WPR18LA170

The pilot was preparing to depart in the helicopter on a local flight. He stated that he powered up the helicopter and that, with the engine/rotor rpm at the “very bottom of [the] green range” and the helicopter still on the ground, the helicopter began to vibrate. The vibrations worsened, and in response to the ground resonance, he lowered the collective and rolled off the throttle to abort the takeoff. The vibrations increased in severity and lasted about 20 seconds. The main rotor and engine then shut down, and the helicopter had rotated about 45° to the left. A nearby witness stated that the helicopter was shaking and that it never came to full power or left the ground. Postaccident examination of the helicopter, including the landing gear dampers, skid attachments, and main rotor blade dampers, revealed no anomalies that would have precluded normal operation. Further, the main rotor dampers were tested about 7 flight hours before the accident flight with no anomalies noted. The Federal Aviation Administration’s (FAA) Helicopter Flying Handbook (FAA-H-8083-21A) stated the following about ground resonance: "If the rpm is low, the only corrective action to stop ground resonance is to close the throttle immediately and fully lower the collective to place the blades in low pitch. If the rpm is in the normal operating range, fly the helicopter off the ground." However, this guidance for ground resonance corrective action was listed in the same paragraph after discussion of hard contact with the ground and when one other landing gear strikes the surface. Although the pilot noted that the engine/rotor rpm was at the very bottom of the green range, the rpm was still in the normal operating range; his decision to lower the collective instead of taking off led to increased (and/or continued) ground resonance vibrations and the subsequent substantial damage to the fuselage.

On June 13, 2018, about 1059 Pacific daylight time, a Schweizer 269C helicopter, N557DC, was substantially damaged when it was involved in an accident near Las Vegas, Nevada. The pilot sustained minor injuries. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot was departing North Las Vegas Airport (VGT), Las Vegas, Nevada, destined for Perkins Field Airport (U08), Overton, Nevada. According to the pilot, he started the engine and, after performing startup and preflight checks, increased the engine/rotor rpm; the helicopter had not yet become light on the skids. He stated that, with the engine/rotor rpm at the “very bottom of [the] green range,” the helicopter began to vibrate while still on the ground. The vibrations initially seemed normal but then worsened. He stated that the helicopter was not yet at the point where he would proceed to hover, which he said would have included getting the helicopter light on the skids, lifting gradually, and having the engine/rotor rpm at the top half of the range. In response to the ground resonance, he lowered the collective and rolled off the throttle to abort the takeoff. The vibrations increased in severity and lasted about 20 seconds. The main rotor and engine then stopped, and the helicopter had rotated about 45° to the left. A nearby witness stated that the helicopter was shaking, with the main rotor blades “flopping up and down.” He said it never came to full power or left the ground and that the helicopter “destroyed itself.” Postaccident examination of the helicopter revealed substantial damage to the fuselage. Examination of the airframe revealed no anomalies or evidence of preimpact mechanical failure that would have precluded normal operation. Flight control continuity was established to the main rotor and tail rotor blades. All of the landing gear dampers were examined and tested within the specified pressure/charge range. No anomalies were observed on the skid attachments. Two main rotor dampers were separated from the main rotor blades and remained attached to the mast; visual examination revealed no anomalies. The attached main rotor damper was removed from the main rotor blade and was examined and tested with no anomalies noted. All of the main rotor blade dampers had been tested about 7 flight hours before the accident and met the specified criteria. Compliance was documented with a special airworthiness information bulletin on landing gear damper inspections (SAIB SW-17-29). The Federal Aviation Administration’s (FAA) Helicopter Flying Handbook in effect at the time of the accident (FAA-H-8083-21A), Chapter 11, Helicopter Emergencies and Hazards, stated the following regarding ground resonance: "If the rpm is low, the only corrective action to stop ground resonance is to close the throttle immediately and fully lower the collective to place the blades in low pitch. If the rpm is in the normal operating range, fly the helicopter off theground." This ground resonance corrective action guidance was listed in the same paragraph after discussion of hard contact with the ground and when one other landing gear strikes the surface. The pilot stated that there are no procedures in the pilot’s operating handbook for the helicopter make/model for dealing with ground resonance on startup and that his training focused on ground resonance during the transition from flight to hover and as the result of skid-to-ground hard contact. The pilot further stated that the FAA guidance in the Helicopter Flying Handbook was focused on ground resonance occurring on skid-to-ground hard contact when the helicopter was already in the air. He also cited the guidance in the handbook regarding corrective action when rpm was low of lowering the collective and closing the throttle. The pilot stated that he had not anticipated that ground resonance could develop so easily without lifting the helicopter.