LAX07LA020 · Aero Commander 112

During the takeoff climb the engine began to run rough, and was not producing enough power to sustain flight. The certified flight instructor (CFI) reported that there was no remaining runway when the engine began to lose power, and he made a forced landing in an open field. An annual inspection had been completed on the airplane 14 days prior to the accident, and this was the first flight since the inspection. During the annual inspection, the fuel system had been inspected. The inspection included de-fueling the fuel tanks to seal the wing leading edge rivets. The fuel tanks were serviced, and no leaks were observed. An inspection checklist for the fuel system identified various items for inspection at the 50 and 100-hour inspections. During the 50-hour inspection, the drain valves, vent lines and vents, filler cap, anti-siphoning valves, and fuel system placards are examined. In the 100-hour inspection, the internal tank inspection, gages, transmitters, fuel line fittings, vent lines, and vents, and the fuel vent line drain assembly beneath the fuselage is removed and cleaned. Following the annual inspection, the airplane sat on the ramp for 2 weeks before the accident flight. On the accident flight, the CFI stated that he and the owner/passenger did a thorough preflight with no discrepancies noted. According to the flight manual, during the preflight inspection, the pilot is to check that both the left and right wing fuel tank vents are unobstructed. The postaccident airframe inspection revealed that the left and right wings' fuel vent lines were clogged with dirt at each end, and their associated unions were also packed with dirt. The vent lines and unions were removed and shipped to the Safety Board's materials laboratory for further inspection. The fuel vent lines were found intermittently clogged with dirt at multiple locations along the length of the lines. The vent lines were cut open and the dirt removed for examination. Insect carcasses were found in the vent lines. According to the airframe manufacturer, there are three fuel vent lines; one at each wing, and one located on the belly of the airplane. If both the main fuel vent lines (left and right wings) become clogged, the belly fuel vent line would not be able to compensate for the clogged vent lines, and fuel flow to the engine would be restricted.

Verbatim NTSB analysis and probable cause from the NTSB dataset