Accident Details
On January 1, 2012, about 1400 central standard time, a Continental Copters Inc. 47G2 helicopter, registration N9021R, impacted terrain during a forced landing following a loss of tail rotor effectiveness near Lohn, Texas. The helicopter was operated by Sky Horse Helicopters Inc. under 14 Code of Federal Regulations Part 91 as an aerial observation flight. Day visual flight rules conditions prevailed, and the flight did not operate on a VFR flight plan. The helicopter had departed on a local flight about 1300.
Pilot Report
The pilot reported that he performed a preflight inspection, including checking the tail rotor oil level through its sight glass. After flying for over an hour, the helicopter yawed slowly to the left, then yawed rapidly. The pilot cut the throttle and flew the helicopter to the ground as fast as possible.
Aircraft Information
The helicopter’s last annual inspection was completed on July 13, 2011, at which time the airframe had accumulated 8,989 total flight hours. At the time of the accident, the helicopter had accumulated 9,057 total flight hours. The pilot stated he had no records regarding the tail rotor gearbox.
Examination Findings
A Federal Aviation Administration inspector examined the wreckage. The tail rotor gearbox did not operate when rotated, and the gearbox shim measured 0.015 inches thick. The gearbox (part no. 47-640-044-39, serial no. A13-3661) was shipped to Scott’s – Bell 47 Inc. for detailed examination under FAA supervision. Disassembly revealed interior surfaces covered with a black oily substance consistent with residual lubricating oil contaminated with liberated metal particles. The oil level sight glass showed no discoloration or obstruction. Damage was noted on the upper aft portion of the main case assembly, where an area was filled with an epoxy-like material. The ring and pinion gears were examined at the NTSB Materials Laboratory. The pinion gear teeth were deformed and worn nearly to the tooth root in some locations. The ring gear showed wear on the top land and drive faces, and wear patterns were present on the coast faces of both gears. Hardness testing indicated increasing hardness with decreasing distance from the bottom land surface.
The commercial pilot and passenger sustained serious injuries. The helicopter’s tailboom was substantially damaged during the forced landing.