On July 24, 1999, at 1830 central daylight time, an Air Tractor 401B, N6006A, operated by Bakke Flying Service, received substantial damage on impact with terrain while spraying a potato field located approximately 2 miles north of McCanna, North Dakota. Visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 137 agricultural flight was not operating on a flight plan. The pilot sustained serious injuries. The flight originated from the Bakke Airstrip, Larimore, North Dakota, at 1815. In a written statement, the pilot reported the following: "July 24 it was hot and humid and flying was not very good wind was light I was spraying the south half of the field going back and forth from the north to south. I was on my 4 or 5 pass on the east end making right turns. In my last right turn the aircraft stalled and hit the ground. ..." The left shoulder harness was found separated following the accident. The shoulder harness was removed and sent to the National Transportation Safety Board's (NTSB's) Materials Laboratory for examination. The NTSB's Material Report stated. "...Generally the harness appeared to be of uniform coloration...". Three sections of the harness were removed and delivered to an independent testing laboratory for tensile testing. The test results for the three sections indicated ultimate loads of 4,920 lbs, 4,760 lbs, 4,670 lbs. The test report also stated, "...In all three cases the failure initiated in the harness adjacent to the double "X" stitching .. A test certificate ... indicated that a harness, part number 500702-403, was tested on April 13, 1989 for Air Tractor. It had a rated strength of 1,500 pounds and failed at 3,300 pounds when an end fitting broke. Testing was reported as in compliance with TSO-C22E (NAS 802), single occupancy." The separated ends of the left shoulder harness had, "...numerous individual filaments protruding from the surface almost obscuring the pattern of the strands..." The area containing the protruding filaments extended approximately one inch below the separated ends above which was the harnesses adjusting roller. Areas along the separated ends exhibited "horsetailing". According to the Materials Laboratory Factual Report,"..."Horsetailing" is a product of the material properties and the differences in the energy stored during separation." The airplane's date of manufacture was 1995. Air Tractor Service Letter #152 applicable for all Air Tractor models and serial numbers states, "... Color fading is an indication of deterioration due to ultraviolet light exposure. Faded or frayed harnesses should be replaced immediately We recommend that the shoulder harness be replaced every five years...".