HISTORY OF FLIGHTOn September 2, 2022, about 1100 Pacific daylight time, a Yakovlev Yak-11, N5943, was destroyed when it was involved in an accident near Shafter, California. The pilot was fatally injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. A witness located at the Shafter-Minter Airport (MIT), Shafter, California, reported that the pilot departed runway 30, and entered a climbing left turn with the intention of orbiting the airport. The witness stated that the pilot continued the left turn, until it was on a course that resembled the left downwind leg for runway 12. Shortly after, the pilot reported to his ground crew, who were monitoring the flight, that he had a low oil pressure indication; the ground crew told the pilot to land. The witness stated that about 3 to 5 seconds later he heard 2 popping sounds from the airplane followed by a loss of engine power. The airplane continued the left downwind and then turned onto the base leg. He stated that as the airplane turned final for runway 12, it appeared to stall. The airplane entered a nose-low, steep left bank attitude and descended rapidly into terrain. Another witness stated that while the airplane was on final, the pilot performed an “S” turn from left to the right, and at the start of a second “S” turn, the airplane stalled, and “rolled up on its back” about 2,000 ft above ground level (agl). The witness stated that the airplane went into an almost straight-down dive, and as it rolled about 30° to the right it remained in a nose-down attitude until about 500 ft agl. The witness recalled that the airplane appeared to start a flare for a field; however, “didn’t round out, stalled, and impacted the ground in a flat attitude.” One witness captured a video of the final moments of the accident sequence. Review of the video showed that at the start of the recording, the airplane was descending rapidly in a steep nose-low, left bank attitude (Figure 1). The airplane began to roll to the right and pitch upward, until just before impact, when the airplane rolled slightly left. Throughout the video, the propeller was observed rotating. Figure 1: A capture of the start of the witness-provided video. AIRCRAFT INFORMATIONThe logbooks were not obtained during the investigation. According to personnel from Anderson Aeromotive, upon receipt of the engine, they removed all the cylinders, changed the oil control rings, and reinstalled the cylinders along with various seals and gaskets. Once the work was complete, they installed the engine on a test stand and ran it at takeoff power for 2 hours with no issues noted. AIRPORT INFORMATIONThe logbooks were not obtained during the investigation. According to personnel from Anderson Aeromotive, upon receipt of the engine, they removed all the cylinders, changed the oil control rings, and reinstalled the cylinders along with various seals and gaskets. Once the work was complete, they installed the engine on a test stand and ran it at takeoff power for 2 hours with no issues noted. WRECKAGE AND IMPACT INFORMATIONExamination of the accident site revealed that the airplane impacted an open field on a heading of about 127° magnetic about 3,777 ft northwest of the approach end of runway 12 at an elevation of 397 ft mean sea level (msl). The wreckage debris path was about 392 ft in length and 60 ft wide. The first identified point of impact with terrain was a large swath of freshly disturbed dirt, which extended about 32 ft to an impact crater that contained 2 of the 4 propeller blades. The empennage separated from the fuselage and was located about 170 ft from the initial impact. The forward portion of the fuselage, which included the instrument panel, engine, and right wing, was located about 244 ft from the initial impact. All major structural components of the airplane were located within the debris path. Examination of the recovered wreckage revealed that the oil line from the outlet port of the oil tank to the inlet port on the engine was separated and exhibited impact damage. The oil tank was impact damaged and breached. The oil line from the outlet port on the engine to the inlet port of the oil tank was separated and exhibited impact damage. The two oil lines (inlet and outlet) that extended from the oil tank aft to the two oil coolers were impact damaged, and mostly remained intact. The line interconnects (rubber tubing) were present. A very small amount of oil was observed on the fuselage directly above the oil coolers, consistent with seeping out of the coolers during postaccident storage. No evidence of leaks was observed throughout the airframe oil system. Examination of the engine revealed that it was separated between the main case and the collector case. The collector case was impact damaged; a separated section remained attached to the main case. Internal examination of the collector case, intermediate rear case, and rear case sections revealed no evidence of thermal discoloration. A slight amount of metallic debris was observed in the internal areas of the collector case and intermediate rear case sections. The induction system and exhaust exhibited varying degrees of impact damage. Cylinder Nos. 8 and 9 were impact damaged. Cylinder Nos. 12 and 13 were separated. Cylinder Nos. 10, 11, and 14 were partially attached. The front case and front accessory case was cracked in various locations, consistent with impact damage. Figure 2: Forward section of the engine with the area of impact damage and displaced cylinders. The intermediate rear case scavenge oil screen was removed and contained several metallic flakes. The main sump drain plug was removed and contained a small amount of metallic debris and dirt. The main oil screen was removed and contained a significant amount of metal particles/debris. The oil pressure relief bypass valve was removed and disassembled. A small amount of metallic debris was observed within the relief valve. The scavenge pump and oil pump were removed and disassembled. Scoring was observed on the inner walls of the scavenge pump body and oil pump body. The internal gears exhibited varying degrees of corrosion. The front case and propeller shaft were removed from the main case. Metallic fragments were observed throughout the front case. Fragments of the front secondary counterbalance bearing was observed on the front case side of the main case. Thermal discoloration of the front secondary counterbalance assembly was observed. The front secondary counterbalance bearing was fused to the crankshaft. The bearing was cut off using a cutting torch. Portions of the removed bearing exhibited thermal discoloration and scoring along the outer area of the bearing. Figure 3: Portion of the front secondary counterbalance bearing. The front cylinders (cylinder Nos. 1, 3, 5, 7, 9, 11, 15, 17) were removed along with the forward section of the main case. The pistons on cylinder Nos. 1, 3, 5, 7, 9, 11, 15, and 17, exhibited light scoring on the piston skirts; however, were otherwise unremarkable. The front link rods and master rod moved freely once the cylinders and pistons were removed. No thermal discoloration was observed on the master rod or link rods. No visible areas of thermal discoloration were observed on any of the rear cylinder link rods, or master rod. The front crankshaft bearing and counterweight were unremarkable. Figure 4: View of the front link rods, master rod, and crankshaft bearing assemblies. The rear support assembly was removed, which exposed the rear counterbalance and counterbalance bearing. The rear counterbalance exhibited thermal discoloration, with scoring within the inner area. The rear counterbalance bearing exhibited severe scoring and displaced bearing material.