N9WZ

HISTORY OF FLIGHTOn July 1, 2023, about 1200 eastern daylight time, a Schweizer 269C helicopter, N9WZ, was substantially damaged when it was involved in an accident near Macclenny, Florida. The commercial pilot sustained minor injuries, and two passengers were not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 aerial observation flight.

The pilot flew the helicopter to the accident site earlier in the day and reported no anomalies. He stated that before the accident flight he briefed and loaded two passengers, performed the engine start, and then “inadvertently stalled the engine” during main rotor engagement. After the rotor stopped, he performed an engine start, rotor engagement, run-up, and ground check with no defects noted. The pilot said that when he started to raise the collective control for takeoff, he felt a “very slight vibration that subsided instantly,” but he lowered the collective, performed a magneto check, checked the gauges, felt for any vibration, and listened for any malfunction. He stated that “everything seemed to be in order” and he subsequently initiated the takeoff.

The pilot reported that as he began to raise the collective an “extremely violent” left-to-right rocking motion began. The pilot considered lifting the helicopter into the air in case the helicopter was in ground resonance, but he believed that the helicopter had experienced “a major malfunction and not ground resonance.” He lowered the collective, reduced the throttle to idle, and the helicopter, which had not left the ground, “disintegrated.”

A witness recorded a 21-second video that showed the seconds before the accident and the accident itself. The video was taken from behind the helicopter and was of low resolution. Sounds consistent with engine and main rotor operating rpm were heard before the engine and rotor noise decreased as the helicopter became light on the skids and rotated nose-left around the main rotor mast about 20° before it settled to the ground. The helicopter then instantaneously rocked left and right. The main transmission and mast assembly became free of its mounts and the turning main rotor destroyed the tailboom. PERSONNEL INFORMATIONThe pilot, who was also the owner of the helicopter, held a commercial pilot certificate with ratings for airplane single-engine land, multi-engine land, instrument airplane, rotorcraft-helicopter, and instrument helicopter. His most recent FAA second-class medical certificate was issued September 1, 2022. The pilot reported 4,908 total hours of flight experience, of which 389 hours were in helicopters. According to the pilot, all his helicopter experience was in the accident helicopter. AIRCRAFT INFORMATIONThe maximum allowable gross weight for the helicopter was 2,050 lbs. The maximum allowable cabin weight was 600 lbs. The estimated gross weight of the helicopter at the time of the accident was 1,931 lbs and within .8 inches of the helicopter’s forward CG limit. The calculations were based on an estimated half-tank of fuel, the known weights of the two adults, and the estimated weight of the minor child. Based on the 2,500-ft density altitude at the time of the accident, the manufacturer’s recommended takeoff weight was 1,890 lbs. Interpolation of performance charts in the Pilot’s Flight Manual revealed that at the estimated gross weight of 1,931 lbs, the helicopter should be capable of in-ground-effect hovering flight up to 3,700 ft density altitude.

Examination of maintenance records revealed that the most recent annual and component inspections were performed referencing superseded and out-of-date maintenance manuals and checklists from the manufacturer. Additionally, the checklist was not applicable by serial number to the accident helicopter.

An entry dated January 1, 2022, stated that an annual inspection was performed referencing “[type certificate data sheets] and manufacturer’s maintenance manuals,” yet none were referenced, and no check sheets were attached.

Entries stated that both the front and rear landing gear dampers were “removed, inspected, recharged with nitrogen, and reinstalled”; however, neither customer service publications nor the helicopter manufacturer’s Handbook of Maintenance Instructions (HMI) contained any instruction for “recharging” the dampers.

The helicopter’s main rotor elastomeric dampers were life-limited components that were subject to inspections every 600 hours of operation, yet no entries that documented a 600-hour extension check of the main rotor elastomeric dampers were provided.

