Northport, WA, USA
N397AS
Air Tractor AT802
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.
On August 14, 2018, about 1500 Pacific daylight time, an Air Tractor AT-802A, 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 the provisions of 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 purpose of the flight was to support of firefighting operations in Colville National Forest, which is about 11 miles northwest of Northport. The accident airplane was one of three airplanes deployed to respond to the fire. The three airplanes had made four successful water drops and had just scooped water from the Columbia River for a fifth drop. The airplanes were climbing through an altitude of 4,600 ft mean sea level when two of the pilots heard the pilot of the accident airplane 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,” which was followed by a total loss of engine power. He stated that the compressor was running and was responsive to throttle inputs but that there was no noticeable thrust. The pilot was initially planning to land on an old logging road, but the landing area was too narrow and was lined with trees on both sides of the road. The pilot opted instead to stall the airplane to the treetops. After impact with the trees, the airplane came to rest upright at the base of the trees adjacent to the road. A Federal Aviation Administration inspector responded to the accident site and reported that the entire airplane came to rest there. The inspector noted that the cowling and one wing were coated with oil and that the engine was damaged. Examination of the accident site by the US Forest Service revealed that the airplane impacted trees adjacent to a fire road The fuselage came to rest upright on a magnetic heading of about 232° and at an elevation of 3,570 ft mean sea level. The first identified point of impact was a 50- to 75-ft tall tree that had damaged limbs near the top of the tree. The debris path was oriented on a magnetic heading of 143° and was about 345 ft in length. Various portions of airplane fragments were distributed throughout the debris path. Additionally, several trees were damaged along the debris path. Postaccident examination of the engine revealed foreign melted debris in the engine combustion chamber with the molten mass stuck to the combustors, turbine vane rings, and turbine blades. The debris was identified as environmental particles, but the source of the debris 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.” Pratt & Whitney Canada also stated that there was no foreign melted debris on the fracture surface of the second-stage power turbine blade. The fracture faces of the first and second stage power turbine blades exhibited overload signatures. Inspections and functional tests were performed on the fuel pump, fuel control unit, flow divider valve, fuel oil heat exchanger, overspeed governor, and fuel nozzles. Additionally, the compressor bleed valve and propeller governor were disassembled and inspected. No mechanical anomalies were found that would have precluded normal operation.
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.
Source: NTSB Aviation Accident Database
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