This January 2008 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: Cessna 172. Location: Spotsylvania, Va. Injuries: 1 Minor. Aircraft damage: Substantial.
What reportedly happened: The pilot was on an IFR flight plan at 5,000 feet in VFR conditions and in contact with approach control when the engine began to run rough. He smelled fuel fumes but was unable to determine the source, so he informed approach control that he needed to land. The engine stopped less than a minute later. The pilot initiated an emergency landing, selecting a field that was 1,050 feet long. He used 10° of flaps and an approach speed of 90 knots. The airplane touched down about halfway down the field then hit a fence and trees.
Examination of the forced landing area revealed that there were more suitable forced landing areas available in the immediate vicinity. According to the Pilot Operators Handbook for the Cessna, the emergency landing procedure calls for full flaps and an approach speed of 65 knots. In this configuration the landing distance required was 610 feet.
The post-accident investigation revealed the No. 4 top spark plug insulator was cracked near the electrode. The No. 4 piston exhibited burn-through erosion signatures consistent with a pre-ignition/detonation event allowing excessive crankcase pressure. The oil supply was lost through the crankcase breather, resulting in the failure of the No. 2 connecting rod cap and loss of engine power.
Probable cause: A loss of engine power due to the cracked spark plug insulator and the failure of the connecting rod cap and loss of oil. The pilot’s failure to follow the checklist and failure to achieve the proper touchdown point during a forced landing was a factor.
For more information: NTSB.gov