Flight Event
On November 16, 2015, about 1040 eastern standard time, a Eurocopter EC120B helicopter, registration N88HA, experienced a loss of tail rotor control while on approach for landing at Goshen Municipal Airport (GSH), Goshen, Indiana. The helicopter-rated pilot and a flight instructor were on board for an instructional flight under Title 14 Code of Federal Regulations Part 91. The flight had originated from Elkhart Municipal Airport (EKM) about 1000. Day visual meteorological conditions prevailed; no flight plan was filed.
The flight instructor reported that during the return leg, an unusual sound was heard. They diverted to GSH for a precautionary landing, about 3 miles away. As they approached the airport, the noise increased. The instructor took the controls. At about 15 feet above ground level and 25 knots, the helicopter began an uncommanded left yaw. Adding right pedal did not stop the yaw. The instructor reduced engine power and initiated a hovering autorotation. The occupants were not injured, but the helicopter sustained substantial damage.
Postaccident Examination
Examination revealed that the tail rotor drive system's Thomas coupling interface with the main gear box tail rotor output shaft had failed. The Thomas coupling was part of an after-market air conditioning system installed under a Federal Aviation Administration (FAA) Supplemental Type Certificate (STC) held by Air Comm Corporation. The STC was installed about 150 flight hours before the accident.
The failed components—Air Comm output flange/pulley, Airbus (Eurocopter) output wheel, and Air Comm compressor drive belt—were examined by the NTSB Materials Laboratory. The internal spline on the pulley and external spline on the output wheel exhibited wear. Rust-colored wear debris was found between worn teeth. Concentricity and hardness measurements were taken; the pulley's hardness met specification. Chemical composition of both components matched their material specifications. Wear debris analysis showed a mixture of debris from pulley, output wheel, and molybdenum disulfide lubricant.
Further examinations compared the accident pulley to exemplars. Four design differences were identified between the OEM Thomas coupling and Air Comm's version:
1. The internal spline on the Air Comm coupling was produced by Electrical Discharge Machining (EDM), which can result in a rough surface finish, whereas the OEM used broaching. 2. The gap between spline teeth at the pitch diameter was larger for the Air Comm coupling; the measured space width exceeded the maximum allowed. 3. The internal spline profile of the Air Comm coupling did not match the involute profile of the OEM shaft; the tooth flanks made initial contact at the edge rather than conforming. 4. Different lubricants were used: the OEM manual specified grease, while the Air Comm instructions specified a jointing compound (Mastinox 8656k or similar).
Regulatory Actions
In response, the manufacturer issued Service Bulletin SB-EC120-111815 on November 20, 2015, and the FAA issued Emergency Airworthiness Directive 2015-24-51 on November 27, 2015. These required immediate and repetitive inspections of the coupling every 25 flight-hours or removal of the STC's Thomas coupling and reinstallation of the OEM coupling. On May 8, 2017, the Emergency AD was superseded by AD 2017-06-11, which revised applicability, terminology, and inspection requirements.