Schweizer 269C N9WZ accident near Macclenny, Florida

Probable cause

The improper maintenance of the landing gear dampers and main rotor elastomeric dampers, resulting in ground resonance during an unsuccessful takeoff attempt with the helicopter near its maximum gross weight for the density altitude.

NTSB analysis

According to the pilot, he flew the helicopter to the accident site earlier in the day with no anomalies noted during the preflight inspection, the flight itself, and the post-flight inspection. He said he briefed and loaded two passengers, which placed the helicopter near its maximum allowable gross weight and near its forward center-of-gravity (CG) limit. The pilot attempted the engine start but “inadvertently stalled the engine” during main rotor engagement. After the rotor stopped, he performed an engine start, rotor engagement, run-up, and a “ground check with no defects noted.”

The pilot said 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. Everything seemed normal so he initiated the takeoff again.

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. Sounds consistent with engine and main rotor operating rpm were heard before both rpm signatures appeared to droop 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. A detailed examination of the wreckage and the maintenance records revealed damage consistent with a ground resonance event, and damage/degradation of main rotor elastomeric dampers and the landing gear dampers that predated the event. It is likely that the degraded condition of the dampers accelerated the helicopter's entry into ground resonance during the pilot's takeoff attempt. The records also revealed improper maintenance and missing entries for mandatory inspections.

The sequence of events, the observed pattern of the helicopter’s self-destruction, and the damage signatures were all consistent with ground resonance. The loading of the helicopter near its maximum gross weight and the relatively high density altitude additionally compromised the control and power margins available to the pilot during the attempted takeoff.

Text from the NTSB. Open the NTSB docket for ERA23LA284 for the full record.