Background
At 1209 mountain daylight time, a Bighorn Helicopters Inc. Bell 206B Jet Ranger helicopter (registration C-GCHE, serial number 1249) departed from the company heliport on the outskirts of Cranbrook, British Columbia, with a pilot and two passengers on board. The passengers were BC Hydro power line technicians. The mission was to visually inspect 60 kVA high-voltage electrical power transmission lines that ran through suburban areas of Cranbrook. The inspection required flying at approximately 20 to 30 feet above power line or pole heights at a ground speed of about 25 knots.
Accident Sequence
At about 1306, while flying southbound at about 120 feet above ground level, the helicopter experienced a sudden loss of engine power, causing a rapid loss of rotor rpm. The helicopter descended quickly and landed heavily on a paved street. It struck a pedestrian on the sidewalk and a motor vehicle travelling westbound on the street. The helicopter broke apart, burst into flames, and was destroyed by a fuel-fed post-crash fire. The three occupants and the pedestrian were fatally injured. The motor vehicle occupant was uninjured. The emergency locator transmitter was consumed by the fire; it is unknown if it transmitted. The accident site was within Cranbrook at latitude 49°29′56″ N, longitude 115°45′40″ W, elevation about 3100 feet above mean sea level.
Loss of Engine Power
The cause of the loss of engine power was not determined. A pilot inadvertently rolling off the throttle or closing the fuel valve was considered unlikely given the flight conditions and pilot experience, except in the case of a complete loss of tail rotor thrust. However, the tail boom and rotor survived and showed no anomalies. Fuel exhaustion was not a factor, as a large amount of spilled fuel was present. Fuel starvation from blocked or fractured lines could not be ruled out. No evidence of pre-impact mechanical failure in any engine module was found. The fuel control unit (FCU) was thermally damaged and could not be functionally tested; examination did not identify a failure mode but did not rule out a malfunction. The power turbine governor (PTG) had over 1100 hours of service since installation in June 2006 with no reported anomalies, but given a shortfall in average service life of such units, a PTG malfunction was possible.
Loss of Flight Control
No evidence indicated any pre-accident malfunction of flight controls or rotor drive system that could have contributed to loss of control. The interruption to engine power caused rapid rotor rpm decay, preventing effective control. The helicopter descended nearly vertically and collided with the street.
Operating Environment
The accident flight occurred at low altitude and low airspeed over built-up areas. The rotorcraft flight manual (RFM) height-velocity diagram (HVD) identifies high-risk zones, labeled "avoid" areas, that represent challenging conditions for recovery from an engine failure. The helicopter frequently operated within these avoid zones. The pilot had limited altitude and time to establish autorotative flight. Obstructions, low airspeed, low height, and low rotor rpm combined to create a rapidly deteriorating situation.
Regulatory Context
Canadian Aviation Regulations (CARs) govern low-level flight over built-up areas. The operator interpreted the power line inspection as either a flight inspection or as not over a built-up area, potentially exempting them from general prohibitions. However, the operation inside HVD avoid areas and any possible non-compliance with CARs were not linked to the loss of engine power. The severity of the outcome was influenced by the low altitude, airspeed, and landing site environment.
Conclusion
Two causal events occurred in sequence: a loss of engine power followed by a loss of flight control. The exact cause of the engine power loss remains undetermined.