What happened
During a patrol flight near an international border, a helicopter was maneuvering at a low airspeed at an altitude of approximately 150 feet above the ground. While performing a right turn, the aircraft impacted a parked pickup truck in a nose-low attitude and came to rest on its right side. Witnesses observed the aircraft dip its nose and enter a right-hand spin, descending nearly straight down. One witness noted that the aircraft appeared to recover from the spin and initiate a climb before entering a second right-hand spin, ultimately impacting the ground in a near-vertical attitude.
Witnesses provided various accounts of the engine and rotor sounds. One observer, experienced as a helicopter mechanic, reported that while the engine was loud, the rotor system sounded as though it was slowing down. Another witness described the sound as if the aircraft were sucking or chopping air.
Weather conditions at the time included winds from 260 degrees at 12 knots, visibility of 10 statute miles, and clear skies. The temperature was 23 degrees Celsius with a dew point of 1 degree Celsius and a barometric pressure of 29.94 inches of Mercury.
The investigation
An investigation included a performance study to determine the controllability and maneuvering capabilities of the aircraft during the observation mission. No discrepancies or pre-existing anomalies were found with the helicopter or the engine that could have prevented normal flight.
Findings
A performance study concluded that at an altitude of approximately 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 aircraft entered a vortex ring state. This unstable condition is characterized by uncommanded pitch and roll oscillations, minimal cyclic authority, and a descent rate that can approach 6,000 feet per minute. Such a state can be entered during any maneuver that places the main rotor in a condition of high upflow and low forward airspeed.