HISTORY OF FLIGHT
On August 16, 2011, about 0900 mountain daylight time, an Air Tractor AT-401 airplane, N1012T, was substantially damaged during a forced landing near Hardin, Montana. The owner/pilot operated the airplane under the provisions of 14 Code of Federal Regulations Part 137 as an aerial application flight. The commercial pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed, and no flight plan was filed.
According to the pilot, he had just taken off and was maneuvering the airplane to start an application pass when the engine quit. During the forced landing to the field, the airplane came to rest upright and sustained structural damage to the entire airplane.
A Federal Aviation Administration (FAA) inspector responded to the accident site and reported that the debris field consisted of fire damaged hay fields about 1 mile behind the main wreckage. A visual inspection of the engine revealed that the compressor section had sustained fire damage, and the turbine section had broken and missing vanes.
AIRCRAFT INFORMATION
According to the engine logbook, on April 4, 2002, the accident engine was installed on the accident airplane, with zero time since overhaul. The engine was started and test flown with no discrepancies.
In March 2010, a GE aviation field support manager, and a technician from Premier Turbine in Neosho, Missouri, performed a borescope inspection of the engine in accordance with combustor outer liner lugs Service Bulletin. At that time they noted that the combustor outer liner lugs were 85-90 percent worn, which rendered the engine unserviceable. They passed on the information to the operator and recommended an engine overhaul due to the condition of the engine hot section and high time since overhaul.
On April 19, 2011, the airplane had undergone a 100-hour inspection and was returned to service in an airworthy condition; no evidence was found indicating that the engine had been worked on or replaced during the 1-year period between when it was deemed unserviceable and when it was returned to service. According to the pilot, the airplane had undergone a 100-hour inspection a week prior to the accident; however, there was no corresponding airframe or engine logbook entry. Recorded time since major overhaul was 2,819.6 hours. According to the engine manufacturer, the engine had three inspection times that were required to be adhered to:
1. Time between Overhaul (TBO) – 2,000 engine flight hours 2. Cycles between Overhaul – 2,250 engine flight cycles 3. Calendar time – Five years
TEST AND RESEARCH
The turbine section assembly and exhaust housing, turbine wheel and blades, and compressor assembly with shaft were shipped to the National Transportation Safety Board's (NTSB) Materials Laboratory in Washington, D.C., for further examination. The materials lab specialist reported that the majority of the first stage turbine wheel blades were missing the bulk of the airfoil material above the platforms. One blade, at the 12 o-clock position had fractured below the airfoil platform, with a blade fragment retained in the wheel slot. This area exhibited features consistent with a fatigue crack that initiated at the forward left side of the blade fir tree. The specialist reported a flat faceted feature that was consistent with stage 1 fatigue crack growth typical after fatigue crack initiation in nickel-based superalloys, was located at the crack initiation site. Ratchet marks observed in this area were also consistent with fatigue crack propagation from the crack initiation site. A detailed examination report is attached to the docket for this accident.