History of Flight
On July 6, 2018, at approximately 1720 eastern daylight time, a Bell 47G-2 helicopter, registration N96195, was substantially damaged when it struck a corn field near Arlington, Indiana. A postimpact fire ensued, and the commercial pilot was fatally injured. The helicopter was owned and operated by Central Indiana Ag Services, LLC, operating under Title 14 Code of Federal Regulations Part 137 for aerial application. Visual meteorological conditions prevailed, and no flight plan was filed. The flight had departed at about 1655 from a loading platform on a farm located approximately 1 mile northwest of the accident site.
According to the co-owner of the helicopter, the pilot had been flying since 0945 that morning and had completed between 10 and 15 spray runs. The pilot was applying a fungicide to corn crops, with each run averaging 20 minutes. When the pilot did not return after 30 minutes, search operations were initiated. The wreckage was located later that evening.
Data from a Lowrance AIRMAP 2000c device installed on the helicopter provided GPS latitude and longitude information for the accident flight, but altitude and time data were not recorded. The last data point of the final flight track corresponded to the wreckage location. No witnesses to the accident were identified.
Aircraft Information
The helicopter had an annual inspection completed on February 15, 2018, at an airframe total time of 13,922 hours. The Hobbs meter at the scene read 323.8, and the helicopter had flown approximately 26.8 hours since the last inspection, resulting in a total airframe time of 13,948.8 hours. On July 9, 2009, the main rotor transmission was replaced. A maintenance release card dated June 24, 2009, indicated that the fan drive quill and clutch assembly were repaired with serviceable parts following Bell 47G-2 procedures, and the transmission was tested and reinstalled per manufacturer publications and FAR Part 43.
Available maintenance records contained only two entries for routine annual/100-hour inspections between July 2009 and January 2014. A log entry dated November 1, 2014, noted "retorqued transmission T adapter plat bolts" during a 100-hour inspection at an airframe total time of 13,875.2 hours. The mechanic who performed this work was unavailable for interview. No other entries referenced the main rotor transmission.
The day before the accident, the co-owner and the pilot performed general maintenance on the helicopter, but this did not involve the main rotor transmission, according to the co-owner.
The Illustrated Parts Breakdown for the Bell 47G-2 specifies that the main rotor transmission housing attachment bolts consist of six AN5-37A bolts, one AN5-41A bolt, and one AN5-44A bolt. These bolts are installed with specific washers and NAS679A5 nuts.
Wreckage and Impact
The wreckage was located at the south end of a corn field at an elevation of 850 feet. The initial impact point was marked by crop damage, with a 20-foot-long swath up to 6 feet wide. The helicopter impacted on an approximate bearing of 002° and came to rest inverted about 20 feet from the initial impact point. The main wreckage included the fuselage, landing skids, engine, lower transmission assembly, tail rotor, and tail boom. A postimpact fire damaged the left side of the fuselage and engine.
The upper portion of the helicopter, including both main rotor blades, the mast, controls, swash plate, and the upper transmission, separated and was found about 75 feet north of the main wreckage, remaining as an assembly. The lower transmission remained attached to the engine, with internal gears rotating freely when actuated. A large scar was observed inside the transmission well.
Examination of the lower transmission housing mounting holes (labeled 1 through 8) revealed damage: holes 1 and 7 were torn and partially separated; holes 2 and 3 were unremarkable; holes 4 and 5 were elongated; holes 6 and 8 were elongated with tearing on the outer edges. No fasteners remained in any of the holes.
The upper transmission portion had corresponding attachment locations. Bolts 1 and 7 remained inserted with nuts and metal attached; bolts 2 and 3 remained inserted without nuts; bolts 4, 5, and 6 were partially inserted without nuts; bolt 8 remained inserted, bent at the thread end, with no nut.
Examination of Transmission Bolts
The upper transmission assembly and eight attachment bolts were sent to the NTSB Materials Laboratory. Washers were observed under the heads of bolts 1, 2, 7, and 8, and also with bolt 4, but not with bolts 3 and 5. Bolts 1 and 7 had nuts remaining and two washers under each nut. The thickness of most washers was approximately 0.060 inch (AN960-516), except for the washer under the head of bolt 7, which was about half that thickness (AN960-516L).
On the lower surface of the freewheeling gear, slight fretting and material transfer were noted near bolt holes 3 through 8. The lower edge of bolt hole 5 was deformed inward.
The attachment bolts had varying numbers of intact and sheared threads. Bolts 1 and 7 had all threads intact. Bolts 2 through 6 and 8 had 1 to 5 intact threads adjacent to the grip, with flattened and smeared thread peaks, consistent with contact with the lower housing bore. The remaining threads were fractured near the roots with smeared surfaces, consistent with shear fracture from nut contact. Sheared bolt threads remained trapped in the separated nuts.
Close examination of bolts 2 and 3 showed intact but damaged threads between the shank and sheared threads, with pressure flanks deformed and missing. Circumferential sliding contact marks were observed. Bolt 5 showed circumferential marks on pressure flanks but no substantial deformation. Bolt 4 showed no marks.
Dimensional measurements on intact threads of bolts 2 through 5 found that the major diameters of bolts 2, 3, and 5 were less than the minimum specified for a new bolt, while bolt 4 was near the lower limit. Thread profile measurements for bolts 3, 4, and 5 were generally comparable to specification, but bolt 2 showed larger deviations: average flank angle about 11° less than specified 60°, average thread depth 0.0055 inch less than specified, and average thread peak width about double the maximum. The thread peaks on bolt 2 had a gold hue consistent with original plating, indicating that the variations were not due to wear or damage.
Medical and Pathological Information
An autopsy was performed on the pilot by Central Indiana Forensic Associates, LLC, on December 12, 2008, as requested by the Shelby County Coroner's Office. The cause of death was listed as "positional asphyxia and left-sided rib fractures," with the manner ruled an accident. Toxicological tests conducted by the FAA Forensic Sciences Laboratory were negative for all tested substances.
Performance Study
A performance study of five previous flights using data from onboard recording devices indicated that the helicopter maintained altitudes between 100 and 150 feet above ground level and groundspeeds between 40 and 60 knots. Calculated load factors varied from about 1.20 g to 1.45 g, within normal flight load factors for most helicopters (2 to 2.5 g).