Schweizer 269C N6148D accident near Calhoun, Georgia

Probable cause

The helicopter owner’s failure to ensure the helicopter’s airworthiness and the inadequate inspection and maintenance of the helicopter's landing gear dampers, which resulted in ground resonance during throttle application. Contributing to the damage of the helicopter were the improper actions of the pilots after encountering ground resonance.

NTSB analysis

The commercial pilot flew with the helicopter's owner earlier that day for a demonstration flight, during which the owner performed several autorotative landings to either touchdown or power recovery. After that flight, the owner exited the helicopter, and a commercial pilot-rated passenger boarded and sat in the left seat of the helicopter. They departed, and, after about 10 to 15 minutes, the pilot landed the helicopter uneventfully on a taxiway. With the engine operating, they discussed a helicopter system. The pilot intended to demonstrate the system that they were discussing, and, while guarding the collective down, he slowly advanced the throttle. As the rpm increased, he reported feeling a shake, then a "boom boom" sound, and noticed a vibration, which he identified from training as ground resonance. He reduced throttle, which was the correct procedure to perform for ground resonance, but the vibration increased. The pilot-rated passenger then advised him to pick the helicopter off the ground and attempted to assist by raising the collective. The throttle was advanced, and, with the main rotor rpm near the green arc, the helicopter began shaking and did not become airborne, although the collective was raised. The main rotor blades contacted the tailboom several times.

The helicopter's latest annual/100-hour inspection was performed about 1 year 2 months before the accident, and the helicopter had been operated about 61 hours since then. The handbook of maintenance instructions for the helicopter specified that the dampers are to be inspected for operation and condition, and for damper extension on airframe, at each 100-hour inspection or every 6 months, whichever occurred first.

Postaccident operational testing of the dampers from the unairworthy helicopter revealed that all failed a load stroke test. Two of the four dampers, which were underserviced, likely would not have passed an on-airframe extension test during the annual/100-hour inspection that was overdue, while one damper, which was slightly underserviced, would likely have passed. The remaining damper was overserviced, but that condition would not have been detected during the on-airframe extension test because there was no test procedure in place to identify that condition. Without the airframe protection provided by the dampers, ground resonance occurred. Further, the Federal Aviation Administration's Helicopter Flying Handbook states that with respect to ground resonance, if the main rotor 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 main rotor blades in low pitch. Thus, raising the collective and advancing the throttle likely contributed to the helicopter's damage.

Text from the NTSB. Open the NTSB docket for ERA16LA261 for the full record.