Incident Details
On July 11, 2022, about 0630 central daylight time, an Air Tractor AT-502B, registration N5850R, was substantially damaged during a forced landing at Decatur Airport (DEC) in Decatur, Illinois. The pilot was not injured. The flight was conducted under Title 14 Code of Federal Regulations Part 137.
During initial climb from DEC, the pilot heard a “pop” and observed white smoke trailing the airplane. A loss of power was noted, and the pilot lowered the nose to maintain airspeed. A forced landing was executed into a bean field, during which the airplane struck an airport perimeter fence, substantially damaging both wings.
Postaccident Examination
Postaccident examination of the engine, a Pratt & Whitney Canada PT6A, revealed that all compressor turbine (CT) rotor blades were fractured at varying spanwise locations. The power turbine (PT) rotor blades immediately downstream of the CT exhibited impact damage and were fractured at varying spanwise locations. Circumferential rub was observed on multiple components, most notably the PT vane ring, consistent with engine rotation at impact.
Materials Analysis
The CT rotor blades were removed for materials analysis. Microstructure analysis identified alloy solutioning consistent with exposure to temperatures in excess of engine operating limits. Scanning electron microscope analysis revealed micro-voids along the material grain boundaries, indicative of creep.
Service Bulletin Information
Pratt & Whitney Canada service bulletin PT6A-72-1767 (revision 5) addresses CT blade creep resulting from operation at higher temperatures and power than recommended. The bulletin introduces a new CT disk balancing assembly with redesigned single-crystal turbine blades to improve creep resistance, though it does not prevent creep if engine limitations are exceeded. Compliance was voluntary as the bulletin was not mandated by Transport Canada or the Federal Aviation Administration.