History of Flight
On July 1, 2023, at approximately 1200 Eastern Daylight Time, a Schweizer 269C helicopter, registration N9WZ, was substantially damaged during an accident near Macclenny, Florida. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 aerial observation flight. The commercial pilot reported minor injuries, and two passengers were not injured.
The pilot had flown the helicopter to the accident site earlier that day without any anomalies. Before the accident flight, he briefed and loaded the two passengers, performed the engine start, and then inadvertently stalled the engine during main rotor engagement. After the rotor stopped, he conducted another engine start, rotor engagement, run-up, and ground check, noting no defects. When he began to raise the collective for takeoff, he felt a very slight vibration that subsided instantly. He lowered the collective, performed a magneto check, checked gauges, and felt for vibrations; everything seemed normal. He then initiated takeoff.
As he raised the collective again, an extremely violent left-to-right rocking motion began. The pilot considered lifting off in case of ground resonance but believed a major malfunction had occurred. He lowered the collective and reduced the throttle to idle. The helicopter, which had not left the ground, then disintegrated.
A witness recorded a 21-second video from behind the helicopter. The low-resolution video showed the helicopter becoming light on the skids and rotating nose-left about 20° around the main rotor mast before settling back down. The helicopter then rocked left and right instantaneously, the main transmission and mast assembly detached from their mounts, and the turning main rotor destroyed the tailboom.
Personnel Information
The pilot, who owned the helicopter, held a commercial pilot certificate with ratings for airplane single-engine, multi-engine, instrument airplane, rotorcraft-helicopter, and instrument helicopter. His most recent FAA second-class medical certificate was issued on September 1, 2022. He reported 4,908 total flight hours, with 389 hours in helicopters, all in the accident helicopter.
Aircraft Information
The helicopter's maximum gross weight was 2,050 lbs, with a maximum cabin weight of 600 lbs. At the time of the accident, the estimated gross weight was 1,931 lbs, within 0.8 inches of the forward CG limit. Calculations were based on half a tank of fuel, known weights of two adults, and estimated weight of a minor child. The density altitude was 2,500 ft; the manufacturer's recommended takeoff weight at that density altitude was 1,890 lbs. Performance chart interpolation showed the helicopter should be capable of hovering in ground effect up to 3,700 ft density altitude at the estimated weight.
Maintenance records showed that the most recent annual and component inspections used superseded and out-of-date manufacturer manuals and checklists, which were not applicable to the accident helicopter by serial number. An entry dated January 1, 2022, referenced type certificate data sheets and manufacturer's maintenance manuals, but none were attached. Entries stated landing gear dampers were removed, inspected, recharged with nitrogen, and reinstalled, but no manufacturer instructions for recharging existed. The main rotor elastomeric dampers were life-limited to 6,000 hours and required inspections every 600 hours, but no records of such inspections were provided. A life-limited component list dated February 7, 2023, noted the main rotor dampers were original and life-limited.
Wreckage and Impact Information
The helicopter was recovered to the pilot's hangar. Examination of photographs showed the cockpit and instrument panel structure intact, but the overhead greenhouses and windscreens were fractured. The main transmission was free of its mounts and lay aft of the engine. The main rotor mast and head were secure in the transmission. All three blade hubs were secure; the blades appeared cut about 3 feet outboard of their grips during recovery. Tailboom remnants were entangled beneath the main rotor transmission and mast assembly.
Postaccident examination by an FAA aviation safety inspector revealed damage to the airframe, main rotor mast, and transmission consistent with ground resonance. All three main rotor elastomeric dampers showed damage consistent with ground resonance; internal elastomer damage predated the accident. Hand-pressure testing of the main landing gear dampers found only the right rear oleo strut offered expected resistance. Two of the remaining four dampers had less than expected resistance, and one showed zero resistance.
Additional Information
The Pilot's Flight Manual warned: "Ground resonance may result if helicopter is operated when the landing gear dampers are not in good condition. If extension is not correct, a detailed inspection procedure must be performed before further flight operations."
According to the FAA Helicopter Flying Handbook, ground resonance is a destructive vibration phenomenon affecting helicopters with articulating rotors, typically with three or more blades. It occurs at certain rotor speeds when on the ground, caused by the airframe's natural frequency being intensified by an out-of-balance rotor. Hard contact with the ground can send a shockwave to the rotor head, altering blade spacing. Low rpm corrective action: close throttle and lower collective. Normal rpm: lift off and allow blades to rephase. The helicopter's Flight Training Manual described ground resonance as developing from ground shocks causing rotor head imbalance, recognized by progressively violent oscillations, with recovery methods depending on rpm.
The accident helicopter had skid-type landing gear, which is generally less prone to ground resonance than wheel-type gear, but damper condition is critical.