Worn magneto leads to forced landing

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.
Location: Miami.
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.

Comments

  1. Pedro Rezende says:

    I think they should tell us the magneto type and model. I work with magnetos Slick and Bendix /TCM for more than 13 years and I know a lot of problens related to each one. So I would like to understand more about this failure.

  2. Was this a desk analysis accident investigation performed by FAA & NTSB? Was anyone on-site to actually see anything other than the left mag? Looks to me like FAA & NTSB wanted this accident report to move off their desk so bigger better things could be accomplished, e.g., coffee break. Also, I haven’t read anything in a POH which claims that a normal operating procedure to clean fouled spark plugs is to run it up and play w/the mixture. We all do it but it isn’t in any book I know about.

  3. I question their deduction that the pilot operated the aircraft with known deficiencies. He performed a normal operating procedure to clean fouled spark plugs. His mag check was within normal limits. Therefore he left the ground with a normally operating engine. Furthermore how does losing one mag cause the engine to drop to 400 rpm (windmilling)? There had to be more to the story. Perhaps the right mag was also bad or carb icing was a factor. It does mention lack of visibility. Was he flying in fog?

  4. Elias Vujovich says:

    If the left mag was only firing intermittently, why did the engine not keep on running on the right mag? I tell my students if it is starting to run rough, determine if one mag is causing it and if it is, then quit using that one and head for the nearest airport–I had a similar case like that in a C-172R and at the runup/mag check, the rpm drop was not acceptable. It seemed to be getting worse the longer the engine ran. Canceled the charter and taxied back to hangar where the owner/mechanic noticed oil leaking from the rear of the mag. Only 1 bolt was left holding it on due to some one else doing lousy maintenance–EJV

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