Accident Sequence
On the day of the occurrence, the pilot of a Bell 206B helicopter (serial number 2118) was accompanied by a provincial forestry officer and a photographer. The flight was in support of video filming of a helicopter water bucketing operation near the Virginia Hills forest fire site, 30 nautical miles north of Whitecourt, Alberta. The forestry officer had requested video footage of a Bell 205 that was performing water bucketing. The Bell 206B pilot was flying above the trees in a westerly direction at about 30 miles per hour, just north of the pond used for water-bucket pick-up. The helicopter suddenly began to rotate to the right. The forestry officer was able to transmit a MAYDAY call. Attempts by the pilot to stop the rotation were unsuccessful, and the helicopter entered the tree canopy while spinning. A main rotor blade cut off the tail boom, and the fuselage struck the forest floor in a nose-down attitude, resulting in substantial damage. The pilot received head injuries, while the passengers were not injured. Other helicopters in the area reported the accident, and the occupants were picked up and taken to hospital.
Damage and Examination
The helicopter came to rest in a steep, nose-down but upright position. The nose was crushed, and the forward windscreen was broken. The main rotor blades were heavily strike-damaged. The tail boom had been cut off by a main rotor blade. The vertical fin, complete with the tail rotor gear box and tail rotor blades, separated and fell to the forest floor. The tail rotor blades did not display the usual heavy damage, though their leading edges showed evidence of tree strikes and the blade skin was torn. Both tail rotor blades were still attached to the root fittings and had been bent near their inboard ends. The tail rotor drive shaft segments had numerous fractures, including one at a bonded coupler.
Examination by the TSB Engineering Laboratory determined a torsional overload failure at the number-4 shaft, indicative of a tail rotor strike. The bonded coupler fracture occurred after the shaft had stopped rotating, and the coupler had been struck by a main rotor blade when the tail boom was cut off. A hole in the left engine cowl resulted from an engine-to-transmission coupler fracture typical of abrupt main rotor blade stoppage. Examinations of the freewheel and splines in the oil cooler fan shafting section revealed no pre-impact failures. The tail rotor control system was examined and no evidence of malfunction was found. Records indicated the helicopter was certified, equipped, and maintained per regulations.
The rotating descent into the tree canopy caused the tail rotor disc (left side of vertical fin) to initially contact upper tree branches horizontally as the tail boom rotated clockwise, producing an unusual damage pattern. Tree strike damage to the tail rotor blade leading edges confirmed the drive system was intact and rotating until tree contact. The torsional overload failure indicated a sudden stoppage of the tail rotor blades early in the impact sequence, with no progressive failure. This resulted in a loss of drive to the tail rotor and reduced blade damage compared to typical cases.
Operational Considerations
Weather at the time (1545 mountain daylight time) was clear sky, visibility obscured in smoke, wind from the southwest at 6 mph, temperature 25°C, and relative humidity 20%.
The Bell 205 being videotaped flew westerly during water bucket pick-up and climb-out. Its pilot observed the Bell 206B on a parallel course about 300 feet horizontally off his right side, at low speed and just above the trees. The Bell 205's track may have converged with the slower Bell 206B, which was below and behind in the rear right quarter. Studies have found that rotor wash from a helicopter in forward flight generates a pair of rotating vortices similar to those from fixed-wing aircraft. Turbulence intensity is proportional to weight and inversely proportional to rotor span and speed. Trailing vortices settle downward and can be dangerous for several minutes after the generating helicopter leaves.
A series of U.S. Army OH-58 helicopter accidents involving loss of directional control led to research identifying a condition known as loss of tail rotor authority (also called loss of tail rotor effectiveness or unanticipated right yaw). This can occur during certain wind conditions in single-rotor helicopter operations at airspeeds of 30 knots or less. It is defined as an uncommanded right yaw rate that does not subside and can result in loss of control. Four characteristics during low-speed flight contribute; one, "Main Rotor Disc Vortex," involves the tail rotor entering the main rotor vortex, reducing blade angle of attack and thrust. Relative wind azimuth in this characteristic is 285–315 degrees. This condition can lead to a rapid flat spin and sudden ground contact. Recovery normally requires collective pitch reduction, but insufficient height may prevent obstacle avoidance.
Since no pre-impact mechanical failure was found, the analysis focused on operational aspects. The loss of directional control could have resulted from self-generated loss of tail rotor authority or from encountering drifting main rotor vortices. The Bell 206B pilot was flying a slow, parallel course low over the trees on the right side. As the Bell 205 climbed away, it flew above and ahead of the Bell 206B. With wind from the southwest, conditions favored main rotor vortices from the Bell 205 drifting into the Bell 206B's path. These vortices could cause a sudden reduction in tail rotor thrust, resulting in uncommanded right yaw developing into a high turn rate. The pilot's low height precluded collective pitch reduction for recovery.
A comparison of the pilot's facial injuries and the forestry officer's lack of injury in the left front seat led the Board to conclude that use of a shoulder harness would have reduced the pilot's injuries.
Conclusion
The pilot was qualified and certified for the flight. Records indicated the helicopter was certified, equipped, and maintained per regulations. No pre-impact mechanical failures were found. The wind conditions and the helicopter's position and speed were favorable for a loss of tail rotor authority condition to develop. The pilot was not wearing the available shoulder harness and suffered facial injuries.
Causes and contributing factors: It is likely that the loss of directional control experienced by the Bell 206B helicopter pilot resulted from a loss of tail rotor authority when he encountered main rotor vortices of the Bell 205 helicopter.