WPR25LA040

On November 17, 2024, about 0930 mountain standard time, a Cessna 172I airplane, N8390L, was substantially damaged when it was involved in an accident near Mayer, Arizona. The pilot sustained minor injuries, and the passenger was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that, while in cruise flight, he thought he heard the engine misfire. He checked the engine instruments and the positions of the throttle and mixture controls and observed no abnormalities. The engine misfired again, and the engine rpm and performance decreased. The pilot advanced the throttle fully, selected full-rich mixture, and applied carburetor heat, after which the engine rpm initially recovered. As the pilot considered returning to the departure airport, the engine began to lose power again, and the airplane could not maintain altitude. He cycled the engine controls and ignition-switch positions, selected the left fuel tank, and then the right tank. He then returned the fuel selector to the BOTH position and completed the emergency checklist, but could not restore engine power. The pilot stated that he initiated a forced landing toward a nearby clearing. While maneuvering toward the clearing, he reported that he was flying on the edge of a stall until just before the airplane impacted the ground. The airplane landed hard on uneven desert terrain, substantially damaging the fuselage and both wings. The owner recovered the nose landing gear, both forward-seat assemblies, the propeller, the instrument panel and associated instruments, and the engine from the accident site. The recovered wreckage was relocated to the owner’s hangar, where a postaccident examination was conducted. The pilot reported that both fuel tanks were topped off before departure and that the airplane departed with about 38 gallons of fuel. The owner reported that the recovery crew drained about 14 gallons from the right tank five days after the accident and observed fuel in both tanks; the left tank had been damaged and was leaking. At the time of the accident, the reported temperature at a ground location about 27 miles from the accident site was 35.6°F, and the dew point was 24.8°F. According to FAA Special Airworthiness Information Bulletin CE-09-35, Carburetor Icing Prevention, the reported temperature and dew point were conducive to carburetor ice at glide and cruise power settings. Postaccident examination of the engine revealed impact damage to the exhaust muffler and exhaust pipe. The induction airbox had been removed prior to the examination and was not present. The upper spark plugs were removed from the cylinders. The electrode areas were undamaged and displayed coloration consistent with a lean fuel/air mixture. Both magnetos remained attached to the engine. The magnetos were timed at about 25° before top dead center (BTDC) on the No. 1 cylinder. The crankshaft was rotated through the vacuum-pump accessorymount and spark was obtained from each ignition lead end, all in firing order. The cylinder rocker covers were removed, and the cylinder overhead components were undamaged and coated with an oily residue. During crankshaft rotation, cylinder “thumb” compression was established on all cylinders and in firing order. Cylinder valve continuity was established, and proper rocker movement was observed. The cylinders were examined with a lighted borescope. Each combustion chamber was mechanically undamaged, and no evidence of foreign-object ingestion or detonation was observed. The valve heads were intact and undamaged and displayed no evidence of contact with their respective piston faces. The carburetor remained attached to the oil sump mounting flange. The fuel inlet line was separated at the carburetor fitting. The throttle cable remained attached to the throttle-linkage arm; however, the castellated nut was finger-loose, and the cotter pin was missing. The throttle- and mixture-cable housing clamping hardware remained secure. The mixture-control arm remained attached to the carburetor. The mixture-control cable was separated near the clamping hardware on the attachment bolt. Examination with a jeweler’s loupe revealed rough cable-separation surfaces covered with carbon residue. The carburetor was disassembled and was free of fuel and contaminants. The carburetor’s internal components were undamaged and displayed no apparent operating anomalies. The fuel accelerator-pump plunger seal remained pliable and intact. No external fuel staining was observed. The carburetor mixture-linkage arm and portions of the separated cable were sent to the NTSB Materials Laboratory for further examination. The examination found that the cable fractured by ductile overstress in unidirectional bending and identified no evidence of fatigue or progressive cracking. Carburetor Icing FAA Special Airworthiness Information Bulletin (CE-09-35) – Carburetor Icing Prevention states that: Pilots should be aware that carburetor icing doesn't just occur in freezing conditions, it can occur at temperatures well above freezing temperatures when there is visible moisture or high humidity. Icing can occur in the carburetor at temperatures above freezing because vaporization of fuel, combined with the expansion of air as it flows through the carburetor, (Venturi Effect) causes sudden cooling, sometimes by a significant amount within a fraction of a second. Carburetor ice can be detected by a drop in rpm in fixed pitch propeller airplanes and a drop in manifold pressure in constant speed propeller airplanes. In both types, usually there will be a roughness in engine operation.