Background
The pilot was maneuvering the airplane south of the airport in preparation for a visual flight rules (VFR) approach. Witnesses observed the airplane in a right spin.
Radar Data
NTAP data indicated the airplane was well above stall speed before disappearing from radar. Examination of radar data revealed a rapid deceleration and descent over two consecutive six-second intervals. In the first interval, ground speed dropped 31 knots (from 200 to 169 knots) and altitude dropped 440 feet (from 11,760 to 11,320 feet), corresponding to a descent rate of 4,400 feet per minute. In the next six seconds, ground speed dropped another 31 knots (from 169 to 138 knots) and altitude dropped 1,020 feet (from 11,320 to 10,300 feet), corresponding to a descent rate of 10,200 feet per minute.
Manufacturer's Input
According to the manufacturer, if the throttles were to be brought back into Beta (flat pitch) range, it is possible that one propeller could go into Beta an instant before the other propeller. If this were to happen, the airplane would instantly snap roll and enter a spiral.
Pilot Experience
The pilot had received an estimated 4 hours of dual instruction in the airplane.
Probable Cause
As per official findings, the probable cause was the pilot's loss of aircraft control inflight for reasons undetermined. Contributing factors were the pilot's inadequate transition/upgrade training and his total lack of experience in aircraft make/model.