Accident Summary
The Robinson R44 Raven II helicopter, registration N313AL (serial number 10056), departed Cranbrook Airport at 1343 mountain standard time (MST) for Revelstoke, following a visual flight rules route north along the Columbia River. At 1415 MST, the helicopter struck steep terrain 33 nautical miles north-northwest of Cranbrook at an elevation of 4200 feet in a mountainous region. The pilot sustained fatal injuries, and the helicopter was destroyed by impact forces and a severe post-crash fire.
Weather Conditions
At 1300 MST, the METAR for Cranbrook recorded cloud at 200 feet above ground level (agl), with a measured ceiling at 1100 feet agl; wind was light from the west at 3 knots and temperature was −4°C. At the accident site, the observed weather was suitable for visual flight rules: an overcast layer at 5000 feet above sea level, no precipitation, visibility of 5 miles, and temperature estimated at −12°C.
Aircraft Details
The helicopter, manufactured in March 2003, had accumulated 343 hours of flight time. It was certificated, equipped, and maintained in accordance with existing regulations. The engine, a Textron Lycoming IO-540-AE1A5, had been removed at 299 hours for sticking exhaust valves and reinstalled 43 hours before the accident. Post-crash examination revealed lipping on intake valve stems caused by an engine overspeed event that occurred within the last 43 hours, likely in the final minutes of operation.
Drive Belt System
The helicopter's power transmission relied on four double-vee drive belts. Post-accident, one belt was damaged but intact, one was broken, and two were missing. The electric actuator that adjusts belt tension showed an extension of 1.3 inches, beyond the normal range of 0.8–1.0 inches. Controlled tests demonstrated that with two belts removed, the actuator extended to 1.3 inches to restore tension, taking 18 seconds to adjust.
Cooling Fan Examination
The engine cooling fan attachment, a taper-fit joint between the shaft and fan socket, exhibited severe galling that occurred within the last few hours of operation. The fan mounting bolts and washers showed fretting and elongation. Galling on this joint is a known factor in vee-belt failure, misalignment, and loss due to induced vibration.
Probable Cause
The Transportation Safety Board of Canada determined that galling on the engine cooling fan taper-fit joint introduced vibration to the belt-drive system, causing misalignment of the belts within the sheave grooves and leading to two vee-belts running off the sheaves in flight. The sudden loss of belt tension caused the remaining belts to slip, resulting in a rapid loss of main rotor rpm. This prevented the pilot from avoiding the trees and led to the collision with terrain and destruction of the helicopter.
Findings as to Risk
The engine cooling fan taper-fit joint is subject to galling damage, which imparts vibration to the vee-belts and sheaves, a known factor in belt failure. Additionally, excessive wear in the exhaust valve guides for cylinders 3 and 5 indicated a deviation from manufacturing quality control.