What happened
The flight was conducted to provide instruction in glider tow operations. Initially, the instructor pilot (CFI) occupied the front seat of the wheel airplane, while the commercial certificated dual student pilot sat in the rear. After completing two glider tow operations, the pilots decided to switch positions. During this process, the student exited the aircraft and stood outside while the CFI climbed over the front seat into the rear; this seat change was performed while the engine was running.
Following a third successful glider tow operation, the engine lost power during the climb of the fourth operation. The pilots attempted to select a muskeg field for a forced landing. As the aircraft reached approximately tree height, the engine surged to full power before stopping again. The pilots were unable to land in the intended muskeg field and instead landed the aircraft in a small lake, where it subsequently sank.
The investigation
Prior to the aircraft being raised from the water, an examination of the cockpit revealed that the fuel selector was positioned between the left and right fuel tank positions. A post-accident inspection of the aircraft and its systems found no evidence of any preexisting mechanical anomaly. To test the system, the engine was provided with fresh fuel and started, running normally. When the fuel selector was moved to the same intermediate position found during the submerged inspection, the engine ran for approximately two minutes before stopping.