Accident Sequence
On September 2, 2022, at approximately 1100 Pacific daylight time, a Yakovlev Yak-11, registered as N5943, was destroyed in an accident near Shafter, California. The pilot, operating the aircraft as a personal flight under 14 CFR Part 91, was fatally injured.
According to a witness at Shafter-Minter Airport, the pilot departed runway 30 and initiated a climbing left turn with the intent to orbit the airport. The witness observed the airplane continue the left turn until it aligned with the left downwind leg for runway 12. Shortly thereafter, the pilot reported to ground crew that he had a low oil pressure indication; the ground crew instructed him to land. The witness then heard two popping sounds from the airplane, followed by a loss of engine power. The airplane continued on the left downwind, then turned onto the base leg. As it turned final for runway 12, the airplane appeared to stall, entered a nose-low, steep left bank, and descended rapidly into terrain.
Another witness reported that while on final, the pilot performed an S-turn from left to right. At the start of a second S-turn, the airplane stalled and "rolled up on its back" at about 2,000 ft above ground level (agl). The witness stated the airplane then entered an almost straight-down dive, rolling about 30° to the right, and remained nose-down until about 500 ft agl, at which point it appeared to initiate a flare but did not round out, stalled, and impacted the ground in a flat attitude.
A video recorded by a witness captured the final moments. Review showed the airplane descending rapidly in a steep nose-low, left bank attitude. It then rolled to the right and pitched upward until just before impact, when it rolled slightly left. The propeller was observed rotating throughout the video.
Aircraft and Engine Background
The aircraft was a Yakovlev Yak-11, registration N5943. Logbooks were not obtained during the investigation. Personnel from Anderson Aeromotive reported that upon receipt of the engine, they removed all cylinders, replaced the oil control rings, and reinstalled the cylinders along with various seals and gaskets. After completion, the engine was installed on a test stand and run at takeoff power for two hours without any issues.
Wreckage Examination
The airplane impacted an open field on a magnetic heading of about 127°, approximately 3,777 ft northwest of the approach end of runway 12, at an elevation of 397 ft msl. The wreckage debris path measured 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 extending about 32 ft to an impact crater containing two of the four propeller blades. The empennage separated from the fuselage and was found about 170 ft from the initial impact. The forward portion of the fuselage, including the instrument panel, engine, and right wing, was located about 244 ft from the initial impact. All major structural components were 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 also separated and impact damaged. The two oil lines extending from the oil tank aft to the two oil coolers were impact damaged but mostly intact, with rubber tubing interconnects present. A very small amount of oil was observed on the fuselage above the oil coolers, consistent with seepage during postaccident storage. No evidence of leaks was found throughout the airframe oil system.
The engine was separated between the main case and the collector case. The collector case was impact damaged, with a separated section remaining attached to the main case. Internal examination of the collector case, intermediate rear case, and rear case sections showed 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 were cracked in several locations, consistent with impact damage.
The intermediate rear case scavenge oil screen was removed and contained several metallic flakes. The main sump drain plug contained a small amount of metallic debris and dirt. The main oil screen contained a significant amount of metal particles and debris. The oil pressure relief bypass valve, when disassembled, exhibited a small amount of metallic debris. The scavenge pump and oil pump bodies showed scoring on the inner walls, and the internal gears had varying degrees of corrosion.
The front case and propeller shaft were removed from the main case. Metallic fragments were found throughout the front case. Fragments of the front secondary counterbalance bearing were observed on the front case side of the main case. Thermal discoloration of the front secondary counterbalance assembly was noted. The front secondary counterbalance bearing was fused to the crankshaft and had to be cut off using a cutting torch; portions of the removed bearing exhibited thermal discoloration and scoring along the outer area.
The front cylinders (Nos. 1, 3, 5, 7, 9, 11, 15, 17) were removed with the forward section of the main case. The pistons on these cylinders showed light scoring on the skirts but were otherwise unremarkable. The front link rods and master rod moved freely after cylinder and piston removal. No thermal discoloration was observed on the master rod or link rods, nor on any rear cylinder link rods or the rear master rod. The front crankshaft bearing and counterweight were unremarkable.
The rear support assembly was removed, exposing 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.