Accident Overview
On an unspecified date, a Bighorn Helicopters Eurocopter AS350 D (registration C-FBHX, serial number 1428) was supporting Alberta Environment for weather station maintenance and snow pack analysis near Blairmore, Alberta. The helicopter departed Cranbrook, British Columbia, at 0830 MST and picked up four passengers in Blairmore before flying to the Allison Pass weather station. After completing tasks there, the helicopter proceeded to the South Racehorse Creek weather station at an elevation of approximately 6300 feet above sea level.
Approach and Accident Sequence
Following a reconnaissance flight, the pilot decided to transport two passengers at a time due to wind conditions. After dropping off two passengers at a cut block, the pilot flew the remaining two to the station. On the first approach, the pilot "toed" the skids onto the mountainside to allow disembarkation under power.
During the second approach to the station, the helicopter encountered variable winds and erratic airspeed indications. At a height of 50 to 75 feet above trees and slowing to hover speed, a gust lifted the helicopter. The pilot lowered the collective to maintain descent, but the helicopter then began to descend excessively. The pilot aborted the approach and turned downhill. The main rotor rpm indicated in the yellow arc (below normal operating range), and the low rotor warning horn sounded momentarily. When the pilot raised the collective to increase power, the engine accelerated, but the helicopter continued to descend, settling into trees and coming to rest on its right side in deep snow. The television cameraman in the right rear seat recorded approximately five seconds of the sequence.
Injuries and Damage
The pilot and front seat passenger sustained serious injuries; the rear passenger received minor injuries. The helicopter was substantially damaged.
Weather Conditions
Weather reports from surrounding stations indicated strong winds. At Cranbrook Airport (45 nm west), wind was 170° true at 13 knots gusting to 24 knots. At Pincher Creek Airport (25 nm east), wind was 260° at 30 knots gusting to 44 knots. At Lethbridge Airport (50 nm east), wind was 240° at 32 knots gusting to 42 knots. Upper winds at 6000 feet were forecast from 220° at 21 knots with a temperature of 1°C. Mountain wave activity was observed in satellite photos, and moderate lee wave turbulence was forecast. Downdraft speeds from winds of 21, 32, and 42 knots could reach 2100, 3250, and 4250 feet per minute, respectively.
Engine and Rotor System
The engine, a Lycoming LTS-101-600A3 (serial number LE43442CE), was tested after the accident and met all specifications. No deficiencies were found in the fuel control unit, power turbine governor, or fuel pump. Main rotor head and blade damage was consistent with considerable power being produced when blades contacted trees. The main rotor operating speed is 385 rpm; the yellow arc is between 320 and 370 rpm; the low rotor warning horn activates between 270 and 335 rpm.
Pilot Information
The pilot was certified for the flight, with approximately 2460 total flight hours (2220 in helicopters, 170 on type). He had completed a 15-hour mountain flying course and had about 780 hours of mountain flying experience (100 on type). His last single-engine pilot proficiency check was on a Bell B206 on 27 April 2001.
Investigation Findings
The investigation focused on atmospheric conditions and a possible power loss. Winds at the site were likely from the south or southwest at 20 to 40 knots, causing subsidence, turbulence, and wind shears on the lee side. An updraft on final approach led the pilot to lower the collective, followed by a downdraft or wind shear causing sudden descent. At 50 to 100 feet above trees and near-hover speed, recovery margin was minimal. The helicopter's maximum rate of climb at that altitude and load was about 1500 fpm, while downdrafts could exceed 3000 fpm.
Engine failure was ruled out because of normal acceleration sounds, rotor blade damage, and continued engine operation after impact. The momentary low rotor warning horn activation indicated rotor speed droop then recovery with only slight collective lowering. The pilot was attempting to land in wind conditions likely exceeding the helicopter's performance limitations.
Conclusion
The helicopter encountered strong downdrafts or wind shear on the second approach that were not present on the first, and the pilot was unable to adequately control the helicopter.