N3386T

On April 28, 2020, about 1752 Pacific daylight time, a Cessna 177, N3386T, was substantially damaged when it was involved in an accident near Lodi, California. The pilot sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The student pilot reported that about 15 minutes after takeoff, at an altitude of 1,400 ft, the engine suddenly began to run rough and make metallic noises along with oil on the windscreen, which was followed by a total loss of engine power. The pilot attempted to restart the engine unsuccessfully and initiated a forced landing to a nearby field. Subsequently, the airplane landed hard and came to rest upright.

Examination of the engine revealed that the engine remained attached to the airframe, with all accessories attached. A hole in the crankcase was observed above cylinder No. 4 and 3. No loose oil lines were found with the exception of the oil pressure line, which was separated from the crankcase, consistent with impact damage. The No. 4 cylinder exhibited extensive damage to the piston skirt area and cylinder wall. The No. 4 connecting rod remained attached to the crankshaft, however, the web was separated from just outboard of the crankshaft to about 1 inch inboard from the piston pin. The area of separation near the piston pin was flat, with no apparent mechanical damage. The separation near the crankshaft exhibited extensive mechanical damage. The middle portion of the connecting rod web was found within the oil sump and exhibited mechanical damage. See figure 1.

Figure 1: View of the No. 4 connecting rod.

The middle web section exhibited multiple areas of locally deformed material, mostly along the protruding flange surfaces. These features were consistent with this remnant having been impacted by adjacent components in multiple locations. The fracture surface between this remnant and the crankshaft bore surfaces exhibited a general rounded but perpendicular orientation, with localized buckling on the flanges. The rough orientation and texture suggested overstress fracture, however, the mating surfaces were damaged from post-fracture impact, with no discernible fracture features present.

The other fracture surface of the connecting rod, toward the piston pin side, was relatively flat and oriented perpendicular to the rod longitudinal direction. The fracture on the pin side remnant was relatively smooth, exhibiting few elevated features. Most of the fracture surface, which was generally flat in orientation, exhibited crack arrest marks, indicative of progressive crack growth, consistent with fatigue. The remaining fracture surface exhibited a rougher, more undulating surface texture consistent with subsequent overstress fracture, as seen in figure 2. Therefore, the fatigue crack covered about 95% of the rod flange-to-flange cross section, measured as approximately 1.086 inches.

Figure 2: The no. 4 connecting rod fracture surface (piston pin side)

The lack of ratchet marks suggested a single initiation site. This site was located at the inside corner of a flange edge. The river patterns and crack arrest marks were consistent with having emanated from this feature. When viewed at an angle, the feature was collocated with an area of local plastic deformation and wear. As shown in figure 3, the outer flange surface had been smoothed, and deformed over the side of the inner flange surface.

Figure 3: Angled view of the crack initiation site (arrow), showing inward deformation of the flange side.

The engine was overhauled on November 10, 1986 and had accumulated 887.86 hours at the time of the accident. No maintenance entries regarding the No. 4 connecting rod were found throughout the engine logbook.