What happened
On November 16, 2015, a Eurocopter EC120B helicopter, registration N88HA, was conducting an instructional flight from Elkhart Municipal Airport to Goshen Municipal Airport (GSH) in Indiana. During the approach to GSH, the flight instructor and the pilot-in-command heard an unusual noise coming from the aircraft.
As the helicopter approached the airport, the noise increased in volume. The flight instructor took control of the aircraft and, while flying approximately 15 feet above the ground at 25 knots, the helicopter experienced an uncommanded left yaw. Despite the instructor applying corresponding right pedal, the yaw could not be stopped. The instructor immediately reduced engine power and initiated a hovering autorotation to land.
The crew sustained no injuries, though the helicopter suffered substantial damage.
The investigation
Investigators examined the tail rotor drive system and discovered that the Thomas coupling interface with the main gearbox tail rotor output shaft had failed. This coupling was part of an aftermarket air conditioning system installed under a Supplemental Type Certificate (STC).
Laboratory analysis of the Air Comm output pulley and the Airbus (Eurocopter) output wheel revealed several design discrepancies between the aftermarket component and the original equipment manufacturer (OEM) part:
- The internal spline on the Air Comm pulley was produced using an Electrical Discharge Machining (EDM) process, which can result in a rougher surface finish compared to the OEM's broaching process.
- The gap between the spline teeth of the aftermarket coupling was greater than the OEM specification.
- The internal spline profile of the aftermarket pulley did not match the involute spline profile of the gearbox output shaft, causing contact at the edge of the tooth flanks rather than a conformal fit.
- The lubrication methods differed; the OEM manual required grease, while the aftermarket instructions specified a jointing compound.
Findings
Technical examination confirmed that the chemical composition and material hardness of the components met specifications. However, the differences in manufacturing processes, spline profiles, and lubrication regimens between the aftermarket STC component and the OEM component likely contributed to increased wear and the eventual failure of the coupling.