N5850RAir Tractor AT-502B · 2004

On July 11, 2022, about 0630 central daylight time, an Air Tractor AT-502B, N5850R, was substantially damaged when it was involved in an accident at Decatur Airport (DEC), Decatur, Illinois. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 137 flight.

During initial climb out from DEC, the pilot heard a “pop” and observed white smoke trailing the airplane. The pilot noticed a loss of power and lowered the nose to gain airspeed, and then made a forced landing into a bean field, during which the airplane struck an airport perimeter fence that substantially damaged both wings.

Postaccident examination of the engine revealed that all CT rotor blades were fractured at varying spanwise locations. The 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.

The CT rotor blades were removed for materials analysis. A microstructure analysis of sampled blades identified alloy solutioning consistent with exposure to temperatures in excess of engine operating limits. A scanning electron microscope analysis of the CT rotor blades revealed micro-voids along the material grain boundaries, indicative of creep.

Pratt & Whitney Canada service bulletin PT6A-72-1767 (revision 5) includes the following information regarding CT blade creep:

There can be blade creep at the CT area when the engine is operated at higher temperatures and power than the Pilot Operating Handbook (POH) recommended power settings. To prevent creep-related events, introduce a new CT Disk Balancing Assembly which incorporates redesigned turbine blades made from a different material and increased gap at the blade inter and under platform.

The new CT blade design features single-crystal material, which is comprised of a continuous crystal lattice that eliminates grain boundaries to improve creep resistance. The new CT blade was not intended to prevent blade creep if the engine is operated outside of the engine limitations. The service bulletin was not mandated by Transport Canada or the Federal Aviation Administration, so compliance was at the discretion of the owner/operator.