NTSB Factual Report

On July 15, 2013, about 1130 central daylight time, an Air Tractor AT-502B airplane, N6113C, was substantially damaged during a forced landing following a loss of engine power near Nauvoo, Illinois. The pilot was not injured. The airplane was registered to and operated by Air Advantage Inc. under the provisions of 14 Code of Federal Regulations Part 137 as an aerial application flight. Visual meteorological conditions prevailed for the flight, which was not operated on a flight plan. The local flight originated from Schilson Field Airport (IS51), Carthage, Illinois, about 1125.

The pilot reported that after takeoff he proceeded 5 miles north toward the assigned agricultural field. As he was setting up for the initial spray run, he heard a "loud explosion" at which point the propeller stopped turning. He noted that metallic debris was emitted from the engine exhaust, which was followed by grinding noises from the engine. He executed a forced landing to a soybean field; however, the airplane overran the bean field and crossed into a corn field, at which time the right main landing gear collapsed. The airplane came to rest upright, with substantial damage to the right wing.

A postaccident engine examination revealed damage to the compressor turbine disc, power turbine disc, and the compressor turbine and power turbine guide vane rings and shrouds. Metallurgical examination determined that one compressor turbine blade exhibited features consistent with fatigue failure initiating at the trailing edge of the blade section, near the root. A majority of the fracture surface was obscured by corrosion products and metal splatter, which precluded a complete evaluation of the fracture surface. Metallographic examination of the trailing edge of the blade fracture surface revealed no material anomalies. The blade material and airfoil trailing edge thickness were determined to be within drawing requirements.

The remaining blades of the compressor turbine displayed gouges and fracture surfaces consistent with secondary impact damage and overstress failures. The power turbine blades were separated from the root to about two-thirds of the blade span. The fracture features were consistent with overstress separations of the blade sections. Five blades were not separated; these blades exhibited gouges consistent with secondary impact damage from the liberated blades. Scraping damage was observed on the compressor and turbine shrouds, which was consistent with blade contact during the engine failure sequence after the initial compressor blade failure.

The airplane was powered by a 750-shaft horsepower Pratt & Whitney Canada PT6A-34AG engine. At the time of the accident, the engine had accumulated about 9,961 hours since new, with about 7,336 hours since overhaul. The most recent recorded engine maintenance consisted of a 100-hour inspection completed on July 6, 2013, at 9,950 hours total time. The most recent engine hot section inspection was completed on December 31, 2012, at 9,782 hours total time.

Maintenance records indicated that the engine was removed and inspected due to excessive vibration in April 2008 at 7,132 hours engine total time. Damage to the power and compressor turbine blades and guide vane rings was observed. Overhauled power turbine blades and an overhauled power turbine guide vane ring were installed. New compressor turbine blades were installed at that time. The most recent engine overhaul was completed in April 2000 at 2,625 hours total time since new.

Based on the available engine maintenance records, the most recent review of engine start cycles was completed on December 22, 2012. At that time, the engine had accumulated 9,752 hours total time, with 5,634 engine start cycles consumed. The compressor turbine start cycle limit was 16,000.

NTSB Probable Cause

A fatigue failure of an engine compressor turbine blade, which resulted in extensive secondary engine damage and a complete loss of engine power.