SEA94LA153

While executing a run-on landing the instructor felt a 'thump' and the helicopter began to oscillate laterally. The instructor pilot diagnosed the problem as ground resonance, took control of the aircraft from the student, closed the throttle, lowered collective, and the helicopter departed the taxiway. Post crash examination of the skid shock struts revealed that all four were charged at a pressure lower than that specified. The maintenance handbook warns that improper shock strut pressure levels may result in ground resonance and destruction of the helicopter.

On June 20, 1994, approximately 1800 hours Pacific daylight time (pdt), a Schweizer 269C helicopter, N7506A, registered to C&H Helicopters, Inc., operated by Hillsboro Helicopters, Inc., and under the control of Neal H. Lawson, a commercially certificated helicopter instructor pilot, was substantially damaged during an encounter with ground resonance while the student pilot was executing a run-on landing to a parallel taxiway at the Hillsboro Airport, Hillsboro, Oregon. The pilot in command and Peruvian student pilot were uninjured. Visual meteorological conditions existed at the time and no flight plan had been filed. The flight, which was instructional in nature, was to have been operated in accordance with 14CFR91 and originated at Hillsboro at 1745 hours.

The pilot in command reported that during the student's run-on landing he "felt a "thump" in the airframe and the aircraft began to oscillate laterally." The pilot took control of the aircraft diagnosing the problem as ground resonance and closed the throttle and lowered collective. The aircraft began to rotate to the right and eventually came to rest at the edge of the taxiway (refer to photographs 01 and 02).

The four landing gear (skid) shock struts were removed from the aircraft and tested at Schweizer Aircraft Corp., under the control of Donald F. Miller, FAA Principal Maintenance Inspector (refer to attached Inspector Statement). The load stroke tests revealed that all four of the shock struts exhibited a charge pressure lower than that specified (refer to pages 1/2 of attached Report No. SA269C-30-7).

Additionally, revision No. R-19 from the Handbook of Maintenance Instructions (HMI) contains a warning stating in part that improper pressure may result in ground resonance and destruction of the helicopter (refer to Attachment R-19).