Hiller UH-12E N3375T accident near Bonners Ferry, Idaho

Probable cause

The pilot’s improper selection of a landing site, which resulted in a main rotor strike. Contributing to the accident was the pilot’s improper performance planning and altitude selection over water, which resulted in an rpm droop.

NTSB analysis

The pilot of the helicopter reported that with two passengers aboard, he approached a mountain lake to a hover above the water and saw the engine and rotor speed decrease to the green arc of the engine and rotor tachometer. He maneuvered the helicopter to the shore, and with the engine and rotor continuing to droop, he landed on some “substantially sized granite rocks” where the skids became lodged in the rocks. The helicopter then tipped forward and the main rotor blades impacted the rocks, which resulted in substantial damage to the main rotor and fuselage.

The pilot reported there were no mechanical malfunctions or failures with the helicopter that would have precluded normal operation. The pilot added that he did not verify the performance capability for an in-ground effect or a hover out of ground effect over water. The Helicopter Flying Handbook, FAA-H-8083-21B, states, “When hovering over surfaces as tall grass, trees, bushes, rough terrain, and water, maximum ground effect is reduced.”

Text from the NTSB. Open the NTSB docket for WPR24LA262 for the full record.