On August 15, 1992, a Hiller UH12-E, SALOY CONVER (registration N9753C) operated by San Joaquin Helicopters, Inc. was involved in an aviation accident near Arvin, CA. Investigators recorded the probable cause as: The main rotor blade tension torsion pin failed due to fatigue, which prevented the pilot from flaring and resulted in a hard landing. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive; 1 related events involving the same aircraft type or operator are linked below.
A helicopter suffered a hard landing after its main rotor blade tension torsion pin failed due to fatigue during an aerial application procedure turn.
What happened
The pilot was conducting an aerial application flight when the incident occurred. While executing a procedure turnaround, the main rotor blade tension torsion pin failed. The failure was attributed to material fatigue. This specific component has a designated life limit of 643 hours. At the time of failure, the pin had accumulated 641.5 hours of service.
The investigation
The mechanical examination focused on the main rotor system. Investigators determined that the torsion pin had reached its operational limit. The part had accrued 641.5 hours, falling just short of the 643-hour life limit. The failure mode was identified as fatigue.
Findings
Following the mechanical failure, the pilot attempted to manage the aircraft's descent. The pilot was able to execute an autorotation. However, the pilot was unable to flare the helicopter prior to ground contact. This inability to flare resulted in a hard landing. The sequence of events indicates that the structural failure directly compromised the pilot's ability to perform a safe touchdown.
Probable cause
The main rotor blade tension torsion pin failed due to fatigue, which prevented the pilot from flaring and resulted in a hard landing.