Accident Details
On September 27, 2008, at about 1300 mountain daylight time, a Bell 206B helicopter (registration N59611) was substantially damaged following a loss of control and impact with terrain approximately 5 miles southeast of Jelm, Wyoming. The certificated commercial pilot, who was the sole occupant, sustained serious injuries. Visual meteorological conditions prevailed for the local flight, which was conducted under 14 CFR Part 91. No flight plan was filed. The flight had departed from the pilot's private residence about 1230.
Pilot and Witness Statements
According to a statement submitted to the NTSB investigator-in-charge (IIC), the pilot reported that while flying over his ranch at about 50 knots looking for stray cattle, "...a strong tailwind or twister caused the ship to vibrate very fast and violent, and dove to the right. My elevation was 200 to 300 feet above ground level (agl)." The pilot stated that several people on the ground observed the helicopter spin 5 to 6 times before impacting the ground. He further stated, "I couldn't get response from the tail rotor, collective, or cyclic." In a telephone interview on the day of the accident, the ranch foreman reported observing the helicopter making a "big circle," followed by 3 or 4 "tight circles" in a clockwise direction before impacting terrain.
Damage Examination
A postcrash examination by a Federal Aviation Administration (FAA) aviation safety inspector revealed damage including the separation of both main rotor blades, the left main cabin door, the left main skid, and the outboard one-third of one tail rotor blade. The tail rotor assembly was severed from the tail boom. The helicopter came to rest partially on its right side on sloping uneven terrain. There was no postcrash fire. The pilot reported no preimpact anomalies with the helicopter.
Relevant Safety Information
On October 31, 1983, Bell Helicopter published Operations Safety Notice (OSN) 206-83-10 regarding loss of tail rotor effectiveness (LTE) in the Model 206B and similar airframes. The OSN describes the phenomenon as an unanticipated right yaw. It contains warnings about maneuvering between hover and 30 mph, particularly regarding tailwinds, loss of translational lift, and high power demand situations. According to FAA Advisory Circular (AC) 90-95, any maneuver requiring high power at low airspeed with a left crosswind or tailwind creates an environment where unanticipated right yaw may occur. The Rotorcraft Flying Handbook (FAA-H-8083-21) defines LTE as an uncommanded yaw rate that does not subside of its own accord and can lead to loss of control if not corrected, noting it may occur in all single rotor helicopters at airspeeds less than 30 knots.