NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.
The pilot reported that the engine began running roughly during cruise flight a few minutes after he switched the fuel selector from the left to the right main fuel tank. He initially switched back to the left main fuel tank, which still contained useable fuel, increased the mixture control to full rich, and turned on the high-boost auxiliary fuel pump. The engine continued to run roughly with occasional backfires and power surges. The pilot then selected the low-boost auxiliary fuel pump and changed back to the right main fuel tank, which contained significantly more fuel than the left main fuel tank. He reported that engine operation briefly improved, before it resumed producing partial power with occasional power surges. The pilot reported that he continued to select different fuel tanks, including the right wingtip tank that only had residual fuel available. Ultimately, the pilot selected the right main fuel tank and the engine continued to operate at partial power. The pilot conducted a forced landing to a nearby interstate highway. Shortly after touchdown, the pilot had to maneuver the airplane to avoid another vehicle and it subsequently entered the grass median separating the northbound and southbound lanes. The airplane’s left wing was substantially damaged when it impacted a grass embankment.A postaccident inspection of the airplane established that there was ample fuel remaining within each main wing fuel tank. The wingtip fuel tanks contained about a gallon of fuel each. Both auxiliary fuel pumps functioned normally when electrical power was applied. An operational engine test run was completed without any modifications to the airframe fuel system and using the fuel remaining in each main tank. The engine performed within its normal operating parameters throughout the engine test run and demonstrated the ability to produce maximum static rpm. Additionally, while at a cruise engine power setting, there was no interruption to engine operation while alternating between the right and left main fuel tanks. During the test run, the fuel selector handle was placed between the right and left main fuel tank positions to simulate an inadvertent misplacement of the fuel selector. Normal engine operation continued for about 30 seconds before there was a noticeable loss of engine power and an associated engine backfire from fuel starvation. The fuel selector was immediately repositioned to the right main fuel tank and normal engine operation was reestablished. Ultimately, the operational test run did not reveal any anomalies that would have precluded normal engine operation.The airplane owner’s manual stipulates that when there is a loss of engine power, the auxiliary fuel pump should be used momentarily until engine power is regained. However, if engine operation does not improve within a few moments, the auxiliary fuel pump should be turned off. A flight manual supplement further stipulates that if a fuel tank becomes fully depleted, the low-boost auxiliary fuel pump should be turned on while changing fuel tanks; however, the auxiliary fuel pump should be turned off once the engine restarts. Additionally, the flight manual supplement cautions that operating with the high-boost auxiliary fuel pump turned on with a functioning engine-driven fuel pump can produce an over-rich fuel mixture and a partial or total loss of engine power.It is likely that the initial engine roughness was caused by the pilot improperly positioning the fuel selector handle between the left and right main fuel tanks, which resulted in a restricted fuel flow to the engine and the initial loss of engine power during cruise flight. After identifying the initial loss of engine power, the pilot likely repositioned the fuel selector handle to a fuel tank containing useable fuel; however, contrary to operational guidance, his continued to use the high-boost auxiliary fuel pump for an extended period, which likely resulted in an over-rich fuel mixture condition that prevented the engine from resuming normal operation.