The pilot reported that about 15 minutes into the flight the Cessna 177’s engine “backfired a couple times” and 5 to 10 seconds later the engine lost total power, although the propeller continued to rotate.
The flight instructor assumed control of the airplane, established a 75-knot glide speed, and attempted to restart the engine, but without success.
The CFI chose a field near Travelers Rest, South Carolina, notified air traffic control of his intention, and landed in the field.
During the landing roll in the recently-plowed field, while at a slow groundspeed, the nosewheel settled into the mud and the airplane nosed over, resulting in substantial damage to the fuselage and both wings.
As a result of the nose over the owner reported warping of the fuselage and both wings and a bent frame at the main landing gear.
Post-accident examination of the engine revealed that the single-drive dual magneto did not produce spark at any of the spark plugs during rotation of the engine using the starter, or when the magneto was rotated using a tool following removal from the engine, or when it was operated on a test bench at the manufacturer’s facility.
Neither of the contact assemblies, which were properly secured, would open during hand rotation of the rotating magnet.
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.
Disassembly inspection of the magneto revealed the left and right cam followers were worn about 0.022 inch and 0.020 inch less than the minimum specification for a new part, which was insufficient to open the contact assembly points.
The contact assembly gap, when open, was specified to be between 0.012 inch and 0.024 inch, with a median value of 0.018 inch. The examination revealed that both cam followers exhibited accelerated wear and heat signatures and both felts were blackened and dry.
Review of the engine maintenance records revealed that a factory overhauled magneto was installed on Dec. 31, 1979, at tachometer time of 180. The magneto remained in service until July 24, 2019, when, at tachometer time 1,542.3, a 500-hour service was performed.
According to the logbook entry, all parts were reused. The magneto remained in service from that date until the accident. The airplane’s last annual inspection was performed on Dec. 12, 2022, at tachometer time 1,635.0.
The mechanic who performed the 500-hour inspection of the magneto reported that at the time of the inspection both cam followers were in satisfactory condition.
He initially indicated that during the inspection he cleaned the oil impregnated breaker cam using electrical contact cleaner, but later indicated that he just wiped it using a clean cloth.
As part of the magneto inspection, he lubricated each cam follower felt with a mixture of STP and heavy weight engine oil.
He did notice a drift of the magneto to engine timing before the service, but he did not notice any drift of the magneto to engine timing during two subsequent annual inspections after the service was performed.
The mechanic who performed the airplane’s last annual inspection reported he did not recall whether there was any drift of the magneto to engine timing. As part of the engine inspection, he did not remove the cover of the magneto or access any internal components of it.
A review of the manual that the mechanic used to perform the 500-hour service of the magneto revealed that the oil impregnated breaker cam was specified to be cleaned using a clean, dry, lint-free cloth, and each cam follower felt was specified to be lubricated using 2 or 3 drops of 10-86527 lubricant (a high-temperature lubricant for breaker cams).
Continental Service Bulletin (SB) 643C, revised on July 21, 2017, specified that magnetos are electro-mechanical devices using rotating parts subjected to the same service treatment, environmental conditions, and wear as the engine.
The SB also specified that magnetos older than June 1, 2015, must be overhauled or replaced at the expiration of five years since the date of original manufacture or last overhaul, or four years since the magneto was placed in service, whichever occurs first, without regard to operating hours.
Service bulletins are not mandatory for Part 91 operators.
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 Identification: 106949
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This March 2023 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.
