History of Flight
On May 25, 2022, about 1750 mountain standard time, a Cessna 172F airplane, N5532R, was substantially damaged when it was involved in an accident near Show Low Regional Airport (SOW), Show Low, Arizona. The pilot and passenger were fatally injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.
Earlier that morning, the pilot flew from Ak-Chin Regional Airport (A39), Maricopa, Arizona, to Falcon Field Airport (FFZ), Mesa, Arizona, to pick up the passenger, and then departed for SOW. The flight arrived at SOW about 0900, and the airplane occupants spent most of the day away from the airport. A witness stated that the pilot and passenger returned to the airport in the late afternoon. The witness assisted them with refueling the airplane at the self-service fuel pump and stated that both wing tanks were filled to the bottom of the filler neck.
Witness Statements
Witnesses at SOW observed the airplane attempt to take off from runway 25 about 1745. During the first attempted takeoff, the airplane became airborne two or three times but did not climb very high. One witness stated that the airplane touched down, “got squirrely,” and almost departed the end of the runway. Another witness stated that the airplane slid sideways and that the engine “did not sound healthy.” Another witness stated that the engine was “sputtering.” The pilot then taxied the airplane back to the approach end of runway 25 and performed a run-up. A witness stated the engine “sounded bad” when the airplane was taxiing.
The pilot then attempted a second takeoff. According to a witness who was also a pilot, the accident pilot “milked it off [the runway], set it back down, and then milked it off [the runway] again.” The witness stated that the engine was “running rich, bogged down” during the second takeoff. The airplane remained at a low altitude, began a left turn toward the downwind pattern, and then “sank.” Another witness stated that, during the left turn, the airplane descended out of sight behind a ridgeline. The airplane subsequently impacted terrain in an open field about 1 mile southwest of the departure end of runway 25.
Aircraft Information
The engine was overhauled on April 29, 2016, and six new Continental 658319A1 cylinders were installed. On October 28, 2020, the pilot purchased the accident airplane. On April 26, 2021, the No. 2 cylinder was removed due to a stuck exhaust valve. The work order for the cylinder repair did not specify a reason for the stuck valve. The valve was repaired, and the cylinder was reinstalled 4 days later. At that time, the engine had accumulated 169.32 hours of tachometer time. The engine logbook contained entries for an annual inspection on September 29, 2021, and an oil change on February 19, 2022. The tachometer displayed 276.79 hours at the time of the accident.
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 the stuck valve. 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 the work. The pilot’s supervisor also stated that, during one flight with the accident pilot, he (the supervisor) was leaning the engine when the pilot stated that he “never leaned the engine” and “didn’t need to lean the engine below 3000 feet.” The pilot indicated that he had “little or no training” on leaning the engine.
Wreckage and Impact Information
The airplane impacted the edge of a stream in an open field. The wreckage was oriented along a 075° heading. The accident site and debris field were confined to the initial impact point. Forward of the empennage, the fuselage was bent downward. All flight control surfaces were attached to the airplane, and flight control continuity was established to the cockpit controls.
The engine was pushed aft, and it protruded into the cockpit. The engine remained attached to the airframe. Engine control continuity was not established due to impact damage. The carburetor was impact separated from the engine. The mixture control at the carburetor was observed in the lean position. Valve train continuity and thumb compression/suction were obtained on all cylinders except for cylinder No. 4. Cylinder No. 4 was removed, and the exhaust valve was observed to be stuck in the open position.
The magneto leads for the bottom spark plugs of cylinders Nos. 2, 4, and 6 exhibited impact damage. Both magnetos were removed, and the magneto leads were cut near the housing. The removed magnetos were rotated by hand, and spark was observed on all the leads. The bottom spark plug of cylinder No. 4 exhibited carbon buildup, and the cylinder contained debris.
The No. 4 cylinder was disassembled, and the exhaust valve stem was mostly covered in carbon buildup. The cylinder and associated exhaust valve components were sent to the National Transportation Safety Board (NTSB) Materials Laboratory for further examination.
Engine Examination
Visual examination of the exhaust valve stem found that the 0.6 to 0.7 inches of the exhaust valve stem (from the seat) exhibited deposits and that the deposits that were 0.5 inches from the seat exhibited a dark brown color with a dull luster. The other 0.1 to 0.2 inches of deposits exhibited a black color that was more reflective. Dark brown and orange-yellow deposits were present over the exhaust valve head. The deposits exhibited a flaky layered morphology that increased the overall diameter of the stem.
The deposits on the exhaust valve stem and valve seat surface were further examined. They showed indications of a single-chained polymer-like organic compound, consistent with deposits of unburned fuel.