ATL94LA079

Upon arriving from a forty minute local flight, the pilot entered a traffic pattern for landing at his private sod airstrip. While on final approach the pilot elected to go around because someone was walking on the sod strip. When the pilot applied power the engine failed to develop full power and he heard a loud back firing sound come from the engine. With reduced power, 1600 RPM, the pilot made a 270 degree left turn and made a forced landing in a plowed field adjacent to his airstrip; at touch down the airplane nosed over. Examination of the engine assembly disclosed that, the outer top or half moon section of the intake boss, on number 3 cylinder head, had separated from the boss. The half moon section of the boss assembly was recovered from inside the rocker cover and examined. The examination of the fracture face revealed areas of fatigue striations, and a post-fracture mark on the outer radius. The examination also determined that the functional ability of the cylinder was not effected unless the separated piece of boss was wedged under one of the moveable components of the cylinder head.

On April 11, 1994, at 0940 central daylight time, a Cessna 150, N11560, collided with the ground and nosed over while attempting an emergency landing near Hazel Green, Alabama. The personal flight operated under 14 CFR Part 91 with no flight plan filed. Visual weather conditions prevailed at the time of the accident. The airplane sustained substantial damage; the pilot and passenger were not injured. The flight departed Hazel Green at 0900 hours.

After flying forty minutes in the local area the pilot and passenger returned to his private airstrip. The pilot completed the before landing check that included the application of carburetor heat. While on short final, the pilot noticed that someone was walking on the runway; at this point he elected to go-around. When he advanced the throttle for full power, the engine would only develop 1600 rpm. The pilot selected an emergency landing area and established an approach for the forced landing. Throughout the maneuver the engine continued to develop power, but not enough to maintain flight. The airplane touched down in a freshly plowed field and nosed over.

During the engine examination the outboard rocker boss for the intake valve rocker arm was broken. The half moon section of the boss assembly was located under the rocker cover. Examination of the fracture faces of the half moon section disclosed that the thinner or bottom portion of the boss assembly failure was typical of an overstress separation. The top or thicker portion of the boss assembly examination revealed isolated areas where a fatigue striation pattern was found (see attached metallurgist's factual report). Examination of the half moon section disclosed that there was a post-fracture mark on the outer radius of the half moon section; the source of the post-fracture mark was not determined. Despite the fracture, normal movement of the intake valve components were possible.

Weather conditions were favorable for the formation of carburetor ice. During the telephone conversation with the pilot, he stated that carburetor heat was applied several times during the flight and on the initial approach. He also stated that normal RPM drops were noted when the carburetor heat was applied.