Incident Overview
During cruise flight, the pilot observed oil on the windshield and decided to perform a forced landing on a dirt road. The pilot reported that his approach speed was 10 knots higher than normal to avoid obstructions, and he did not use flaps because he would not have made it to the road. Realizing he was going too fast and would overshoot the road, the pilot turned toward an adjoining field. He maneuvered the airplane over a buffalo herd and landed. During the landing roll, the airplane collided with a fence, rupturing the right wing fuel tank.
Preflight and Oil System
The pilot stated that during preflight, he checked the oil and it was about one-third down from the top, equating to approximately 6 quarts (capacity is 8 quarts). The oil filler cap was easily visible and in place at that time, but the pilot did not physically check it for security. After the incident, the oil filler cap was found in a recess atop the right bank of cylinders, where it had fallen. Subsequent examination of the engine revealed that 3 quarts of oil remained in the crankcase.
Findings
The missing oil filler cap was located on top of the engine. The pilot's preflight inspection did not include physically verifying the cap's security.