What happened
During an IFR cross-country flight, a certificated flight instructor was conducting multiengine training with one of the two owner/pilots aboard. Upon reaching cruise altitude, the crew switched both the left and right engine fuel selectors from the main tanks to the auxiliary tanks. Shortly after this change, the left engine lost power; however, power was restored once the selector was returned to the main tank, prompting the instructor to continue the flight.
During the approach to the destination airport, the instructor performed a simulated left engine failure by reducing power on the left engine to zero thrust, intentionally leaving the propeller unfeathered. While on the base leg and preparing to turn final, the left engine began to surge, causing the aircraft to drift away from the airport. The instructor took control and increased power on both engines, but the aircraft continued moving away from the runway. During this period, the left engine continued to surge and the right engine began to lose power. Although the instructor moved the boost pumps for both engines from the low position to the high position, there was no noticeable change in power. The crew ultimately performed a forced landing in a sandy ravine off the airport. All occupants were not injured, and all pilots reported that ample fuel was on board and selectors were set to the main tanks at the time of the engine issues.
The investigation
Post-accident inspection revealed that the left main and auxiliary fuel tanks were breached during the impact. The right main and auxiliary tanks remained intact, with usable fuel remaining in the right main tank. While the right engine was successfully run on a test stand, an inspection of its spark plugs indicated the engine had been operating in an overly rich state just prior to the accident. The left engine sustained damage that prevented a test run; following disassembly, no evidence of pre-impact mechanical anomalies was found, though the left engine's fuel boost pump was noted to have run noisily before overheating.
Findings
- The right engine operated in an overly rich condition prior to the accident.
- The aircraft flight manual contains a caution stating that moving the boost pump switch to the high position while the engine-driven fuel pump is operating may result in an overly rich condition and total loss of engine power.