Cessna 177 N13612 accident near Travelers Rest, South Carolina

Probable cause

Maintenance personnel’s improper servicing of the magneto, which resulted in its accelerated wear, failure, and the total loss of engine power.

NTSB analysis

About 15 minutes after takeoff during an instructional flight, the engine backfired several times and 5 to 10 seconds later the engine lost total power. The flight instructor assumed control of the airplane, maintained a 75-knot glide speed, and attempted to restart the engine, but without success. The flight instructor landed in a recently plowed field and, while at a slow groundspeed, the nose wheel settled into the mud and the airplane nosed over, resulting in substantial damage to the fuselage and both wings.

The postaccident examination of the engine revealed that the single-drive dual magneto was not delivering power to the spark plugs. The examination of the magneto revealed that, while the left and right contact assemblies of the magneto were properly secured, neither of them opened during rotation of the rotating magnet assembly. Thus, with rotation of the magnet and the contact assembly points closed, the electrical path was shorting to ground, and not the normal path to the ignition leads, which likely resulted in the total loss of engine power. The left and right breaker cam followers, which would open each contact assembly after contacting a lobe of the breaker cam, was worn to less than the minimum specified for a new part and was insufficient to open the contact assembly points. Both cam followers exhibited accelerated wear and heat signatures consistent with lack of lubrication. Each felt pad, which provided lubrication to the oil-impregnated breaker cam, was dark and dry consistent with lack of lubrication.

The magneto had operated satisfactory for nearly 40 years and 1,362 hours and then underwent a 500-hour inspection about 112 hours and 3 years before the accident, during which each cam follower was in satisfactory condition and displayed no abnormal wear. During that inspection the mechanic likely improperly cleaned the breaker cam and used incorrect oil to lubricate the felt of each cam follower. Thus, the improper servicing of the magneto likely resulted in the accelerated wear and heat signatures observed, both of which were consistent with lack of lubrication of each cam follower.

Text from the NTSB. Open the NTSB docket for ERA23LA162 for the full record.