Accident Overview
On July 29, 2011, at approximately 1630 eastern daylight time, a Schweizer 269C helicopter, registration N39SH, experienced a ground resonance encounter at a private helipad near Darlington, Maryland, resulting in substantial damage. The commercially certified pilot sustained minor injuries. Visual meteorological conditions prevailed, and no flight plan was filed for the business flight, which had departed from the Harford County Fairgrounds in Bel Air, Maryland, around 1600. The flight was conducted under 14 CFR Part 91.
Pilot and Aircraft Information
The pilot held a commercial pilot certificate with ratings for airplane single-engine land and rotorcraft-helicopter, as well as a flight instructor certificate with ratings for helicopter and instrument helicopter. He reported 968 total flight hours, including 541 hours in the accident helicopter make and model. His most recent flight review was in February 2011 in a Schweizer 300C helicopter, and his most recent FAA second-class medical certificate was issued in April 2011.
The helicopter was manufactured in 1987 and registered to the operator in August 2010. It was equipped with a single reciprocating 190 hp Lycoming HIO-360 engine. The most recent annual/100-hour inspection, completed on August 4, 2010, recorded a total aircraft time of 2907.2 hours, performed in accordance with the Schweizer Helicopter Maintenance Instructions (HMI).
Accident Sequence
The pilot had spent the day providing helicopter rides at the county fair. The accident flight was to reposition the helicopter from the fairgrounds to the company's base for the night. The pilot reported an uneventful flight and a visual approach to a field adjacent to the helipad, followed by a hover-taxi to the pad. He described the touchdown as light; he reduced the collective to full down and applied collective friction for cool down per the checklist. Approximately one minute after touchdown, the helicopter began to vibrate violently. The pilot attempted to further reduce power, but the vibration continued until the helicopter broke apart.
Maintenance and Examination Findings
The helicopter's landing gear consisted of forward and aft crossbeams and left and right stabilizers, attached through four skid struts with four corresponding oleo-type landing gear dampers. The pilot performed a preflight inspection that day, including visual and manual checks of the dampers, and noted no discrepancies.
According to the HMI, daily preflight inspections required checking the helicopter's stance with full fuel tanks; an unusually nose-down or nose-up stance would require examining damper extension. The HMI also mandated examination of landing gear dampers for operation, condition, and extension at each 100-hour inspection, with overhaul required only when dampers failed specified limits. Maintenance logs from January 2009 onward did not specifically indicate any damper maintenance or overhaul.
After the accident, the minority owner expressed concern to an FAA inspector that maintenance was overdue and that the helicopter had accrued over 150 hours since its last 100-hour/annual inspection, based on fuel consumption and receipts. He also reported that the Hobbs meter had been disconnected during operation.
The main rotor dampers and landing gear dampers were removed and examined at Schweizer Aircraft Corp. on September 20, 2011, with an FAA inspector present. The main rotor dampers displayed fractures of the attaching clevis turnbuckle rod consistent with static overload failure. The landing gear dampers were tested for compression load versus stroke length. The forward left damper was within specifications; the forward right damper was approximately 125 lbs below the specified range; neither aft damper met specifications.
The Rotorcraft Flight Manual included a warning: "Ground resonance may result if the helicopter is operated when the landing gear dampers are not in good operating condition." The HMI also warned that incorrect fluid levels, improper pressure, or inoperable valving could deteriorate damping capabilities, potentially causing ground resonance and destruction of the helicopter.