History of Flight
On September 18, 2015, at about 1700 Alaska daylight time, a Robinson R-44 Astro helicopter, registration N7162R, experienced a separation of the tail rotor gearbox while hovering over a remote bay on Umnak Island, approximately 24 miles southwest of Fort Glenn, Alaska. The commercial pilot, who was the sole occupant, sustained no injury. The helicopter was registered to 62 Romeo, LLC, of Van Nuys, California, and operated by Bering Pacific Ranches, Ltd., of Calgary, Alberta, Canada, as a Title 14 Code of Federal Regulations Part 91 visual flight rules aerial mustering flight. Visual meteorological conditions prevailed at the accident site, and no flight plan was filed for the local flight, which originated from the operator's base at Fort Glenn at 1500.
The pilot reported that after departing Fort Glenn and traveling about 30 miles, he arrived at Russian Bay. The aerial mustering operation involved cattle in a flat valley of marsh and sandbars. While in a hover between 10 to 15 feet above ground level behind a group of cattle over a flat gravel sandbar, he heard metallic grinding noises from behind his position in the front right seat. The pilot stated that the helicopter was stable with engine power, rotor rpm was within normal limits, and he had normal responding controls and tail rotor authority. He immediately landed on the sandbar, completed the shutdown process, and exited without further incident. During a postflight inspection, he observed that the tail rotor gearbox had separated from the tailboom attachment point and was on the ground. A second company helicopter retrieved the pilot and returned him to Fort Glenn.
Damage and Examination
The helicopter sustained substantial damage to the tailboom and tail rotor system. One main rotor blade also sustained substantial damage. Post-accident photographs showed the tailboom attached to the fuselage, and the vertical stabilizer attached to the tailboom with the tail rotor system guard and skid intact. The helicopter was recovered, though the tips of the two tail rotor blades were not recovered. The tailboom, its mounting flanges, the tail rotor gearbox, and the two tail rotor blades were sent to the NTSB Materials Laboratory for examination.
At the laboratory, the tail rotor gearbox was submitted with three of its four mounting flanges separated—both top flanges and the bottom port (left) flange. The separated top flanges were submitted with the mating bolts still assembled. The bolt from the separated bottom flange was also submitted, but the flange was not recovered. The tailboom was submitted with the other ends of the top and bottom bolts from the port side still assembled. All bolts had corrosion on fracture surfaces; those still assembled inside the tailboom were most heavily corroded. The bolt from the remaining attached bottom starboard (right) flange was removed for examination. Fracture surfaces were relatively flat with directional smearing. After cleaning, the starboard bolt was examined with a scanning electron microscope; features were consistent with shear overstress (microvoid coalescence). Light fret damage was observed on mating surfaces of gearbox flanges and tailboom, mostly obscured by secondary contact damage and corrosion.
Both tail rotor blades showed extensive deformation, and their tips had separated. Deformation, fracture paths, and surface features were consistent with overstress. Red marks near the separations were likely paint transfer from an object struck while rotating. No fatigue cracks (per Robinson Service Bulletin SB-83) were found. Black light examination revealed small fluorescent spots on leading edges consistent with insect debris; no snarge or evidence of bird strike was observed.
Additional Information
A review of airframe maintenance records showed an annual inspection on July 21, 2015, at 2,490.5 total hours, with no preexisting malfunctions or failures noted for the tailboom, tail rotor drive shaft, gearbox, or blades. The helicopter was not equipped with an emergency locator transmitter or a skid-based emergency floatation system, neither of which was required.
The R-44 Pilot's Operating Handbook describes the tail rotor system as having two all-metal blades and a teetering hub, with the tail rotor drive shaft having a lightly-loaded damper bearing. Emergency procedures indicate loss of tail rotor thrust is usually indicated by nose-right yaw uncorrectable with left pedal. The manufacturer confirmed that even aggressive pedal inputs will not cause tail rotor blades to contact the airframe short of component failure.
Title 14 CFR Part 27.661 requires sufficient clearance between rotor blades and other structure to prevent striking during any operating condition.