On June 21, 1994, a Schweizer 269C (registration N7506A) operated by Hillsboro Helicopters, Inc. was involved in an aviation accident near Hillsboro, OR. Investigators recorded the probable cause as: THE IMPROPER SERVICING OF THE SKID SHOCK STRUTS. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive; 2 related events involving the same aircraft type or operator are linked below.
During a run-on landing, an instructor pilot felt a thump and lateral oscillation. Diagnosing ground resonance, he took control and shut down. Post-crash examination revealed all four skid shock struts were under-inflated.
Incident Overview
A helicopter experienced an abnormal event during a run-on landing. The instructor pilot reported feeling a thump, followed by lateral oscillations. Recognizing the symptoms of ground resonance, the instructor took control of the aircraft from the student, closed the throttle, and lowered the collective. The helicopter subsequently departed the taxiway.
Findings
Post-crash examination of the skid shock struts revealed that all four were charged at a pressure lower than that specified in the maintenance handbook. The handbook explicitly warns that improper shock strut pressure levels may result in ground resonance and destruction of the helicopter.
Conclusion
The incident involved a run-on landing during which the instructor pilot diagnosed ground resonance and took corrective action. The subsequent investigation identified low shock strut pressure as a significant finding.