History of Flight
On May 24, 2012, at approximately 1830 eastern daylight time, a Schweizer 269C helicopter, registration N760DA, was substantially damaged during a hard landing following a loss of tail rotor effectiveness. The accident occurred in Piney Flats, Tennessee. Both the certificated commercial pilot and his pilot-rated passenger were not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the local personal flight, which was conducted under 14 CFR Part 91.
The pilot reported that the flight originated from his farm in Piney Flats. After departing for a local sightseeing flight with a friend, they performed a visual approach and landing at Elizabethton Municipal Airport (0A9) in Elizabethton, Tennessee, then took off and returned to the farm for landing. While on short final approach for landing, the pilot heard a "high-pitched whine" and the tail rotor authority degraded. He immediately initiated a full autorotation to landing. During the landing, the helicopter touched down in a grass-covered field approximately 100 yards from the pilot's hangar. The touchdown point was uneven and sloped away from the hangar and the direction of flight. The struts and main landing gear collapsed, and the tailskid assembly and vertical tail struck the ground. Prior to the event, the pilot did not see any warning lights or note any abnormalities.
Personnel Information
According to FAA and pilot records, the pilot held a commercial pilot certificate with ratings for rotorcraft-helicopter, private privileges for airplane single-engine land, and instrument airplane. He also held a flight instructor certificate with a rating for rotorcraft-helicopter. His most recent FAA third-class medical certificate was issued on June 4, 2010. He reported 967 hours of total flight experience, 586 of which were in the accident helicopter make and model.
The pilot-rated passenger held a commercial pilot certificate with ratings for airplane single-engine land, airplane multi-engine land, and instrument airplane. His most recent FAA second-class medical certificate was issued on October 1, 2008.
Aircraft Information
The accident aircraft was a single-engine, three-seat, light utility helicopter powered by a 190-horsepower air-cooled Lycoming HIO-360-D1A engine, constructed primarily of aluminum alloy. It was equipped with conventional collective and cyclic control sticks and tail rotor control pedals. The main rotor was a fully articulated three-bladed design, and the tail rotor was a two-bladed semi-rigid anti-torque rotor design. Power was transmitted from the engine to the rotor system through a V-belt drive incorporating a free-wheeling sprag clutch, a main drive transmission, a tail rotor transmission, and shafts.
The helicopter was manufactured in 1988. At the time of the accident, it had accrued 6,523.4 total hours of operation. The last recorded maintenance was cleaning and repacking of the lower pulley bearings at 6,494.5 hours, completed on December 29, 2011. The most recent annual inspection was completed on December 9, 2011, at 6,488.6 hours.
Meteorological Information
The recorded weather at Tri-Cities Regional Airport (TRI), Bristol, Tennessee, located 6 nautical miles northwest of the accident site, at 1853 (approximately 23 minutes after the accident), included calm winds, 10 miles visibility, few clouds at 6,500 feet, scattered clouds at 10,000 feet, temperature 27°C, dew point 13°C, and an altimeter setting of 29.96 inches of mercury.
Wreckage and Impact Information
Examination of the wreckage revealed substantial damage. The fuel tanks were not compromised and were approximately half full; the right fuel tank was displaced outboard in its retention straps. The cabin was intact, but the aft cabin wall upper beam was distorted on both sides of the mast. The instrument panel remained attached to the floor but was tilted to the left. The main frame was intact with no breaks but exhibited kinking and bending in several tubes. The tail boom was intact and attached; the vertical stabilizer exhibited compression bending and was folded to the right.
Examination of the flight control system revealed no preaccident anomalies or failures. Control continuity existed from the cyclic and collective controls to the main rotor and from the tail rotor pedals to the tail rotor blade pitch links. All three main rotor blades were undamaged and attached. The tail rotor assembly was intact and undamaged.
Examination of the drive train revealed that the tail rotor driveshaft (TRDS) had become uncoupled from the drive assembly and main rotor belt transmission. The TRDS could be rotated freely by hand. The main transmission input (pinion) shaft aft nut was missing, along with its cotter pin. The cotter pin was found fragmented and captured in the grease cavity of the TRDS end fitting; it was shorter and thinner than the specified 1½-inch-long, 3/32-inch-diameter cotter pin. The grease in the coupling was soft and black, while the grease on the pinion was discolored, caked, and dry in the aft pinion splines. The forward TRDS retention nut, although safety wired, was less than hand tight. The pinion splines exhibited significant wear and fretting corrosion.
Additional Information
On June 6, 2013, Sikorsky Aircraft Corporation issued Safety Advisory SSA-269-13-001 advising mechanics and pilots that improper maintenance of the main transmission aft pinion nut and belt drive system installation could cause decoupling of the tail rotor driveshaft and loss of tail rotor thrust, resulting in loss of aircraft control, loss of the aircraft, and injury to or death of the occupants.
Tests and Research
The mechanic who maintained the helicopter since 2009 had performed the last three annual inspections, the last three 100-hour inspections, the last 1200-hour inspection, and the last maintenance action (cleaning and repacking of lower pulley bearings). According to the Handbook of Maintenance Instructions, the 1200-hour inspection required removal of the upper pulley, inspection of the pinion, and torquing and safeying of the aft pinion nut during reassembly. When shown the broken cotter pin, the aft pinion nut which had lost torque, the damaged pinion splines, dried-out grease, fretting corrosion, and tooth wear, the mechanic advised that during the inspections he had never gotten into the assembly to check the torque on the aft pinion nut or inspect the cotter pin.