Accident Sequence
A helicopter conducting a patrol flight near an international border was maneuvering at a low airspeed at an altitude of approximately 150 feet above ground level. According to local authorities, the helicopter impacted a parked pickup truck in a nose-low attitude while in a right turn, coming to rest on its right side.
Witness Observations
Multiple witnesses reported observing the helicopter's unusual behavior. One witness stated that the helicopter "appeared to dip the nose-down and enter a spin to the right nearly straight down." Another witness added that the helicopter seemed to recover from the spin and initiate a climb before it began to spin to the right again, impacting the ground in a near-vertical attitude. Several witnesses noted that the engine sounded as though it were "screaming." A witness with experience as a helicopter mechanic reported that while the engine was screaming, the rotor system sounded as though it was slowing down. Another witness described the sound as "sucking or chopping air."
Weather and Environmental Conditions
Recorded weather near the accident site included winds from 260 degrees at 12 knots, visibility 10 statute miles, clear skies, temperature 23 degrees Celsius, dew point 1 degree Celsius, and a barometric pressure of 29.94 inches of Mercury. The density altitude was computed to be 5,433 feet above mean sea level.
Investigation Findings
Examination of the helicopter and its engine revealed no discrepancies or pre-existing anomalies that could have precluded normal flight.
Performance Study
A performance study was conducted to evaluate the controllability and maneuvering capabilities during the assigned observation mission. The study concluded that at the approximate altitude of 150 feet AGL and an airspeed of 20–30 knots, there may not have been sufficient time or altitude for the pilot to recover after the helicopter entered a vortex ring state. A fully developed vortex ring state is characterized by an unstable condition where the helicopter experiences uncommanded pitch and roll oscillations, has little or no cyclic authority, and achieves a descent rate that, if allowed to develop, may approach 6,000 feet per minute. The study noted that a vortex ring state may be entered during any maneuver that places the main rotor in a condition of high upflow and low forward airspeed.