N731DH

On June 29, 1993, at about 0805 central daylight time a Cessna 188B, N731DH, lost engine power during an aerial application maneuver near Centre, Alabama. The airplane was registered to Charles Timmons, dba Aero Dusters, and operated by Vaughn Flying Service, Inc., under visual flight rules and 14 CFR Part 137. A flight plan was not filed for the aerial application flight. There were no injuries to the airline transport pilot, while the airplane was substantially damaged. Origination of the flight was Centre, Alabama, at about 0700 on the same day.

The pilot reported that the engine lost power, at 200 feet, and the propeller ceased rotating, during the climb following a swath run. He force landed the airplane in a wet, slick, soybean field. Unable to stop, the airplane was intentionally ground looped to avoid colliding with a house. The left main landing gear collapsed and the wing collided with the ground. On site examination of the engine oil screen revealed metal in the oil.

The aircraft's engine was examined and fretting was discovered along the main bearing support parting surfaces. The number two and three main bearing lock slots were displaced .06 inches and particles of the number two main bearing shell were found in the oil sump. The engine exhibited wear in several locations which indicated passage of bearing shell fragments throughout the engine during operation. The crankshaft was fractured from the number two main crankshaft journal to the number three crankcheek. The number three and four camshaft intake lobes and corresponding valve lifters were spalled and the number five counter weight pin was found in the oil sump. The number five counterweight retaining plate and snap ring were not found.

On May 22, 1993, a top overhaul was performed on the engine, all cylinders were removed, and the number three cylinder was replaced. According to John V. Moeller, Aviation Safety Inspector, Manufacturing, this type of maintenance requires loosening of the crankcase throughbolts. He stated that the lack of sufficient torque placed on the crankcase throughbolts resulted in the fretting which was found on the crankcase. He also stated that the rubbing between the crankcase parting surfaces which causes this type of fretting allows the main bearings to move fore and aft, which eventually results in the breakup of the crankshaft bearings. Once a main bearing breaks up, the crankshaft is subject to surface origin fatigue like that found on the number two journal and the crankshaft may fail.