Background
A flight instructor (CFI) and a commercial certificated dual student pilot were conducting glider tow operations. The CFI initially occupied the front seat, and the student pilot was seated in the rear. The purpose of the flight was to instruct the student in glider tow procedures.
Sequence of Events
The pilots completed two glider tows and then decided to exchange seats. With the engine running, the student exited the rear of the airplane and stood outside while the CFI climbed over the front seat into the rear. A third glider tow operation was performed without incident.
During the fourth glider tow, while climbing, the engine lost power. The pilots selected a muskeg field for a forced landing. As the airplane descended to about tree height, the engine surged to full power briefly and then stopped again. The pilots were unable to land in the muskeg field and instead landed the wheel-equipped airplane in a small lake, where it sank.
Postaccident Examination
Examination of the cockpit before the airplane was raised revealed that the fuel selector was positioned between the left and right fuel tank positions. Postaccident inspection of the airplane and its systems found no evidence of a preexisting mechanical anomaly. The engine was supplied with fresh fuel and started, running normally. The fuel selector was then moved to the position where it had been discovered, while the engine was submerged. Under that configuration, the engine ran for approximately two minutes and then stopped.