Accident Summary
On March 23, 2023, at about 1830 eastern daylight time, a Cessna 177B, N13612, was substantially damaged during a forced landing near Travelers Rest, South Carolina. The flight instructor and private pilot were not injured. The flight was conducted as a Title 14 Code of Federal Regulations Part 91 instructional flight.
The pilot reported that about 15 minutes into the flight, the engine backfired a couple times, and 5 to 10 seconds later, total engine power was lost, though the propeller continued to rotate. The flight instructor took control, established a 75-knot glide speed, and attempted to restart the engine without success. He selected a nearby field, notified air traffic control, and landed in the recently plowed field. During the landing roll at slow groundspeed, the nose wheel settled into the mud, causing the airplane to nose over. The resulting damage included warping of the fuselage and both wings, and a bent frame at the main landing gear.
Postaccident Examination
Examination of the engine revealed that the single-drive dual magneto did not produce spark at any spark plug when rotated using the starter, when rotated with a tool after removal, or when tested on a bench at the manufacturer's facility. Neither contact assembly would open during hand rotation of the rotating magnet. With the contact points closed, the electrical path was shorting to ground instead of reaching the ignition leads. Disassembly showed that the left and right cam followers were worn about 0.022 inch and 0.020 inch less, respectively, than the minimum specification for a new part. This wear prevented the contact points from opening, as the specified open gap should have been between 0.012 inch and 0.024 inch. Both cam followers exhibited accelerated wear and heat signatures, and both felts were blackened and dry.
Maintenance History
A factory-overhauled magneto was installed on December 31, 1979, at tachometer time 180. It remained in service until July 24, 2019, when at tachometer time 1,542.3, a 500-hour service was performed. The logbook entry stated that all parts were reused. The airplane's last annual inspection was on December 12, 2022, at tachometer time 1,635.0.
The mechanic who performed the 500-hour service reported that both cam followers were in satisfactory condition at that time. He initially stated that he cleaned the oil-impregnated breaker cam with electrical contact cleaner but later indicated he simply wiped it with a clean cloth. He lubricated each cam follower felt with a mixture of STP and heavy-weight engine oil. He observed a drift of magneto-to-engine timing before the service but not during two subsequent annual inspections. The mechanic who performed the last annual inspection did not recall any timing drift and did not remove the magneto cover or access internal components.
A review of the manual used for the 500-hour service specified that the breaker cam should be cleaned with a clean, dry, lint-free cloth, and each cam follower felt should be lubricated with two or three drops of 10-86527 lubricant, a high-temperature lubricant for breaker cams.
Continental Aerospace Technologies Service Bulletin (SB) 643C, revised July 21, 2017, stated that magnetos are electro-mechanical devices requiring service and replacement intervals. The SB specified that magnetos older than June 1, 2015 (like the accident magneto) must be overhauled or replaced within five years since original manufacture or last overhaul, or four years since placed in service, whichever occurs first, regardless of operating hours. Service bulletins are not mandatory for Part 91 operators.