Failure To Lean

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

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.

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.

Before multi-probe engine monitors, we were lucky to have a single-probe EGT and a cylinder head temperature gauge monitoring only one cylinder. There was little guidance on leaning, and traditional techniques were passed down in the cockpit. What was passed down sometimes was bad advice, especially when it ignored CHT.

There are a lot of reasons to lean our gasoline-fueled engines proactively, not least of which is the cost of fuel. In engines originally designed for 80 octane fuel (red), even the reduced lead levels in 100LL (blue) avgas—when compared to its forerunner 100/130 octane (green)—can become problematic. In addition to lead fouling of the spark plugs, heavier carbon deposits can result from improper leaning and create mischief elsewhere in the cylinder.

Proper mixture management of spark-ignition engines requires someone willing to teach the techniques as well as someone open to learning something new, and using it.


On May 25, 2022, at about 1750 Mountain time, a Cessna 172F Skyhawk impacted terrain shortly after takeoff from Show Low, Arizona. The pilot (male, 59) and passenger were fatally injured.

On the morning of the accident, the pilot flew from Maricopa, Arizona, to Mesa, Arizona, to pick up the passenger. The pair then departed for Show Low, with the accident airplane arriving at about 0900. They spent most of the day away from the airport, returning in the late afternoon. A witness assisted with refueling the airplane and later stated that both wing tanks were filled to the bottom of the filler neck.

The airplane attempted to take off from Runway 25 at about 1745, becoming airborne two or three times, but did not climb very high before and the takeoff was aborted. One witness stated the engine was “sputtering.” The pilot taxied back to the approach end of Runway 25 and performed a run-up, which a witness stated “sounded bad,” and then attempted a second takeoff. A pilot-rated witness said the accident pilot “milked it off [the runway], set it back down, and then milked it off [the runway] again,” and added that the engine was “running rich, bogged down” during the second takeoff.

The airplane remained at low altitude, began a left turn toward the downwind pattern and then “sank” out of sight behind a ridge line. The airplane impacted open terrain about a mile southwest of the departure end of Runway 25.


The accident site and debris field were confined to the initial impact point. All flight control surfaces were attached to the airplane, and control continuity was established to the cockpit. Engine control continuity was not established, due to impact damage. The mixture control at the carburetor was observed in the lean position. Both magnetos were removed and generated spark when turned by hand. Valve train continuity and thumb compression/suction were obtained on all cylinders except for cylinder #4, which was removed. Examination revealed the exhaust valve was stuck open due to carbon buildup. The deposits on the exhaust valve stem and valve seat were consistent with deposits of unburned fuel.

The engine was overhauled on April 29, 2016, and six new cylinders were installed. On October 28, 2020, the pilot purchased the accident airplane. On April 26, 2021, the #2 cylinder was removed to repair a stuck exhaust valve, and then was reinstalled four days later. Between then and the accident, the engine accumulated an additional 107 hours, averaging slightly more than 35 hours a year.

The pilot’s supervisor, who was also a pilot and had flown with the accident pilot, thought an exhaust valve also became stuck sometime between January 16 and February 27, 2022, and that the pilot had difficulty finding a mechanic to fix it. The supervisor further stated that he explained to the pilot the procedure for resolving the stuck valve, but he did not know if the pilot attempted that procedure himself or found a mechanic to perform it.

At one point in his ownership, the accident pilot told his supervisor that he never leaned the engine, and had little to no training in the procedure.

Probable Cause

The NTSB determined the probable cause(s) of this accident to be: “The pilot’s improper leaning of the engine during an extended period of time, which caused an exhaust valve to become stuck and led to the partial loss of engine power during the accident flight. Contributing to the accident was the pilot’s decision to attempt a second takeoff without having the engine further examined.”

According to the NTSB, “The stuck valve led to a partial loss of power during the accident takeoffs.” The NTSB added, “The partial loss of engine power that resulted from the No. 4 cylinder exhaust valve becoming stuck was likely due to the pilot’s improper leaning of the engine over an extended period. Excessively rich mixtures can lead to a buildup of unburned hydrocarbons, which can foul engine components with deposits. The accident could likely have been avoided if the pilot had the airplane examined by maintenance personnel before attempting the second takeoff.”

Somewhere between the extremes of the student pilot’s admonishment to “put the mixture control in the full-rich position and leave it there until shut down” and routinely running the engine lean of peak EGT is a happy compromise. In our view, the key is keeping the engine hot enough to ensure complete combustion while keeping it cool enough that the cylinder’s aluminum alloy retains its characteristics.

The only way to do that consistently is by using a multi-probe engine monitor and employing constant management of the engine’s fuel/air mixture. Some training won’t hurt, either.


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