N59611Bell 206B · 1974

On September 27, 2008, about 1300 mountain daylight time, a Bell 206B helicopter, N59611, was substantially damaged following a loss of control and impact with terrain about 5 miles southeast of Jelm, Wyoming. The certificated commercial pilot, the sole occupant, sustained serious injuries. Visual meteorological conditions prevailed for the local flight, which was being operated in accordance with 14 Code of Federal Regulations (CFR) Part 91, and a flight plan was not filed. The flight departed from the pilot's private residence about 1230.

In a statement submitted to the NTSB investigator-in-charge (IIC), the pilot reported that while he was 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. The pilot further stated, "I couldn't get response from the tail rotor, collective, or cyclic." In a telephone interview with the IIC 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.

A postcrash examination of the helicopter by a Federal Aviation Administration (FAA) aviation safety inspector revealed that damage to the helicopter included the separation of both main rotor blades, left main cabin door, 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, and there was no postcrash fire. The pilot reported no preimpact anomalies with the helicopter.

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. The notice contains the following warnings when maneuvering between hover and 30 miles per hour (mph): "Be aware that a tail wind will reduce relative wind speed if a downwind translation occurs. If loss of translational lift occurs it can result in a high power demand and an additional anti-torque requirement. Be alert during hover (especially OGE) and high power demand situations. Be alert during hover in winds of about 8-12 knots (especially OGE) since there are no strong indications to the pilot, to the possibility of a reduction of translational lift. This reduction results in an unexpected high power demand and increased anti-torque requirements. Be aware that if a considerable amount of left pedal is being maintained, that a sufficient amount of left pedal may not be available to counteract an unanticipated right yaw."

According to FAA Advisory Circular (AC) 90-95, "Any maneuver which requires the pilot to operate in a high power, low airspeed environment with a left crosswind or tailwind creates an environment where unanticipated right yaw may occur." The AC also advised of greater susceptibility for loss of tail rotor effectiveness (LTE) in right turns and the phenomena may occur in varying degrees in all single main rotor helicopters at airspeeds less than 30 knots.

The Rotorcraft Flying Handbook (FAA-H-8083-21) states that unanticipated yaw or loss of tail rotor effectiveness (LTE) is the occurrence of an uncommanded yaw rate that does not subside of its own accord and, which, if not corrected, can result in the loss of helicopter control. It further states, that LTE is not related to an equipment or maintenance malfunction and may occur in all single rotor helicopters at airspeeds less than 30 knots. The recovery technique for LTE is full left pedal while simultaneously moving cyclic control forward to increase airspeed. If altitude permits, reduce power.