, based at 6TN3.
A company pilot reported that he flew the helicopter on approximately 10 local sightseeing flights on the morning of the accident. The accident pilot took over between 1300 and 1400 and completed five sightseeing flights before the accident flight. The company pilot checked the fuel level before the accident flight and reported it was "just below the 6-inch line," corresponding to about 300 pounds of fuel.
The pilot of another helicopter operating in the area reported that he did not hear any distress calls but heard the accident pilot make a normal landmark position report over "wonderworks."
A witness near the accident site reported observing the helicopter at a low altitude in a descent and that it "didn't sound right." The witness described the sound as if "the engine was wound tight" and it "lost the rotor sound." The engine then went silent, "as if the pilot cut the power," followed by sounds of impact. Another witness reported hearing the impact and seeing the accident site engulfed in fire.
Personnel Information
According to FAA records, the pilot held commercial pilot and flight instructor certificates with ratings for rotorcraft-helicopter and instrument-helicopter. He reported 550 hours of total flight experience, with 300 hours in the preceding 6 months, on his most recent FAA second-class medical certificate application, issued April 21, 2015.
Company records indicated the pilot was hired in April 2015. He completed a factory Bell Helicopter 206L pilot transition course on April 10, 2015, and received a logbook endorsement for satisfactory completion of a flight review in accordance with 14 CFR section 61.56 on that date. His logbook showed about 1,310 hours of total flight experience as of March 25, 2016, including about 875 hours in Bell 206-series helicopters. The operator's log indicated the pilot flew the accident helicopter on several occasions between March 25 and the accident, but the accumulated flight time could not be determined.
Aircraft Information
The seven-seat helicopter was manufactured in 1977 and issued an FAA standard airworthiness certificate in the normal category on March 1, 1977. It was equipped with a two-blade main rotor system and a two-blade tail rotor system powered by a 420-horsepower Rolls-Royce (formerly Allison) 250-C20B turboshaft engine.
At the time of the accident, the helicopter had accumulated about 22,562 total hours, and the engine had accumulated about 8,550 total hours. The helicopter had been operated for about 40 hours since its most recent 100-hour and annual inspections, signed off concurrently on March 4, 2016.
Engine records indicated that the engine fuel pump, model number 386500-5, serial number T103542, manufactured by TRW, Inc. (later Argo-Tech and Eaton Corporation), was installed on June 23, 2009, at an engine total time of 7,472.0 flight hours. The fuel pump had 0 flight hours since overhaul at installation. Based on the engine total time at the accident, the fuel pump had accumulated about 1,078 flight hours since its last overhaul. According to the Rolls-Royce M250-C20 series maintenance manual, the fuel pump had an overhaul interval of 4,000 hours. The last overhaul was performed at International Governor Services (IGS) in Broomfield, Colorado, in accordance with Argo-Tech Component Maintenance Manual (CMM) No. 73-10-10, revision 0, dated November 2000. Signed-off inspections included dimensional checks, fluorescent penetrant inspection of filter housings and pump cover, and magnetic particle inspection of drive shafts and gears. No anomalous findings were recorded.
Wreckage and Impact Information
The helicopter impacted trees near the top of a ridge at about 1,100 ft mean sea level (msl). The main wreckage came to rest on its left side on a heading of about 340° magnetic in a wooded area near the bottom of the ridge. An initial tree strike was identified about 405 ft south of the main wreckage at an elevation of about 1,100 ft msl. Pieces of Plexiglas and the front-left piece of skid tube were found near the initial tree strike. The debris path extended down to the main wreckage, which was mostly consumed by a postcrash fire. All major structural components were located at the site.
Examination of the caution and warning panel (CWP) via X-ray showed no illuminated bulbs at impact. Manual rotation of the main rotor blades in the direction opposite normal rotation resulted in rotation of the main rotor shaft, drive shaft, engine No. 4 turbine wheel, and tail rotor drive output shaft. Freewheeling functionality was confirmed. The flexible couplings between drive shaft flanges were intact with no evidence of power at impact. The tail rotor gearbox remained mounted to a section of tail boom; manual rotation of tail rotor blades resulted in gearbox input rotation. Chip detectors from the main transmission and tail rotor gearbox showed no large ferrous particles.
Both main rotor pitch links were fractured about mid-length with overload signatures. Control linkages were attached but fractured and thermally distressed. The hydraulic pump remained mounted to the main transmission. The collective control stick and jackshaft were loose with thermal distress. The cyclic stick was not recovered; its base was loose with thermal distress. Due to impact and fire damage, seats and restraint systems could not be evaluated.
Engine Examination
The engine remained installed and exhibited crush damage to the left casing and exhaust port. The first stage compressor showed no hard or soft body impact damage but had sooting. No scoring was observed between compressor blades and blade track. The 4th stage turbine wheel blades had no visible impact damage. The gas producer fuel control indicator was in the 0° ("cut off") position. Disassembly of the core engine (compressor, combustion, turbine sections, and accessory gearbox) revealed no anomalous damage that would have precluded normal engine operation.
Fuel Pump Examination
The engine fuel pump was removed and disassembled. The drive shaft small splines exhibited severe damage with significant material loss. The drive shaft spacer showed thermal distress and indentations consistent with contact with the drive gear internal splines. Remnant pieces consistent with the retaining clip were later identified in photographs. The drive gear internal splines also exhibited severe damage. Hardness and material composition of the drive shaft and drive gear met specifications. Remnant material on the splines was consistent with mineral-oil-based/soap-thickened lubricants.
The drive shaft spacer was consistent with a P/N 215981-8 spacer, about 0.240 inches thick. The original build record from 1985 indicated a 0.120-inch spacer. Records of previous overhauls were not available, and it could not be determined whether other spacer sizes were used or if the drive shaft, drive gear, and driven gear were original.
Additional Information
Discussions with IGS personnel revealed that they had seen drive shaft spline wear similar to that observed on the accident pump, but not to the same severity. An IGS mechanic stated he had seen such wear about 4 or 5 times since 2008, often associated with lack of lubricant or corrosion. He noted wear only on the smaller splines of the drive shaft. Fuel pumps submitted for overhaul are visually examined for spline or gear tooth wear; if observed, the part is rejected.
The helicopter was not required to meet improved fuel system crash resistance standards (14 CFR 27.952) introduced in 1994. The Bell 206L type certificate predated these standards. The original fuel system consisted of thin-wall bladder-type fuel cells not drop-tested. A retrofit kit (Service Instruction No. BHT-206-SI-2043) introduced in 1994 offered improved fuel bladders, stainless steel braided hoses, and frangible attachments, but no record of modification was found.
The FAA Rotorcraft Flying Handbook describes autorotation as a descending maneuver where the engine is disengaged from the main rotor system. The Bell 206L rotorcraft flight manual specifies that in the event of engine failure, an autorotation should be performed by adjusting collective to maintain rotor speed of 90-107% rpm and cyclic to obtain desired airspeed, with a normal autorotation airspeed of 70 mph (61 knots).
Flight Recorders
The helicopter was not equipped with any crash-resistant data or image recorders, nor was it required to be.
Medical and Pathological Information
Autopsies performed by the Regional Forensic Center Knox County indicated that all five occupants suffered blunt force injuries. Three died primarily from thermal injuries, one from combined thermal and blunt force injuries (pilot), and one from blunt force injuries. Toxicological testing on the pilot was negative for all tested drugs.