Failure To Lean

Low-power/compression avgas engines must be leaned properly to prevent combustion byproduct buildups.

Tuesday, April 30, 2024 at 08:00 AM ET

Verified

The bottom spark plug of the accident airplane’s six-cylinder Continental O-300 mounted in cylinder #4 exhibited carbon buildup, as shown in the image at right. Since carbon is conductive, these deposits likely reduced the spark produced and reduced the engine’s power output. But this plug was only a symptom of what ailed the engine; the cylinder itself had accumulated substantial carbon deposits resulting from incomplete combustion. Its exhaust valve ultimately stuck open, reducing engine output and contributing to the accident.

Key Takeaways:

Improper engine mixture management, especially consistently running too rich, leads to harmful carbon deposits and can cause critical engine failures like stuck exhaust valves.
A fatal Cessna 172 accident was directly attributed by the NTSB to the pilot's long-term failure to lean the engine, resulting in a stuck exhaust valve and partial power loss during takeoff attempts.
Effective mixture control is crucial for engine longevity, fuel efficiency, and safety, requiring both proper pilot training and the use of modern multi-probe engine monitoring systems.

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As a student pilot who wasn’t paying directly for engine maintenance, I was firmly told to put the mixture control in the full-rich position and leave it there until shutdown, including when in cruise on solo cross-countries. At the time, I was flying Cessna 150s and Cherokee 140s and always filled the tanks before takeoff. No one paid all that much attention to leaning the mixture back then, at least when burning the 80 octane avgas that was still available. Occasionally someone would need to “burn off a plug” to get a clean magneto check, but that was about it.

After I earned the private certificate, I flew with other pilots and instructors who, over time, taught me how to use the mixture control, even on the ground. These days, I lean my Debonair’s IO-520 aggressively on the ground and in the air, running either lean of peak EGT or at it, at lower power levels. Through three engines and many different spark plugs, I’ve never fouled one, and the only time there was debris in a cylinder involved a valve performing an unscheduled disassembly procedure.