Accident Overview
On July 1, 2009, at approximately 1246 central daylight time, an Ayres Corporation S2R-G6, registration N60015, operated by Air Advantage Inc., received substantial damage on impact with terrain during a forced landing near Ainsworth, Iowa. The 14 CFR Part 137 aerial application flight was not operating on a flight plan. The local flight had originated from Washington, Iowa, about 1200. The pilot sustained serious injuries.
Pilot's Account
The pilot stated that he was completing his second to last spray pass when the engine power decreased while the airplane was at an altitude of 5-10 feet above ground level in a pitched-up attitude with a right bank of 10-15 degrees. He did not recall the airspeed at the time of power loss but noted it should have been about 140 mph. The airspeed was "bleeding off" and there was a "definite drag." He was not thrown forward when the airplane decelerated. The loss of power was described as "subtle" with no "residual" power present. He did not hear or see any abnormalities before the power loss. He attempted to regain engine power by turning the boost pump on but did not turn the igniter on, which had been off during previous spray passes. He also moved the power lever from the 3/4 position to full forward. Unable to regain power, he performed a forced landing on a field that did not initially appear terraced.
Aircraft and Engine Information
The 1993 Ayres Corporation S2R-G6, serial number G6-110, was purchased on April 10, 1993, and registered to Farm & Ranch Aerial Service, Inc. It was sold on November 5, 2008, to Air Advantage, Inc. The engine was a Honeywell TPE331-6-252M, serial number P-20457C, but the data plate indicated both "251" and "252" equipment codes, with neither crossed out. The engine was configured with a Woodward FCU, part number 897770-10, serial number 1587010. Engine logbook records showed the engine was installed on N60015 on July 6, 2004, at a total time of 7,123.2 hours. The last hot section and gear box inspections were performed on February 5, 2009, at a total time of 9,703.9 hours. The Hobbs time at the accident was 1,238.2 hours. The engine was equipped with a Negative Torque Sensing System (NTS). Logbook entries showed NTS trip pressures were set in 2003, 2004, and 2005, but there was no record of NTS checks as per the Honeywell Maintenance Manual.
Examination of Engine and Fuel Control Unit
Disassembly and examination of the engine and gearbox revealed evidence of rotation. Metal spray was present on the suction side of the turbine rotor blades and stator vanes. Debris consistent with soil from the accident site was found in the combustor section. The FCU was broken off at its base, and the fuel inlet was broken open; the fuel inlet screen was not found. Prior to flow testing, the FCU was rotated by hand and noted to bind at varying points. The input shaft assembly was partially disassembled, reducing the resistance. Flow testing was performed per Woodward Governor Test Specification TSP-7970. Disassembly of the FCU revealed debris and smearing that was brown/black in color. A majority of the overspeed governor ball head bearing retainer was not present, and the p2 bellows spring was fractured. Metallurgical examination of the ball head bearing showed it within dimensional specifications but with a misalignment of about 0.0035 inches due to material on its shoulder. The p2 bellows spring exhibited fatigue fracture. Debris was found in the flow divider screen, with FTIR analysis showing similarity to the ball head bearing retainer. A lesser quantity of silver debris produced an Energy Dispersive X-ray spectrum similar to aluminum alloy. Flow testing of the primary and secondary fuel nozzle manifolds revealed debris, with secondary manifold debris spectrum also similar to the ball head bearing retainer. A representative of Propulsion Controls Company stated that Gulf Coast Turbines submitted FCU serial number 1587010 for maintenance in 2003, and that the unit underwent repair/overhaul with time since overhaul recorded as 0 hours. The representative noted several returns of FCUs due to ball head bearing spacer disintegration. Searches of the FAA Service Difficulty Reporting System yielded 24 engine-related records and 25 FCU-related records, but none specifically identified a failure from a ball head bearing spacer. The Woodward representative stated no record of ball head bearing retainer failures.