Air Tractor Inc AT 301 N8593S accident near Pearsall, Texas

Probable cause

The failure of the No. 1 cylinder rod at the link pin. Contributing to the accident was oil starvation to the rod bearing due to the improper flushing of the oil cooler, which allowed metal and other debris in the oil to clog the drain and the led to the piston’s failure.

NTSB analysis

The pilot reported that, while returning to the airport after spraying a cornfield, he noticed that the engine cylinder head temperature was high. After landing, he checked the engine cylinders for discrepancies and found none, so he decided to continue the spraying operations at the cornfield. As the pilot pulled up the airplane from a spray run, the engine started to vibrate and lose power. He subsequently made a forced landing in a nearby field, which resulted in the airplane sustaining substantial damage to both wings.

During postaccident examination, a considerable amount of metal was found in the oil drain. The No. 2 cylinder front spark plug was loose. After removing all of the spark plugs, an attempt was made to rotate the engine, but it seized. After removing the No. 1 cylinder, it was discovered that the piston rod had broken at the link pin; no other rods were broken. The No. 1 cylinder link pin was scored, and there was evidence of oil starvation at the rod bearing. According to the operator, the engine had accrued about 100 hours since it had received extensive maintenance, including the replacement of the crankshaft. It is likely that the oil cooler was not properly flushed when the crankshaft was replaced and that metal and other debris remained in the oil, clogged the drain, and caused the piston to fail.

Text from the NTSB. Open the NTSB docket for CEN14LA287 for the full record.