On June 16, 2011, at 1330 central daylight time, N30902, an Ayres Corporation S2R-T34, sustained substantial damage when it made a forced landing to a soybean field after an uncontained engine failure and in-flight fire. The commercial pilot sustained minor injuries. The airplane was registered to and operated by Swan Lake Flying Service, Altheimer, Arkansas. No flight plan was filed for the local flight that departed a private airstrip near Altheimer, about 1320. Visual meteorological conditions prevailed for the agricultural spraying flight conducted under 14 CFR Part 137. According to the pilot, he departed with 2,400 pounds of fertilizer and flew to a 23-acre field approximately one-mile from where he departed. Just as he started his first pass, and without any warning, the engine "exploded" and the propeller "slammed" to a stop. He also saw flames around the cowling and exiting the exhaust stack. The pilot immediately established best glide speed and headed toward a soybean field while dumping the fertilizer. He said that as he was approaching the field, there was a lot of black soot coming from the engine and he could still see flames and pieces of metal exiting the exhaust stack. As he touched down in the field, he wanted to avoid hitting a canal, but the smoke was too thick for him to see. The landing gear struck the canal and was torn off the airplane. It then skidded for approximately 75 feet before coming to a stop, and the pilot was able to exit the airplane before it was consumed by fire. The engine was examined at Pratt & Whitney Canada, St, Hubert, Quebec, Canada on October 31, 2011, under the supervision of a NTSB powerplant engineer. The teardown revealed a failure in the first stage planetary gear-set of the propeller reduction gearbox (RGB). All the gear teeth of the sun gear were ground down as far as the gear tooth roots and the three meshing planetary gear teeth were battered and mutilated with only 2/3 of the gear teeth height remaining. The review of the documentation of the last repair revealed that, contrary to guidance in Pratt & Whitney Canada overhaul and service bulletin instructions, which state that the 1st stage planetary gears should be replaced with a new set, the gearset was actually replaced with a used set from another engine. The shop which sold the used gearset had misrepresented the gearset as being zero-timed. The hours accumulated on the used gearset were beyond the allowed time between overhaul (TBO) time of the engine in which it was installed.