Incident Summary
A certificated flight instructor (CFI) was providing multiengine training to one of two owner/pilots during an IFR cross-country flight. The flight experienced engine power anomalies and concluded with a forced landing off airport.
Sequence of Events
Upon reaching cruise altitude, both left and right engine fuel selectors were turned from the main tanks to the auxiliary tanks. Shortly after selecting the auxiliary tanks, the left engine lost power. Power was restored after the fuel selector was returned to the main tank, and the CFI elected to continue the flight.
During approach to land at the destination airport, the CFI simulated a left engine failure by reducing power on that engine to "zero thrust." The left propeller was intentionally not feathered. While on the base leg, about to turn final, the left engine began to surge, pulling the airplane away from the airport. The CFI stated that he took the controls and increased power on both engines, but the airplane continued to move away from the runway. He noted that the left engine continued to surge and the right engine began to lose power. He turned the boost pumps for both engines from low to "ON" (high) position without any noticeable difference in power. He did not feather either propeller, and made a forced landing off airport in a sandy ravine.
All pilots indicated that there was ample fuel aboard the airplane at the time of the engine problems and that the fuel tank selectors were on the main tanks.
Postaccident Inspection
Postaccident inspection disclosed that the left main and auxiliary fuel tanks had been breached during the accident. The right main and auxiliary tanks were intact, with a usable quantity of fuel remaining in the right main tank. The left engine had damage that precluded a test run. It was disassembled, and no evidence of any preimpact mechanical anomaly was discovered. The left engine driven fuel pump was not tested. The left engine's fuel boost pump ran noisily for a short period and then overheated.
The right engine was successfully run on a test stand. An inspection of the right engine's spark plugs prior to the engine run revealed that the engine had operated in an overly rich state just prior to the accident. The right engine's fuel boost pump functioned normally.
The airplane's flight manual cautions that if the boost pump switch is moved to the high position while the engine driven fuel pump is operating, an overly rich condition may result in a total loss of engine power.