Accident Details
On July 6, 2011, at approximately 1030 central daylight time, an Ayres S2R600 G6 single-engine airplane, registration N3101S, sustained substantial damage following an impact with terrain near Pine Bluff, Arkansas. The aircraft was owned and operated by Swan Lake Flying Service, Inc., and was conducting a local agricultural flight under 14 Code of Federal Regulations Part 137. Visual meteorological conditions prevailed, and no flight plan was filed. The pilot reported no injuries.
The pilot stated that the takeoff and initial climb were normal. While turning at about 150 feet above ground level, the engine suddenly lost power. The pilot executed a forced landing near the end of the runway, where the airplane struck terrain and a drainage ditch before coming to rest upright. The impact caused substantial damage to the engine mounts and forward fuselage, and both main landing gear partially separated from the airframe.
Engine Examination
Airframe and engine maintenance logbooks were not available for review. One maintenance record from a repair facility indicated that the engine had been disassembled and repaired three years prior due to a torque sensor failure. That record showed that the torque sensor and several other parts had been installed in an overhauled condition, and the engine was approved for return to service on September 13, 2008.
A postaccident examination of the engine revealed that four of five torque sensor assembly mounting bolts were loose. One of the loose bolts was undersized, of incorrect configuration, and not the specified part. The torque sensor in the engine matched the serial number listed in the September 2008 maintenance record. Heavy wear was observed on the torque sensor housing around two bolt holes, and the corresponding mounting hardware also exhibited wear. The difference in bolt shank diameter allowed motion of the torque sensor housing relative to the gear case, as evidenced by separation of anti-rotation tabs that left deep wear grooves in the housing, indicative of tab contact. Three teeth were found separated from the torque sensor spur gear, which is part of the assembly that drives the fuel pump and fuel control. Metallurgical examination determined that the failure mode of the separated teeth showed features consistent with high cycle fatigue initiation and tip loading after the torque sensor mounting bolts became loose. According to the manufacturer, when the fuel pump and fuel control are no longer driven by the gears in the gear train—which are driven by the torque sensor spur gear—the engine stops running.