Accident Sequence
The helicopter was substantially damaged when it struck the helipad during an uncommanded yaw encountered during the initial hover after liftoff from a roof-top hospital heliport. The pilot reported that he picked up into a 4 to 6-foot hover and initiated a right pedal turn. He stated that as the helicopter reached a west heading "the aircraft would not turn any more" despite his continued application of right pedal. He said: "As I continued to apply right pedal the aircraft then went into [a] sudden and uncommanded yaw to the left. I was unable to stop the yaw." The helicopter subsequently impacted the helipad and roof structure. It came to rest at the east edge of the helipad oriented on a southeast heading.
Inspection Findings
A post-accident inspection revealed that the Fenestron (tail rotor) drive shaft had failed approximately 6 inches aft of the main gearbox. The failure occurred at the point where the drive shaft entered a tunnel formed by the left and right engine firewalls. The firewalls and drive shaft segments in the vicinity of the point of failure exhibited scrape marks. Examination of the forward section of the drive shaft revealed features characteristic of an overload failure.
The main gearbox output shaft assembly and rear transmission coupling connected the tail rotor drive shaft to the gearbox. Further examination revealed that the coupling flange could be moved laterally relative to the pinion approximately 3/32 (0.094) inch. Allowable lateral play in the drive flange was 1 millimeter (0.039 inch). Disassembly of the transmission coupling determined that the nut which secured the drive flange to the output assembly pinion gear was improperly installed.
Additional Technical Details
Wear patterns indicated that the locking tangs on the cup washer did not engage the corresponding slots on the shaft, allowing the nut to loosen over time. In addition, the condition of the locking tangs indicated that they were folded over during installation causing them to separate from the cup. The resulting wear had removed material to such an extent that the contact face was no longer perpendicular to the longitudinal axis of the shaft. This allowed excessive radial play in the transmission coupling, which permitted contact between the tail rotor drive shaft and the firewalls.
The FAA Rotorcraft Flying Handbook, FAA-H-8083-21, provided information related to failure of the anti-torque system on a helicopter. The handbook stated: "The loss of antitorque normally results in an immediate yawing of the helicopter's nose. The helicopter yaws to the right in a counter-clockwise rotor system and to the left in a clockwise system... The severity of the yaw is proportionate to the amount of power being used and the airspeed. An antitorque failure with a high power setting at a low airspeed results in a severe yawing." The main rotor system of the accident helicopter rotated clockwise as viewed from above.