Accident Overview
On August 14, 2018, at about 1500 Pacific daylight time, an Air Tractor AT-802A, registration N397AS, was substantially damaged when it was involved in an accident near Northport, Washington. The pilot was seriously injured. The airplane was operated as a public use aircraft under Title 14 Code of Federal Regulations Part 91.
According to the Washington State Department of Natural Resources, the airplane was operated under a call-when-needed contract as a single-engine air tanker. The flight was in support of firefighting operations in Colville National Forest, about 11 miles northwest of Northport.
Flight and Loss of Power
The accident airplane was one of three aircraft deployed to respond to a fire. The three airplanes had made four successful water drops and had just scooped water from the Columbia River for a fifth drop. While climbing through 4,600 ft mean sea level, two pilots heard the accident pilot announce “mayday” and state that his airplane was losing engine power.
The pilot of the accident airplane stated that the engine was running nominally until he heard “a rather loud bang,” followed by a total loss of engine power. The compressor was running and responsive to throttle inputs, but there was no noticeable thrust. Initially planning to land on an old logging road, the pilot found the area too narrow with trees on both sides and instead stalled the airplane to the treetops.
Impact and Wreckage
After impact with trees, the airplane came to rest upright at the base of trees adjacent to the road. A Federal Aviation Administration inspector reported the entire airplane came to rest there, with the cowling and one wing coated in oil and the engine damaged. Examination by the US Forest Service revealed the airplane impacted trees adjacent to a fire road. The fuselage came to rest upright on a magnetic heading of about 232° at an elevation of 3,570 ft mean sea level. The first identified point of impact was a 50- to 75-ft tree with damaged limbs near the top. The debris path was oriented on a magnetic heading of 143°, about 345 ft in length, with various airplane fragments and several damaged trees along the path.
Postaccident Examination
Postaccident examination of the engine found foreign melted debris in the combustion chamber, with molten mass stuck to combustors, turbine vane rings, and turbine blades. The debris was identified as environmental particles, but the source was not identified. A second-stage power turbine blade exhibited fatigue signatures. According to Pratt & Whitney Canada, the fatigue appeared to have initiated from several regions of “microshrinkage porosities.” No foreign melted debris was on the fracture surface of that blade. The fracture faces of the first- and second-stage power turbine blades showed overload signatures.
Inspections and functional tests of the fuel pump, fuel control unit, flow divider valve, fuel oil heat exchanger, overspeed governor, and fuel nozzles were performed. The compressor bleed valve and propeller governor were disassembled and inspected. No mechanical anomalies were found that would have precluded normal operation.