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: Cessna 172.
Location: Green Valley, Ariz.
Aircraft damage: Substantial.
What reportedly happened: During cruise flight the engine suddenly started to run rough and backfire. The pilot attempted to troubleshoot the rough-running engine without success. He decided to initiate a precautionary landing on a nearby road. During the landing roll out, the airplane hit two road signs and a wire fence.
Examination of the engine revealed that the number three cylinder exhaust rocker arm was disconnected from the cylinder due to the rocker arm hold-down nut backing off.
Probable cause: The loss of engine power due to the separation of the number three cylinder exhaust rocker arm.