What happened
During a flight at altitudes between 2,000 and 2,500 feet mean sea level, the pilot was operating with fuel being supplied from the main tank. Approximately one hour into the flight, the pilot activated the boost pump and switched the fuel selector to include the auxiliary fuel tank. About 30 to 45 minutes later, while both fuel tank on/off valves were in the "on" position, the engine experienced a loss of power.
The pilot applied carburetor heat, but the engine power was not restored. The pilot attempted a forced landing on an east-west oriented dirt road but determined that trees prevented a landing on the roadway itself. Consequently, the pilot maneuvered for a landing south of and parallel to the road. During the descent, the left wing of the aircraft collided with a tree approximately 20 feet above ground level. The pilot was not injured.
The investigation
An FAA airworthiness inspector examined the aircraft at the scene and found that the original fuel tank was empty, while an aluminum auxiliary tank installed in place of the hopper tank was full. In the cockpit, yellow placards near the two fuel on/off valves indicated usable fuel quantity and the "push-on" and "pull-off" positions; however, no placard warned that both valves should not be operated simultaneously.
Check valves were installed at the outlets of both tanks and were oriented correctly for flow direction. Laboratory testing conducted with FAA oversight revealed that the check valve from the original fuel tank opened at less than 1/4 psi, whereas the check valve from the auxiliary tank required 3/4 psi to open.
Maintenance records from May 25, 2003, showed that a mechanic had removed agricultural equipment and installed a 77-gallon aluminum fuel tank and boost pump. The mechanic stated the installation was modeled after a system used on another aircraft and utilized check valves purchased from a yacht company. While the mechanic claimed to have installed a placard prohibiting the simultaneous use of both valves, no such warning was found in the cockpit. Furthermore, the FAA inspector-in-charge determined that the installation of the 77-gallon auxiliary tank was performed without FAA-approved data and that the airplane flight manual lacked a supplement for operating the modified system.