History of Flight
On June 2, 2012, at approximately 1135 mountain daylight time, a Schweizer 269C helicopter, registration N2196F, entered ground resonance after setting down on the ramp at Rocky Mountain Metropolitan Airport (BJC) near Denver, Colorado. The commercial pilot and a passenger were uninjured. The helicopter sustained substantial damage to its tail boom during the event. The helicopter was registered to Top Flight Rotors LLC and operated by TYJ Global under 14 CFR Part 91 as an instructional flight. Day visual flight rules (VFR) conditions prevailed, and the flight was operating on a company VFR flight plan. The local flight was originating from BJC at the time.
The pilot reported that the flight was an introductory discovery flight for the student passenger. The pilot stated that he lifted the helicopter into a two-foot skid height hover. An airplane began taxiing in front of the helicopter. To avoid his downwash disturbing the airplane or the airplane's prop wash disturbing him, the pilot set the helicopter back onto the ground. He set down the left skid first, then the right, executing a two-point touchdown. When the right skid contacted, the helicopter tried to drift right. The pilot applied slight left cyclic input to counter the drift while lowering collective. As the collective reached full down and the skids began to settle, he noticed a slight low-frequency vibration that worsened rapidly, indicating the onset of ground resonance. The engine RPM was too low and collective too low to lift off and restore rotor blade phase. With collective full down, the pilot rolled the throttle to idle. Within seconds, the helicopter shook itself apart.
Personnel Information
The pilot held an FAA commercial pilot certificate with rotorcraft helicopter and instrument helicopter ratings, as well as a Flight Instructor certificate with rotorcraft helicopter and instrument helicopter ratings. He was issued a second-class FAA airman medical certificate on January 3, 2012, with no limitations. The operator reported the pilot had 259 total flight hours, including 212 hours in the same make and model.
Aircraft Information
N2196F was a 1994 Schweizer 269C helicopter, serial number S1647. It was equipped with fixed skids and configured for two occupants. The engine was a Lycoming HIO-360-D1A, serial number L-26088-51A, driving a three-blade main rotor assembly. The front landing gear struts were replaced on August 8, 2011, at a Hobbs time of 1185.6; rear struts were replaced on November 14, 2011, at Hobbs 1439.7. The manufacturer's maintenance procedures require checking forward and aft landing gear dampers for operation, condition, and extension at each 100-hour inspection. The most recent 100-hour inspection was completed on May 17, 2012. According to the operator, total time at accident was 1,720 hours. The mechanic who performed the inspection stated that the periodic inspection procedure was used and damper measurements were within limits.
Meteorological Information
At 1050, BJC reported wind variable at 4 knots, visibility 50 statute miles, scattered clouds at 7,000 and 20,000 feet, temperature 23°C, dew point 4°C, and altimeter 30.00 inHg.
Wreckage and Impact Information
The helicopter came to rest upright on a parking pad adjacent to a taxiway. Damage included the main rotor blades, tail rotor blades, and tail boom (aft end consistent with a main rotor strike). The main rotor mast separated from the upper aft fuselage and came to rest behind and left of the fuselage. A postaccident examination by FAA inspectors revealed no preimpact mechanical failures or malfunctions.
Additional Information
The FAA Rotorcraft Flying Handbook notes that ground resonance develops when rotor blades move out of phase, causing an unbalanced disc, which can occur after a firm touchdown on one skid. Low rotor speed corrective action is to immediately close throttle and fully lower collective; if rotor speed is normal, lift off to hover to allow blade realignment. The helicopter manufacturer issued a safety advisory on February 22, 2012, noting that landing gear dampers slow airframe rocking motion and that dampers must meet design specifications to prevent ground resonance. The maintenance manual overhaul procedure for dampers did not include functional testing after overhaul. A representative of the repair station that overhauled the accident dampers stated they followed manual procedures, which do not require a test of overhauled dampers.
Medical and Pathological Information
The pilot's postaccident toxicology testing was negative for the tests performed.
Tests and Research
On July 3, 2012, landing skid dampers were tested at the manufacturer's facility under FAA supervision. A load versus displacement test was performed. The left forward damper (part 269A3150-19, serial HT13324-1) had minor bending and was out of limits on compression stroke at lower pressure test point but within ultimate load. The right forward damper (serial HT13324-2) was intact and out of limits on compression stroke at lower pressure test point. The left rear damper (part 269A3150-21, serial HT13443-1) was intact and high out of limits at ultimate load. The right rear damper (serial HT13443-2) was intact with a dent in the lower barrel and failed due to a high ultimate load point.