This March 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 150.
Injuries: 2 Minor.
Aircraft damage: Substantial.
What reportedly happened: A CFI and pilot receiving instruction were practicing airwork when the engine began to run rough. They diverted to a nearby airport and made a precautionary landing.
The CFI performed an engine run-up and noticed a rough-running left magneto. He attempted to fix the problem by running up the engine to a high rpm setting with the fuel/air ratio leaned in an effort to clear what he thought was lead-fouled spark plugs. The left magneto drop was within limits after the procedure.
The flight resumed. When the Cessna was about six miles from the original departure airport, power began to fluctuate, then dropped to no more than 400 rpm.
The CFI took control of the airplane. He attempted to maneuver the airplane towards a grass airstrip that he knew about, but because of the low light and visibility restrictions, he flew past it. He elected to land in a field perpendicular to the airstrip. While approaching the field, the airplane hit powerlines.
The post-accident examination of the engine revealed the left magneto operated intermittently. Disassembly of the left magneto revealed the point cam was worn, and the points exhibited erosion.
Probable cause: The CFI’s continued operation of the airplane with known deficiencies. The lack of suitable terrain for a landing was a factor.
For more information: NTSB.gov/ntsb/brief.asp?ev_id=20070313X00278&key=1.