Accident Summary
A helicopter impacted trees and terrain while maneuvering during a sightseeing flight over a national park. The aircraft was in a descending left turn with a high angle of bank, and the main rotor rpm was decreasing. Several seconds prior to impact with the trees, a low rpm warning horn was heard.
Acoustical Analysis
An acoustical analysis of a video recording made by a passenger revealed a decrease in engine gear box and planetary speeds, while the engine speed remained steady in the seconds before the end of the recording. The video ended at the moment of initial contact with the trees.
Examination Findings
Post-accident examination of the engine and airframe revealed no anomalies that would have contributed to the event.
Conclusion
The helicopter struck trees and terrain while maneuvering in a descending left turn with a high bank angle and decreasing main rotor rpm. No causal factors were identified in the investigation.