AIR TRACTOR INC AT 401 near Pearsall, TX — 2017-08-24

Probable cause

The fatigue crack of an engine cylinder, which resulted in a partial loss of engine power.

NTSB narrative

The commercial pilot was conducting a local, agricultural application flight when the engine experienced a partial loss of power during a spray pass. The pilot's corrective actions did not restore normal engine operation, and he conducted a forced landing to a nearby field. The airplane descended into tall vegetation and landed in soft, sandy soil and then decelerated rapidly and nosed over. Postaccident engine examination revealed a circumferential fracture of the No. 9 cylinder. Further examination determined that the cylinder fractured from a fatigue crack that had initiated at multiple locations at a valley between two cooling fins on the outside surface. The fatigue crack progressed inward and circumferentially around the cylinder before finally cracking in overstress. No discrepancies, such as corrosion pits, wear marks, or machining marks, were found at the crack initiation sites. The cylinder met the airplane manufacturer's material and hardness specifications. The fracture would have allowed combustion gasses to escape during engine operation and led to a corresponding loss of cylinder compression, which would have led to erratic engine operation and a partial loss of power. According to engine logbook documentation, no cylinder cracks were found during an airworthiness directive inspection completed about 64 flight hours before the accident.

Analysis

Primary failure mode

Engine power loss

First missed decision gate

Pilot could have aborted the spray run upon initial engine surging.

NTSB coding

• Loss of engine power (partial) · Maneuvering-low-alt flying

Evidence available

• 5 docket documents

Docket documents5

• Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1form
• Airworthiness Directive No. 99-11-02other
• Engine Logbook (Excerpts)other
• Service Bulletin No. 1787other
• Materials Laboratory Factual Reportreport

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