This October 2007 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.
Aircraft: Mooney M20F. Location: Alamo, Nev. Injuries: None. Aircraft damage: Substantial.
What reportedly happened: Before departing on a cross-country flight, the pilot added one quart of oil to the engine, bringing the total oil level to seven quarts. Approximately 90 minutes into the flight he noticed a rise in oil temperature, so he opened the cowl flaps and increased the mixture in an effort to cool the engine. This resulted in a drop in exhaust gas temperature but the oil temperature continued to rise. The pilot then noticed that the oil pressure was reading low. He declared an emergency, and prepared to land on a dirt road. The engine backfired, then quit. The airplane rolled sharply to the right. The pilot was able to correct back to level flight, then made a forced landing in a sagebrush field close to a road.
The post-accident inspection detected a hole in the crankcase above and below the number four cylinder. An attempt was made to manually rotate the crankshaft. Only partial rotation was possible about 8 inches in either direction. No oil was found on the outside of the airplane or on the cowl. Investigators drained the oil out of the oil sump for a total of 0.75 to 1.25 quarts of oil. Along with the oil, portions of crankcase, lifter, piston, and connecting rod material were in the oil sump. The oil filter was opened. It contained metallic debris throughout the filament and in the can. All of the cylinders exhibited internal mechanical damage, as well as thermal damage to the number four connecting rod journal. The number two piston compression ring remained in its ring land, but had broken into several pieces. Extensive heat signatures were noted around the circumference of the number two piston at the ring land, which is consistent with oil burn.
Probable cause: Failure of the number two cylinder piston compression ring, which resulted in oil exhaustion and a catastrophic internal engine failure.
For more information: NTSB.gov.