What happened
On a flight from Cranbrook to Revelstoke, British Columbia, a Robinson R44 Raven II helicopter, registration N313AL, struck steep, forested terrain approximately 33 nautical miles north-northwest of Cranbrook. The aircraft was operating under visual flight rules along the Columbia River when it impacted a tree in its flight path, causing the helicopter to break up before striking the ground in a steep, nose-down attitude. The impact and a subsequent intense fire resulted in one fatality and the total destruction of the aircraft.
The investigation
Investigators examined the engine and the drive system to determine why the aircraft lost control. The investigation focused on the four double-vee drive-belts used to transfer power from the Textron Lycardi engine to the main transmission. During the post-crash inspection, investigators found that one belt remained on the shafts, while a second had broken and become trapped in the wreckage. The remaining two belts were missing entirely.
Analysis of the upper sheave revealed heavy rubber deposits on the forward grooves, suggesting the belts had slipped significantly. The engine was also found to have experienced an overspeed event shortly before the crash. Furthermore, investigators identified significant galling on the engine cooling fan's taper-fit joint and fretting on the mounting bolts, damage that had developed within the final hours of flight.
Findings
- Galling on the engine cooling fan taper-fit joint introduced vibrations into the belt-drive system.
- These vibrations caused the misalignment of the belts, leading to two vee-belts running off the sheaves during flight.
- The sudden loss of belt tension caused the remaining belts to slip, resulting in a rapid decay of main rotor RPM and an engine overspeed.
- The simultaneous loss of rotor power and the engine overspeed created a condition that was too difficult to diagnose and manage, making the collision with the trees and terrain unavoidable.