WPR24LA229

A failure of the No. 2 cylinder due to fatigue cracking as the result of an undersized fillet radius around the edge of the cylinder head dome.

The pilot reported that he completed a preflight inspection and engine run-up with no anomalies noted. Shortly after takeoff, while making a right turn from the crosswind to downwind leg of the airport traffic pattern, he heard a loud “pop,” and noticed brown smoke coming from the engine compartment. The airplane began shaking and the engine subsequently lost total power. The pilot declared an emergency and landed on the runway; however, the airplane overran the departure end of the runway and nosed over, resulting in substantial damage to both wings and the engine mount. Postaccident examination of the engine revealed that the No. 2 cylinder head cracked due to an undersized fillet radius around the edge of the dome. The undersized fillet increased local tensile stresses in the head under normal operating loads, which resulted in fatigue crack initiation and growth. The undersized fillet radius was less than that of the mating barrel corner radius, indicating that the undersized fillet was likely machined into the head at the time of manufacture. The remaining three cylinders from the accident engine were examined and revealed a discontinuous fillet in each of the heads that was inconsistent with the manufacturer’s engineering drawings. At the time of the accident, the engine had over 1,400 hours in service in the 4 years since the cylinder was manufactured. The accident engine is likely a relatively high-time engine and could be among the fleet leaders for other engines operating with cylinders manufactured around the same time as the accident cylinders.

On July 2, 2024, about 1200 Pacific daylight time, a Cessna 172N, N74C, was substantially damaged when it was involved in an accident near San Diego, California. The pilot sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that he completed a preflight inspection and engine run-up with no anomalies noted. He taxied to and departed runway 28R. During the right turn from crosswind to the downwind leg of the airport traffic pattern, about 1,000 ft above ground level, he heard a loud “pop” and noticed brown smoke coming from the engine compartment. The airplane began shaking and the engine subsequently lost total power. After several attempts to contact the tower controller, the pilot declared an emergency and the tower controller cleared the airplane to land on runway 10L. The pilot stated that he performed the engine failure checklist, landed on runway 10L, overran the departure end of the runway, and the airplane nosed over. Postaccident examination of the airplane revealed substantial damage to the engine mount and both wings. Examination of the airplane’s maintenance records revealed that new cylinder assemblies were installed on the accident engine at the last major overhaul, completed on August 14, 2020. The engine was installed on the airplane with zero time since overhaul on April 1, 2022. At the time of the accident, the engine had accumulated 1,408.3 hours in service since overhaul. Postaccident examination of the engine revealed normal operational characteristics of the fuel, electrical, induction and exhaust systems. The No. 2 cylinder cooling fins on the forward lower quadrant of the cylinder head, near the intake port, were stained dark, and a crack was observed in the head between the 6th and 7th fins from the barrel side of the head in the stained area. The cylinder was removed and sent to the NTSB Materials Laboratory for further examination. Examination of the No. 2 cylinder revealed that the fracture at the lower forward quadrant had relatively smooth fracture features, with radial marks and curving crack progression lines, consistent with fatigue. The fatigue features emanated from a broad origin area located at the interior surface. Unlabeled arrows in figure 1 indicate the general directions of fatigue crack progression emanating from the origin area, and dashed lines indicate the approximate fatigue boundary where crack growth transitioned to fast fracture. The remainder of the fracture surface beyond the fatigue boundaries had rough features consistent with overstress fracture. Figure 1: Fracture features on the dome side of the fracture. Dashed lines indicate approximate fatigue boundaries, and unlabeled arrows indicate general fatigue crack propagation directions. The engineering drawing for the cylinder head that was obtained from a representative for Superior Air Parts indicated that the head should have a smooth transition between the dome surface and the barrel shrink band contact face. The engineering drawing for the head included a radial cross-section that detailed the transition between the dome face and the adjacent shrink band contact face where the fillet between the two surfaces had a specified radius of 0.050 inch. Examination of the No. 2 cylinder revealed that the transition between the dome face and the adjacent shrink band contact face where the fillet between the two surfaces had exceeded the manufacturer-specified radius of 0.050 inches (in), as shown in figure 2. Figure 2: Cut surfaces on the aft side of the cylinder after the longitudinally oriented vertical cut was made through the assembly showing the barrel and head at the outboard end of the barrel. A measurement of the radius between the shrink band contact face and the outboard face of the barrel is shown. Cylinder Nos. 1, 3, and 4 were also examined by the NTSB Materials Lab. The area of the heads on all four cylinders, where the outer edge of the dome transitioned to the barrel shrink band contact face, was examined. A discontinuous fillet was observed in each of the heads, as shown in figures 3 through 5. The portion of the fillet with the larger radius was located adjacent to the dome face. The larger fillet radius in each case was measured using a microscope, and results showed the fillet radius next to the dome measured approximately 0.053 in to 0.055 in, slightly larger than the specified fillet radius in that area as shown in the engineering drawing for the cylinder head. Adjacent to the shrink band contact face, an undersized fillet radius was observed as indicated in figures 3 through 5. All of the cylinders from the accident airplane were manufactured in June 2020. Figure 3: Cut surface of cylinder 1 showing the fillet in the head between the dome and the shrink band contact face at the outboard end of the barrel. Figure 4: Cut surface of cylinder 3 showing the fillet in the head between the dome and the shrink band contact face at the outboard end of the barrel. Figure 5: Cut surface of cylinder 4 showing the fillet in the head between the dome and the shrink band contact face at the outboard end of the barrel.