On September 25, 2004, a Schweizer 269C-1 (registration N61413) operated by Group 3 Aviation was involved in an aviation accident near Burbank, CA. Investigators recorded the probable cause as: The helicopter's forward landing gear dampers were found with pistons fully extended and fluid in excess of specified levels, which prevented them from attenuating vibrations and preventing ground resonance. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive; 4 related events involving the same aircraft type or operator are linked below.
A helicopter experienced ground resonance after a hard touchdown during refresher training, resulting in substantial damage. Post-accident examination revealed overfilled forward landing gear dampers.
Incident Overview
A helicopter undergoing refresher training encountered ground resonance after landing, resulting in substantial damage. The pilot-undergoing-instruction (PUI) was receiving refresher training. The approach was normal, but the touchdown was harder than normal. After the helicopter was on the ground, the certified flight instructor (CFI) began discussing the touchdown with the PUI. At that point, the helicopter began to vibrate. The CFI did not have time to react before the helicopter self-destructed.
Post-Accident Findings
A post-accident examination of the helicopter's components was conducted. The examination found that in the forward landing gear dampers, the pistons were fully extended and contained fluid in excess of the levels specified in the maintenance instructions. According to the examination, dampers in this condition will not compress or attenuate airframe motion and vibrations, nor will they prevent ground resonance. The last inspection on the helicopter was a 100-hour inspection, which was completed 4.5 hours prior to the accident flight.
Probable cause
The helicopter's forward landing gear dampers were found with pistons fully extended and fluid in excess of specified levels, which prevented them from attenuating vibrations and preventing ground resonance.