Bell 407 N407HL

The single-main-rotor helicopter was dispatched on a search and rescue (SAR) mission in response to a snowmobile accident in remote, mountainous terrain. The helicopter was on lease to the local sheriff's office, and was being flown by the chief pilot of the company that owned it. Two SAR personnel were passengers. After an initial but fruitless search to locate the snowmobile victim, the helicopter located members of the snowmobile party. After a landing and brief discussion with the snowmobilers, they agreed to lead the helicopter to the accident site on their snowmobiles. The helicopter lifted off to follow the snowmobilers, flying about 100 to 200 feet above the trees. Because the helicopter was faster than the snowmobiles, the pilot stopped several times in a hover to allow the snowmobiles to catch up. During one hover, the pilot experienced a slight left yaw, which he believed he corrected. The helicopter then began to spin rapidly and descended into the trees. The sheriff's office dispatchers used a commercial flight following system to track the helicopter, but system difficulties prevented them from detecting the accident via that system. The injured pilot climbed a nearby hill, and notified the dispatchers of the accident via radio. The recovered wreckage was examined, and no evidence of any preimpact deficiencies or failures that would have prevented normal operation and continued flight was discovered. Data recovered from the engine control unit indicated that the engine operated normally until the accident, when parameter exceedances and torque spikes, consistent with main rotor blade strikes, were recorded. The directional control actuator (DCA), which was the hydro-mechanical unit used to control the tail rotor, was removed and sent to the helicopter manufacturer for testing and examination. The DCA passed the functional checks, and no evidence of any anomalies that would have prevented normal operation was detected. A damaged tree near a trail sign in the search area suggested the possibility that the tail rotor might have struck the tree and been damaged just before the accident. However, examination of the damage to the tree and the tail rotor did not support that hypothesis. Single-main-rotor helicopters are susceptible to a phenomenon known as "loss of tail rotor effectiveness" (LTE), which can occur at low airspeeds, and is a function of relative wind direction. The loss of control occurred in a hover, but the relative wind direction could not be determined. LTE is more likely at high density altitude and/or gross weight. The accident occurred at a density altitude of about 9,000 feet, and at a weight about 800 pounds below the maximum certificated operating weight. In the accident helicopter, LTE would result in a nose-right spin direction. Both the pilot and surviving passenger recalled that the spin direction was nose left, which is contrary to LTE. However, neither was certain of the spin direction, and a ground witness reported that the spin direction was nose right, consistent with LTE. Examination of the wreckage did not provide conclusive evidence as to the direction of the spin, but did indicate that both the main and tail rotor were rotating under power at the time of impact. Based on the witness observation and the uncertainty of the helicopter occupants about the direction of spin, the high density altitude, and the lack of indications of a tail rotor mechanical failure, it is likely that the loss of control was due to LTE.

Quoted verbatim from the NTSB record.