On June 26, 1995, a Hughes 369D (registration N1089N) operated by Hsi Helicopters, Inc. was involved in an aviation accident near Highland, CA. Investigators recorded the probable cause as: A high strain rate crack growth, consistent with a one-time overload, on the tail rotor gearbox casting. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A helicopter with three doors removed, engaged in survey positioning, experienced a loud 'whump' and loss of tail rotor control during departure from mountainous terrain. The pilot landed hard on a ridgeline, and the helicopter rolled down the slope. Metallurgical analysis indicated a one-time overload on the tail rotor gearbox.
Incident
A helicopter pilot was operating in mountainous terrain with three doors removed, positioning survey personnel to re-mark survey sites. During departure from the mountainous terrain, the pilot briefly noticed something in his skid mounted mirror. At that point, the pilot and three passengers heard a loud 'whump' followed by a loss of tail rotor control. The pilot maneuvered the spinning helicopter over some power lines and onto a downward sloping ridgeline. The helicopter landed hard and rolled down the mountain approximately 100 to 150 feet.
Investigation Findings
Investigators were unable to recover the tail rotor blades along with half of the severed gear box. The left rear seat passenger was missing a sweat jacket. An abundance of 12-inch reinforced marker tape was found at the accident site. Metallurgical analysis revealed that a high strain rate crack growth had occurred as in a one-time overload on the tail rotor gearbox casting.
Probable cause
A high strain rate crack growth, consistent with a one-time overload, on the tail rotor gearbox casting.
Frequently asked questions
What happened in the 26 Jun 1995 Hughes 369D accident near Highland, CA?
A helicopter with three doors removed, engaged in survey positioning, experienced a loud 'whump' and loss of tail rotor control during departure from mountainous terrain. The pilot landed hard on a ridgeline, and the helicopter rolled down the slope. Metallurgical analysis indicated a one-time overload on the tail…
What aircraft was involved and where did it happen?
The accident on 26 Jun 1995 involved a Hughes 369D, registration N1089N, operated by Hsi Helicopters, Inc., at Highland, CA.
What was the probable cause of the accident?
A high strain rate crack growth, consistent with a one-time overload, on the tail rotor gearbox casting.
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