N528MBAir Tractor AT-802A

On August 9, 2017, about 0730 central daylight time, an Air Tractor AT-802A airplane, N528MB, was substantially damaged when it was involved in an accident near Galesburg, Illinois. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations (CFR) Part 137 agricultural flight.

The pilot reported that the accident occurred about 1 hour into his first aerial-application flight of the day. He stated that he felt a slight "thud or bump" that he initially believed was the airplane flying through its own propeller wash during a turn. The pilot stated that the engine gauges were indicating normal parameters as he continued to line-up for another spray pass; however, before his next spray pass the engine experienced a similar "thud" that he identified as a compressor stall. The pilot entered a climb to gain safe altitude and maneuvered the airplane away from nearby homes. The engine experienced several more compressor stalls and the pilot saw sparks departing the engine exhaust ducts before it experienced a total loss of power. The pilot completed a forced landing in a nearby field, during which the airplane ground looped in the dense soybean crop. The landing gear collapsed, and the outboard portion of the right wing and aileron were substantially damaged during the forced landing.

Examination of the engine revealed that a single 1st stage compressor rotor airfoil had fractured near the root. The fatigue crack originated from a thinned leading edge due to significant erosion at the bottom of the airfoil pressure side; however, the exact location of the fatigue origin could not be determined due to erosion damage. The observed fracture features were consistent with a crack that had progressed in high-cycle fatigue. Dimensional measurements of the fractured airfoil and the two adjacent airfoils revealed that they were up to 0.050 inch less than the minimum requirement for an airfoil. The airfoils also exhibited evidence of fine wear scratches, characteristic of fine abrasive particles impacting the airfoil, and thinning of the leading edge of the airfoils. The remaining damage observed through the remaining gas path was considered secondary to the separation of the single 1st stage compressor rotor airfoil.

According to maintenance documentation, the engine had accumulated 4,349.3 hours since new, and 1,808.6 hours since being partially disassembled for inspection and repair on December 16, 2013; however, according to the logbook entry, the gas generator was not inspected at that time. The most recent inspection of the gas generator was completed during an annual/100-hour inspection completed on May 1, 2017, about 99.3 hours before the accident. Based on the available maintenance documentation, the engine had not been overhauled during its service history. The engine manufacturer recommends that the engine be overhauled after 3,000 hours. Federal Aviation Administration regulations do not require Part 137 operators to comply with an engine manufacturer's recommended time between overhaul interval provided the engine meets annual and 100-hour requirements.