Accident Overview
A multi-engine instructional flight ended when the airplane entered a spin and descended into a wastewater treatment facility. The flight school personnel indicated that the intended flight plan was for the instructor to demonstrate Vmc maneuvers to the private pilot undergoing instruction (PUI) and a passenger.
Flight History
Radar data showed that the flight departed and flew in a south-southeasterly direction. The airplane climbed to approximately 4,400 feet mean sea level and then showed a decrease in true airspeed before disappearing from radar. Performance calculations using the airplane's presumed radar track indicated that during the last portion of the flight, the calibrated airspeed decreased to 110 knots and the rate of descent was 400 feet per minute. The track also showed a turn to the left prior to the airplane dropping off radar.
Post-Accident Examination
Post-accident examination revealed that the left engine was producing no to low power at the time of impact, while the right engine was producing mid to high power. This power configuration is consistent with a Vmc demonstration, during which power is reduced on the critical engine (left) and the airplane is recovered prior to loss of directional control or stall. Examination of the engines and propellers found no evidence of a mechanical malfunction or failure. Control continuity was confirmed.
Pilot Experience
The private pilot undergoing instruction had accrued 2.7 hours in multi-engine airplanes at the time of the accident. The passenger had accrued 8.3 hours in multi-engine airplanes. The calibrated airspeed of 110 knots noted during the final portion of the flight is greater than the Vmc speed of 69.5 knots.