Tail rotor assembly separates from Hughes 369D during flight

Casualties unknown • Messalinka Camp, British Columbia, CA

A Hughes 369D helicopter experienced a severe tail rotor vibration and subsequent assembly separation during a flight following a passenger drop-off.

What happened

Following a stop at the Messalinka Camp to disembark a passenger, a Hughes 369D helicopter departed for its next destination. While climbing to approximately 300 feet, the pilot experienced intense vibrations through the tail rotor pedals. In response, the pilot initiated a steep descent and reduced airspeed to roughly 60 knots to attempt a precautionary landing.

At an altitude of approximately 75 feet, the tail rotor assembly detached from the aircraft. This separation caused the helicopter to pitch nose-down and yaw sharply to the right. The pilot executed an autorotation, successfully landing the aircraft in a clearing. There were no injuries to the pilot or the two remaining passengers, and no further damage was sustained beyond the initial separation.

The investigation

Investigators determined that the tail rotor and its associated transmission had detached from the airframe during flight. The separation caused one tail rotor blade to strike the lower vertical stabilizer repeatedly before the transmission disengaged. The impact also damaged the horizontal stabilizer and the aft tail boom. The transmission and fragments of a tail rotor blade were found a significant distance from the crash site.

Examination of the components revealed that the tail rotor blades had previously undergone a modification to include leading edge abrasion strips to combat erosion. While the aircraft had complied with all relevant Airworthiness Directives, including the installation of rivets intended to act as a secondary failsafe, these measures failed to prevent the incident.

Findings

  • The primary cause of the forced landing was the separation of the tail rotor abrasion strip from one of the blades.
  • The abrasion strip failed adhesively, and the retaining rivets pulled free from the blade.
  • The loss of the strip's weight created a massive imbalance, generating vibrations severe enough to tear the tail rotor transmission assembly from the helicopter.
  • The transmission retaining studs failed due to rapid, low-cycle fatigue cracking.
  • The operator had performed the required daily pre-flight visual inspections, which failed to detect the bond separation before it occurred in flight.
  • The rivets added as a failsafe did not prevent the strip from releasing.

Probable cause

The separation of the leading edge abrasion strip from a tail rotor blade, caused by the failure of both the adhesive bond and the secondary retaining rivets, created a severe imbalance that led to the structural failure of the tail rotor transmission assembly.

Frequently asked questions

What happened in the 1994-05-10 Hughes Helicopter 369D C-GPDH accident near Messalinka Camp, British Columbia, CA?

A Hughes 369D helicopter experienced a severe tail rotor vibration and subsequent assembly separation during a flight following a passenger drop-off.

What aircraft was involved and where did it happen?

The accident on 1994-05-10 involved a Hughes Helicopter 369D C-GPDH, operated by Northern Mountain Helicopters Ltd., at Messalinka Camp, British Columbia, CA.

What was the probable cause of the accident?

The separation of the leading edge abrasion strip from a tail rotor blade, caused by the failure of both the adhesive bond and the secondary retaining rivets, created a severe imbalance that led to the structural failure of the tail rotor transmission assembly.

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