On September 16, 1999, about 0930 central daylight time, an Air Tractor AT-502B, N5092G, registered and operated by Clifford Flying Service, operating as a 14 CFR Part 137 aerial application flight, experienced a reported total loss of engine power during a swath run. The pilot made a forced landing to a cotton field. Visual meteorological conditions prevailed and no flight plan was filed. The airplane sustained substantial damage. The commercial pilot reported serious injuries. The flight originated from Hollandale, Mississippi, about 20 minutes before the accident. The pilot stated he was making a swath run from north to south at about 10 to 15 feet, when the airplane experienced a total loss of engine power. He applied full power, ensured the boost pump was on, and made a left turn to avoid trees. He lowered the flaps to the full down position. The airplane collided with a cotton field, and nosed over inverted. Examination of the engine assembly was conducted by the FAA and parties to the NTSB investigation. The Pratt and Whitney representative stated in his written report, "The following was noted during the process of investigating the cause of the accident. (See attached P&W Report No. 99-09-17) 1. P3 air line P/N3028739 was found disconnected at the P3 filter housing. There was no apparent damage to the threads on the nipple coming from the housing. There was no apparent damage to the "B" nut internally or externally. There was no evidence of safety wire securing the "B" nut.... 2. The P3 air filter and bowl were found intact, filter was clean and not damaged. 3. Airframe fuel boost pump operated properly when power was applied. 4. Airframe fuel filter contained a small amount of what appeared to be clean jet fuel. 5. Engine driven fuel pump filter bowl was full of fuel, and the filter appeared to be clean and unrestricted.... 6. Fuel line from the engine fuel filter to the fuel control unit was full of fuel. 7. All engine control rods and cables were properly connected. Some controls appeared to have moderate impact damage, but were not disconnected. 8. The accessory gear box was turned by rotating the air conditioner adapter. The gear box and compressor rotated freely with all components installed. 9. The propeller was bent in the direction of rotation and the blades appeared to be in the feathered position." An interview with an airframe and power plant mechanic with inspection authority revealed the mechanic had performed the last recorded 200-hour inspection on the airplane. The mechanic stated that he had performed a compressor wash on the airplane in November 1998, but he had not removed the P3 line as stated in the P&W Maintenance Manual. Instead he had removed the P3 bowl, filter, and completed the compressor wash. When asked if he had checked the area around the fuel control unit to ensure that the lines were tight, and safety wire/lock wire had been installed, the mechanic stated, the "B" nut line slipped by without being safetied, however he did not remove the line. Review of the P&W Canada Maintenance Manual, paragraph 5. Internal Washing, A. General, CAUTION: states, "Make sure the P3 line to the filter and FCU is disconnected before performing a motoring compressor wash to avoid contamination.... It further states in (2) Installation of Air Pressure Lines: (b) 4 The coupling nut of the insulated line (19) is to be tightened and fastened with lock wire after the pressure check (Ref. Para.10.)."