Scappoose, OR, USA
N61442
Hughes 269C-1
The pilot initiated the flight at maximum gross weight. After picking the helicopter up to a hover and stabilizing RPM, the RPM began to dip. Unable to recover RPM, the helicopter settled to the ground and subsequently encountered a ground resonance condition. After closing the throttle and lowering the collective, the helicopter began to shudder and turned 45 degrees to the left before the ground resonance condition subsided. The pilot was successful in his attempt to pick the helicopter up again to a hover, stabilizing engine RPM, coming to a stable hover, and sustaining rotor RPM during a left pedal turn. While attempting forward flight from this position the helicopter's RPM dipped again, resulting in the pilot having to set the helicopter down again. The aircraft entered ground resonance a second time, this time worse than the first, resulting in the helicopter being shook apart around the pilot and his passenger. Post-accident tests indicated that all four landing gear dampers, having failed a load stroke test, were set too hard, resulting in a ground resonance condition being encountered after a landing due to reduced rotor RPM.
On July 29, 2003, approximately 1630 Pacific daylight time, a Hughes 269C-1 helicopter, N61442, was substantially damaged after impacting terrain while hovering at the Scappoose Industrial Airpark, Scappoose, Oregon. The commercial pilot and his sole passenger sustained minor injuries. The helicopter was registered to and operated by Hillsboro Aviation, Hillsboro, Oregon. Visual meteorological conditions prevailed for the 14 CFR Part 91 personal flight, and a flight plan was not filed. The flight was originating at the time of the accident, and its destination was the Portland-Hillsboro Airport, Hillsboro, Oregon. In a telephone interview with the NTSB investigator-in-charge (IIC), and according to the Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2), the pilot reported that he initiated the flight at maximum gross weight and had not experienced any problems with the helicopter that day. The pilot stated that after boarding his passenger and checking his gauges, he "picked the helicopter up" and was stabilizing the RPM when he noticed that they started to "dip". The pilot reported that because he wasn't able to maintain RPM, the aircraft settled to the ground and subsequently encountered a ground resonance condition. The pilot stated that he then lowered collective and closed the throttle, but the helicopter continued to shake and turned about 45 degrees to the left. The pilot related that after the helicopter stopped shaking, he checked his gauges again and determined that he could attempt another takeoff. After lifting the aircraft off the ground a second time, he was able to stabilize the RPM in a hover. The pilot stated that he then initiated a left pedal turn, but when he attempted to fly forward he observed the rotor RPM dipping again, requiring him to land the helicopter a second time. The pilot reported that after the helicopter settled to the ground, he again lowered collective and rolled off throttle; however, the aircraft entered into a ground resonance condition again, this time worse than the first. The pilot further stated that, unable to get the helicopter out of ground resonance, "...it "shook itself apart around us." An FAA inspector, who traveled to the accident site, reported substantial damage to the mounts that retain the gearbox. The inspector also reported the main rotor blades had struck the top of the cabin and the tail boom. On August 20, 2003, subsequent to an examination of aircraft component parts, Schweizer Aircraft Corporation reported that all four of the aircraft's landing gear dampers were load stoke tested. Results of the examination indicated that the dampers failed the test, exceeding the specified load to reach the full compression point and higher than specified at the intermediate point. The dampers were not in compliance with specifications, being set too hard. Damage to the aircraft revealed that a ground resonance condition was encountered during landing with reduced rotor RPM.
The helicopter encountering a ground resonance condition following a failure to maintain hovering flight. Contributing factors to the accident included the pilot's failure to maintain rotor RPM and the high pressure state of the landing gear shock absorbing dampers.
Source: NTSB Aviation Accident Database
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