On October 18, 1993, an Aerospatiale SA-315 (registration N49511) operated by Pacific Western Helicopters was involved in an aviation accident near Beckwourth, CA. Investigators recorded the probable cause as: Internal leakage in the collective servo caused a hydraulic lock, and the servo extension and retraction time exceeded the manufacturer's specifications. 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.
During an external load logging operation, the pilot heard a loud noise followed by an uncontrolled right turn and could not lower the collective due to a hydraulic lock in the collective servo.
Incident Overview
The pilot was performing an external load logging operation using a helicopter. After lifting two logs, a loud 'thunk' noise was heard. Subsequently, the helicopter experienced an uncontrolled turn to the right. The pilot found it impossible to lower the collective, which prevented maintaining adequate rotor RPM.
Examination Findings
Post-incident examination of the collective servo revealed internal leakage. This leakage caused a hydraulic lock within the servo, restricting its movement. Additionally, the servo's extension and retraction times exceeded the manufacturer's specifications.
Conclusion
The investigation determined that the collective servo had internal leakage resulting in a hydraulic lock, and its response times were outside specified limits.
Probable cause
Internal leakage in the collective servo caused a hydraulic lock, and the servo extension and retraction time exceeded the manufacturer's specifications.