Overview
On May 20, 2015, at approximately 1440 central daylight time, an Ayres Corporation S2R-G6 agricultural airplane (registration N3300U) made a forced landing into a field following a loss of engine power near Tivoli, Texas. The commercial pilot, the sole occupant, sustained minor injuries, and the airplane was substantially damaged. The aircraft was registered to AT Rental Properties and operated by AT Ag Services, LLC, under 14 Code of Federal Regulations Part 137 as an aerial application flight. Visual meteorological conditions prevailed, and no flight plan was filed. The local flight originated from a private strip near Tivoli.
Sequence of Events
The pilot reported that the accident flight was the twelfth aerial application flight of the day. While carrying a load of fertilizer to the destination field, the engine made an uncommanded power acceleration. The pilot noticed an increase in engine temperature and retarded the power lever, but the engine did not respond. He jettisoned the fertilizer load and shut off the fuel supply to the engine. Subsequently, he performed a forced landing into a field, during which the airplane nosed over and came to rest inverted.
Postaccident Examination
A postaccident examination of the airplane was conducted by the responding Federal Aviation Administration (FAA) inspector and a representative from Honeywell Aerospace. The examination revealed no anomalies with the engine rigging, but the engine's fuel control unit stub shaft spline appeared worn. No other anomalies were noted.
On June 9, 2015, the fuel control unit was examined by Woodward, Inc., along with representatives from the NTSB, FAA, and Honeywell Aerospace. The examination confirmed that the stub shaft spline was worn beyond allowable limits. The internal spline was checked using a specific go/no-go gauge, which should not fit between the teeth of the spline; however, the gauge rotated freely. This wear can allow the fuel control underspeed governor to be driven at a speed lower than the engine speed, subsequently increasing fuel flow and engine speed.
Maintenance History
A review of the aircraft maintenance records revealed that Airworthiness Directive (AD) 2006-15-08, applicable to Honeywell TPE331 series turboprop engines, was accomplished on March 16, 2015. The AD addresses reports of loss of the fuel control drive, which can lead to engine overspeed, overtorque, overtemperature, uncontained rotor failure, and asymmetric thrust in multi-engine airplanes. The AD specifies inspections of the fuel control drive for wear or damage, with repair or replacement as necessary. The records noted that the next inspection was due in 1,000 hours.