Air Tractor AT802 N397AS accident near Northport, Washington

Probable cause

A total loss of engine power during a climb due to an accumulation of molten environmental particles, the source of which could not be determined based on the available evidence, which resulted in a forced landing.

NTSB analysis

The accident airplane was one of three public aircraft that were supporting firefighting operations. The three airplanes had made four successful water drops and had just scooped water from a river to make a fifth drop. The airplanes were in a climb en route to the drop location when the pilot of the accident airplane heard a “loud bang” followed by a total loss of engine power. The pilot made a forced landing to treetops in a heavily forested area.

During the on-site examination, a coat of oil was observed on the cowling and one of the wings. Damage was also observed to the engine. The engine examination revealed that a second-stage power turbine blade exhibited fatigue signatures, and molten debris was identified throughout the engine. The debris was subsequently identified as environmental particles, but the source of the debris was not determined from the available evidence for this investigation. The loss of engine power was likely due to the accumulation of foreign melted environmental debris on the engine’s hot section components, which resulted in the first- and second-stage power turbine blades fracturing from tensile overload.

Text from the NTSB. Open the NTSB docket for WPR18LA222 for the full record.