Accident Overview
On the date of the accident, a Precision Helicopters Inc. Bell 206B Jet Ranger (registration C-GPGX, serial number 1362) was departing from a prepared helicopter landing area adjacent to the Nose Mountain, Alberta, fire observation tower at approximately 1815 mountain daylight time. The pilot and three initial attack firefighters were on board. The landing area was in a clearing on a mountain plateau at the north edge of a steep escarpment.
Departure and Loss of Control
After lift-off, the pilot hover-taxied around a brush pile on the west side of the clearing and departed westward toward the escarpment. When the helicopter overflew the rim of the escarpment, it began to yaw to the right. The pilot was unable to control the yaw with full left pedal. As the helicopter rotated through 180°, the pilot lowered the collective to regain directional control. The helicopter descended onto the escarpment, rolled over, and came to rest on its left side. One firefighter sustained fatal injuries, another sustained serious injuries, and the pilot and the third firefighter sustained minor injuries. The impact forces activated the onboard emergency locator transmitter. The helicopter was substantially damaged, but there was no post-impact fire.
Helicopter and Crew Information
Precision Helicopters Inc. operates two Eurocopter AS 350s and one Bell 206B under Canadian Aviation Regulations sections 702 and 703. The helicopter had been chartered by the Alberta Ministry of Sustainable Resource Development Forest Protection Branch (ASRD-FPB) and was dispatched from Graham Fire Base at 1614. After encountering thunderstorms and downdrafts en route, the pilot diverted to Nose Mountain for fuel. Another thunderstorm forced a brief landing six miles south before arriving at 1803. The helicopter was shut down, 22 US gallons of Jet A were added (total 45 US gallons), and the same crew boarded for the accident flight.
Weather and Landing Area
Visual meteorological conditions prevailed, with widespread thunderstorm and lightning activity. Hourly winds and temperatures were not recorded on site; estimated temperature was 15°C to 28°C, with a density altitude of about 7000 feet using 22°C. Winds shifted from northeast to southeast at up to 10 knots. The landing area measured about 200 by 150 feet on the northeast side of a clearing, surrounded by trees up to 25 feet tall. A brush pile 6 feet high and 20 feet in diameter was west of the landing area. Distance from the landing area to the escarpment was about 200 feet; from the take-off position to the rim, about 125 feet. Elevation is 4879 feet asl. Prevailing winds were normally from the west, and departures to the west over the escarpment were typical. The pilot had departed that way many times.
Pilot Experience
The pilot held valid commercial helicopter and aircraft maintenance engineer licences, with about 5250 total flying hours, 3750 on Bell 206s. He had worked for Precision since 1981, flying since 1986, and was Chief Pilot and Operations Manager. He met training requirements, was knowledgeable about insufficient power and unanticipated right yaw, and had used the landing area several hundred times. He was well rested and described as competent and cautious.
Helicopter Weight and Balance
The pilot estimated firefighter weights (200, 150, 130 lbs) and baggage (100 lbs) for a calculated gross weight of 3104 lbs at take-off. Maximum take-off weight for the Bell 206B with internal load is 3200 lbs. Post-accident weighing of gear (239 lbs) and estimated passenger personal gear (14-17 lbs each) indicated the actual weight was approximately 3245 lbs. The engine (Rolls-Royce 250 C20) had been modified with a 250-C20B compressor and turbine for improved high-altitude performance. No pre-existing flight control or tail rotor drive system malfunction was found.
Passenger Weighing Practices
Canadian Aviation Regulations Section 703.37 requires a weight and balance system meeting Commercial Air Service Standards. Precision's operations manual required actual weights for computed load when the pilot estimates weights differ from standard. ASRD-FPB's handbook states the pilot is responsible for payload calculation, but a government representative must provide accurate manifests. At the time, ASRD-FPB had no formal system to provide actual individual firefighter weights including personal gear, and firefighters were not informed that excess weight was critical on smaller helicopters. Firefighters' annual fitness tests recorded self-weighed weight but not extra gear (boots, hard hat, coveralls, gloves, safety belt with first-aid kit and water bottle, and radios for team leaders). ASRD-FPB had preferred larger helicopters like the AS 350 but used Bell 206Bs during high demand.
Engine Power and Unanticipated Right Yaw
The maximum permitted take-off torque is 100% for five minutes, with 110% for five seconds. Before forward flight, torque to hover in ground effect was 96%, higher than the typical 85-90% for similar conditions. The helicopter did not attain translational lift before the escarpment rim, where ground effect was lost. Given weight and density altitude, more than 96% torque would have been required to maintain level flight after clearing the rim. The phenomenon of unanticipated right yaw (URY) is highly publicized in training literature, including the FAA Rotorcraft Flying Handbook, which describes it as an adverse yaw tendency.