DEN83LA118

While engaged in seismil work, the plt was contacted by radio & was asked to assist in freeing 2 crew trucks that had gotten stuck in sand dunes. He decided to assist since the stuck trucks were delaying the entire operation. He planned to partially raise & pull one end of a truck by using a 100 ft long line. The helicopter was facing the aft end of the truck & long line was at about a 25-30 deg angle when continuous collective power was applied. By the plt. The helicopter then yawed left. The plt attempted to compensate with right anti-torque pedal, reduced power & induced right forward cyclic. Reportedly, the helicopter then turned 360 deg & came to rest on its right side. At some point after the left yaw, the plt released the long line. The company safety officer reported that if a 150 ft long line had been used, there would have been a larger margin for error.