Cessna 172F N5532R accident near Show Low, Arizona

Probable cause

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.

NTSB analysis

The pilot and passenger were departing on a personal flight. Witnesses observed the airplane attempt to take off, but the takeoff was aborted, and the airplane was then taxied back to the approach end of the runway so that the pilot could perform an engine run-up. The witnesses stated that the engine was “sputtering” and “sounded bad” during the takeoff and the taxi to the runup area. The pilot subsequently attempted a second takeoff. The airplane remained at a low altitude, began a left turn toward the downwind pattern, and then “sank” out of sight behind a ridgeline before impacting terrain. A pilot witness stated that the pilot had “milked it [off the runway], set it back down, and then milked it off [the runway] again.” This witness also described the engine as “running rich, like it was bogged down” during the second takeoff.

Postaccident examination of the engine found that the No. 4 engine cylinder exhaust valve was stuck due to a buildup of material that enlarged the overall diameter of the valve’s stem. Examination of the valve stem deposits showed indications of an organic compound that was consistent with deposits of unburned fuel. The stuck valve led to a partial loss of power during the accident takeoffs.

The accident pilot reported to another pilot that there had been previous engine problems that had been identified as stuck valves. The accident pilot also reported that he ”never leaned the engine” and that he had littleto-no training on how to lean the engine.

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.

Text from the NTSB. Open the NTSB docket for WPR22FA188 for the full record.