Yakovlev YAK-11 N5943 accident near Shafter, California

Probable cause

The pilot’s exceedance of the airplane’s critical angle of attack, which resulted in an aerodynamic stall and subsequent impact with terrain. Contributing to the accident was the loss of engine oil pressure shortly after takeoff.

NTSB analysis

The pilot had departed in the accident airplane to orbit the airport following recent maintenance to the airplane. Shortly after takeoff, the pilot transmitted to his ground crew that the oil pressure had decreased; one witness told the pilot to land. A witness reported that the airplane was on the downwind leg for the runway when the engine lost power. Witnesses observed the airplane turn from the base leg to final for the runway, and shortly after, it stalled and entered a steep nose-down descent. One witness reported the airplane stalled while performing “S” turns During the descent, when the airplane was about 500 ft above ground level the airplane appeared to flare. The ground scars and wreckage fragmentation patterns indicated that the airplane impacted terrain in a nose-down attitude with high forward velocity.

A video of the airplane showed that the airplane was in a left bank attitude during the nose-down dive. The airplane rolled right and pitched up just before impact. The video also showed that the propeller was rotating through the duration of the video.

The airplane had not been flown in about 6 years before the accident flight. Before the flight, the engine was removed from the airplane and sent out for maintenance. The cylinders were removed, the oil control rings were replaced, and the engine was reassembled with new seals and gaskets. The engine was placed on a test stand and ran at takeoff power for about 2 hours before it was reinstalled on the airplane.

Postaccident examination of the engine revealed that the front and rear counterbalances and counterbalance bearings were thermally discolored consistent with abnormal wear. The front secondary counterbalance bearing failed and was fused to the crankshaft. The rear counterbalance bearing exhibited severe scoring and displaced bearing material. An abundance of metallic debris was located throughout the engine and oil system, including the scavenge pump filter, engine oil filter, and sump plug.

While the engine had accumulated metal debris within the engine oil filter, there was evidence of the oil pressure bypass operating, which would allow oil to continue to flow throughout the oil system. The reason for the reduction in oil pressure and the subsequent loss of engine power was not determined.

Text from the NTSB. Open the NTSB docket for WPR22FA331 for the full record.