N88HASocata TBM 700 · 2017

***This report was modified on February 20,2020. Please see the docket for this accident to view the original report.***

On November 16, 2015, about 1040 eastern standard time, a Eurocopter EC120B helicopter, N88HA, experienced a loss of tail rotor control while on approach for landing at the Goshen Municipal Airport (GSH), Goshen, Indiana. The helicopter-rated pilot and the flight instructor were not injured and the helicopter sustained substantial damage. The helicopter was registered to Hanscopter LLC and operated by the private individual under the provisions of Title 14 Code of Federal Regulations Part 91 as an instructional flight. Day visual meteorological conditions prevailed for the flight, which was not operated on a flight plan. The local flight originated from the Elkhart Municipal Airport (EKM) about 1000.

The flight instructor reported they were on a return instructional flight when they heard a noise come from somewhere in the helicopter. The pilot stated it was an "unusual sound." They elected to divert to GSH for a precautionary landing, which was about 3 miles from their current position. The instructor explained that as they approached the airport, the noise level increased. It was at that time that he took the controls. About 15 feet above ground level and 25 knots, the helicopter began an uncommanded left yaw. The instructor added corresponding right pedal, which was not effective in stopping the left yaw. The flight instructor said he immediately reduced engine power and initiated a hovering autorotation.

A postaccident examination revealed the tail rotor drive system's Thomas coupling interface with the main gear box tail rotor output shaft failed. The main gear box tail rotor output shaft transmits torque to the tail rotor via the tail rotor shaft through the Thomas coupling. The Thomas coupling was part of an after-market air conditioning system. A review of maintenance records revealed that the air conditioning system installation was approved under a Federal Aviation Administration (FAA) Supplemental Type Certificate (STC) in 2003; the STC was held by Air Comm Corporation. The STC was installed on the incident helicopter about 150 flight hours prior to the event.

The Air Comm output flange/pulley, Airbus (Eurocopter) output wheel, and Air Comm compressor drive belt from the accident helicopter were sent to the NTSB Materials Laboratory for examination. The pulley and output wheel were examined as part of a group examination with representatives from NTSB, FAA, Airbus Helicopters (the Original Equipment Manufacturer, OEM) and Air Comm Corp. The internal spline on the pulley and external spline on the output wheel exhibited wear of the spline teeth. On the pulley, rust-colored wear debris was observed in the space between the worn teeth. The concentricity of the pulley was examined by measuring the diameter and center location at three positions: the internal diameter forward of the spline, the outer diameter of the body at the aft end of the pulley, and the groove between the second and third pulley ribs. The hardness of the pulley material was measured and found to be in accordance with the hardness requirement for quenched and tempered 4340 steel. The chemical composition of the pulley and output wheel was measured using a portable x-ray fluorescence spectrometer. The reported composition for each component was consistent with each component's respective material specification. A sample of wear debris from the pulley spline was examined with a scanning electron microscope (SEM) equipped with energy dispersive x-ray spectroscopy (EDS). The detected elements were consistent with a mixture of wear debris from the pulley, wear debris from the output wheel, and molybdenum disulfide dry lubricant.

After the conclusion of the group examination, additional examinations were conducted on the accident pulley, an exemplar pulley from Air Comm, an exemplar output flange from the OEM, and a pulley that was removed from service and returned from the field. The findings from the NTSB Materials Laboratory determined there were four areas of design difference between the OEM Thomas Coupling and Air Comm's Thomas coupling, to include:

1. The production process for the internal spline on the Air Comm Thomas Coupling was different than on the OEM component. The Air Comm process was an Electrical Discharge Machining (EDM) process. This process can produce a rough surface finish. The OEM exemplar output flange exhibited linear tool marks on the internal spline tooth flanks, consistent with a broaching operation.

2. The gapping between the spline teeth of the Air Comm Thomas Coupling and the spline teeth of the OEM's main gear box tail rotor drive shaft at the pitch diameter was found to be greater than that of the OEM's Thomas Coupling half. The measured space width and tooth width values for the exemplar Airbus output flange and output wheel were in accordance with the limits for the fit class. The Air Comm pulley pitch diameter tooth space width exceeded the maximum space.

3. The internal spline profile of the Air Comm Thomas Coupling did not match the involute spline profile of the OEM's main gear box tail rotor drive shaft. By comparison, the tooth flanks of the pulley, with the constant radius form, were not conformal to the tooth flank of the output wheel. Instead, the internal spline teeth made initial contact with the output wheel spline along the edge where the internal spline tooth flank met the internal spline top landing.

4. Examination of the accident pulley and another pulley removed from service revealed different lubricating substances were used. The aircraft maintenance manual of the OEM main gear box tail rotor specified that the output flange spline joint should be lubricated with grease when the flange is installed. The Air Comm pulley installation instructions specified that the pulley internal spline be coated with a jointing compound, Mastinox 8656k or similar.

In response to the accident, the manufacturer published a Service Bulletin (SB-EC120-111815) on November 20, 2015, and the FAA issued an Emergency Airworthiness Directive (Emergency AD 2015-24-51) on November 27, 2015. The SB and EAD called for an immediate and repetitive inspection of the coupling every 25 flight-hours or deactivating the air conditioning system by removing the STC's Thomas coupling and re-installing the original equipment manufacturers (OEM) Thomas coupling half. Installing the OEM would return the tail rotor shaft drive system to its original design. On May 8, 2017, the Emergency AD was superseded by FAA Airworthiness Directive 2017-06-11 that revised the applicability, some of the terminology, and the inspection requirements of the Emergency AD.