The life-limited component list attached to an entry dated February 7, 2023, revealed that the main rotor dampers were original to the helicopter and life-limited to 6,000 hours. AIRPORT INFORMATIONThe maximum allowable gross weight for the helicopter was 2,050 lbs. The maximum allowable cabin weight was 600 lbs. The estimated gross weight of the helicopter at the time of the accident was 1,931 lbs and within .8 inches of the helicopter’s forward CG limit. The calculations were based on an estimated half-tank of fuel, the known weights of the two adults, and the estimated weight of the minor child. Based on the 2,500-ft density altitude at the time of the accident, the manufacturer’s recommended takeoff weight was 1,890 lbs. Interpolation of performance charts in the Pilot’s Flight Manual revealed that at the estimated gross weight of 1,931 lbs, the helicopter should be capable of in-ground-effect hovering flight up to 3,700 ft density altitude.

Examination of maintenance records revealed that the most recent annual and component inspections were performed referencing superseded and out-of-date maintenance manuals and checklists from the manufacturer. Additionally, the checklist was not applicable by serial number to the accident helicopter.

An entry dated January 1, 2022, stated that an annual inspection was performed referencing “[type certificate data sheets] and manufacturer’s maintenance manuals,” yet none were referenced, and no check sheets were attached.

Entries stated that both the front and rear landing gear dampers were “removed, inspected, recharged with nitrogen, and reinstalled”; however, neither customer service publications nor the helicopter manufacturer’s Handbook of Maintenance Instructions (HMI) contained any instruction for “recharging” the dampers.

The helicopter’s main rotor elastomeric dampers were life-limited components that were subject to inspections every 600 hours of operation, yet no entries that documented a 600-hour extension check of the main rotor elastomeric dampers were provided.

The life-limited component list attached to an entry dated February 7, 2023, revealed that the main rotor dampers were original to the helicopter and life-limited to 6,000 hours. WRECKAGE AND IMPACT INFORMATIONThe helicopter was recovered to the pilot’s hangar. Examination of photographs revealed that the structure of the cockpit and the instrument panel were intact, but the overhead greenhouses and the pilot’s and copilot’s windscreens were fractured. The main transmission was free of its mounts and lay aft of the engine. The main rotor mast, with main rotor head attached, was secure in the transmission. All three blade hubs were secure in their grips. All three main rotor blades appeared to have been cut about 3 feet outboard of their respective grips during recovery operations. Remnants of the tailboom appeared entangled beneath the main rotor transmission and mast assembly.

The helicopter was recovered and moved to a recovery facility in Jacksonville, Florida, where it was examined by an FAA aviation safety inspector.

The postaccident examination revealed damage to the airframe, main rotor mast, and transmission consistent with a ground resonance event. All three main rotor elastomeric dampers displayed damage consistent with ground resonance. The displayed damage to the internal elastomer was consistent with damage that predated the accident impact/extension to the installed dampers.

Hand-pressure testing of the main landing gear dampers revealed only the right rear oleo strut offered the expected resistance. Two of the remaining four dampers demonstrated less than the expected resistance, and one damper displayed zero resistance.

According to the Pilot’s Flight Manual, 4-2, Pilot’s Preflight inspection:

WARNING

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. ADDITIONAL INFORMATIONAccording to the FAA Helicopter Flying Handbook, Chapter 11, Helicopter Emergencies and Hazards:

Helicopters with articulating rotors (usually designs with three or more main rotor blades) are subject to ground resonance, a destructive vibration phenomenon that occurs at certain rotor speeds when the helicopter is on the ground. Ground resonance is a mechanical design issue that results from the helicopter’s airframe having a natural frequency that can be intensified by an out-of-balance rotor. The unbalanced rotor disk vibrates at the same frequency (or multiple thereof) of the airframe’s resonant frequency, and the harmonic oscillation increases because the engine is adding power to the system, increasing the magnitude (amplitude) of the vibrations until the structure or structures fail. This condition can cause a helicopter to self-destruct in a matter of seconds.

Hard contact with the ground on one corner (and usually with wheel-type landing gear) can send a shockwave to the main rotor head, resulting in the blades of a three-blade rotor disk moving from their normal 120° relationship to each other. This movement occurs along the drag hinge and could result in something like 122°, 122°, and 116° between blades. When another part of the landing gear strikes the surface, the unbalanced condition could be further aggravated.

If t...