NTSB Factual Report

On July 09, 2013, about 2000 mountain daylight time, a Stinson 108-3, N703C, experienced a total loss of engine power and landed in a field in Blackfoot, Idaho. The airplane was registered to, and operated by, the non-certificated pilot under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The non-certificated pilot, the sole occupant, was not injured; the airplane sustained substantial damage. The personal local flight departed from McCarley Field Airport, Blackfoot, about 1945. Visual meteorological conditions prevailed, and no flight plan had been filed.

The pilot stated that he departed for a short practice flight with about 8-9 gallons of fuel on board. After about 15 minutes airborne, with the airplane about 1,000 feet above ground level (agl), he heard a knocking sound from the engine compartment. Shortly thereafter, the engine power reduced about 50 percent and the airplane began to lose altitude. He switched the fuel selector to the other fuel tank and selected a wheat field where he could make a forced landing. During the landing roll, the airplane flipped over and came to rest inverted. The pilot estimated that he had about 4 to 5 gallons of fuel on board at the time of the accident.

A Federal Aviation Administration (FAA) certificated airframe and powerplant mechanic completed a post accident examination.

He noted that at the accident site, the right fuel cap remained secure in place on the wing and the right fuel tank was not breached. There was an eight inch diameter area of blithe and fuel-soaked soil near the right wing. Upon recovery he was able to drain 30 ounces of fuel. The left fuel cap was located in the soil adjacent to the wing; the tanks were not breached. The mechanic stated there was a 4.5 foot diameter area of blithe and fuel-soaked soil near the left wing. Upon recovery, the mechanic obtained 10 ounces of auto fuel and 2 tablespoons of water from the left tank. Disassembly of the gascolator revealed that the bowl was about half full of cloudy fuel and the screen was free of debris.

The fuel selector rotated freely, although the mechanic noted that the detents could not be felt, only a click could be heard. The mechanic found no evidence of mechanical malfunctions or failures that would have precluded normal operation.

The airplane's fuel system was designed to supply the engine via two aluminum fuel tanks located in both the left and right wing. Each tank holds 25 gallons, of which 2 gallons was unusable in level flight conditions. A strainer was located in the outlet of each tank and from there, fuel was gravity fed to the fuel selector valve, which contained an option of the following selections "23 GAL L," "23 GAL R," and on the bottom "FUEL OFF." The selector handle was shorter on the indicating/pointing side. From the selector, fuel would flow to another strainer and continue to the engine's carburetor.

NTSB Probable Cause

A partial loss of engine power during cruise flight for reasons that could not be determined because postaccident examination revealed no mechanical malfunctions or failures that would have precluded normal operation.