HISTORY OF FLIGHT On September 21, 2000, at 1015 central daylight time, a Pzl-Mielec M-18A agricultural airplane, N81548, was destroyed when its right wing separated in flight while maneuvering near Jacksonville, Arkansas. The airplane was registered to and operated by Bullock Flying Service of Dewitt, Arkansas. The commercial pilot, who was the sole occupant, received minor injuries. Visual meteorological conditions prevailed and a flight plan was not filed for the 14 Code of Federal Regulations Part 137 aerial application flight. The airplane departed a private air strip near Dumas, Arkansas, at 0951. According to the pilot, he was initiating a spray run over a field when he heard a loud pop. The pilot stated that the airplane started to roll and nose down. The airplane impacted the ground, coming to rest inverted. The pilot stated that he removed his helmet, unbuckled his safety belt, and exited through the right door. According to the FAA inspector, who responded to the accident site, the right wing was located approximately 50 yards from the main wreckage area. The FAA inspector stated that the right lower outer wing attachment fitting had failed and displayed dark corrosion and a flat fracture surface. The aircraft wreckage was taken to Dawson Aircraft in Clinton, Arkansas, for further examination. AIRCRAFT INFORMATION The M-18A has three wing sections; a center wing section and two outboard wing sections (left and right). The outboard wing sections each contain a forward upper, a forward lower, and a single aft wing attachment fitting, which are mated to corresponding fittings on the center wing. The center wing attachment fittings consist of dual lugs, which straddle the singular lugs of the outboard wing attachment fittings. When the forward upper and lower wing attachment fittings are aligned during installation, they are secured together by use of expansion mandrels, which are placed through the center of the aligned lugs, and conical ended bolts, which are inserted through the mandrels and torqued, causing the expansion mandrels to expand and secure the fittings in place. At the time of the accident, the manufacturer called for the placement of the expansion mandrel's gap to be at the 6 o'clock position. The aft fittings are secured together using a straight bolt. The wing attachment fittings are made from 30HGSA steel. The Service Manual for the M-18 series airplanes and service bulletins, K/02.142/91 and E/02.152/94 required a 3,000-hour inspection to extend the airframe service life to 6,000 hours, which was the aircraft's service life limit. According to the Service Manual and service bulletins, all of the wing attachment fittings shall be examined by flourescent penetrant or magnetic particle techniques during the 3,000-hour inspection. There was no indication in the aircraft maintenance records that the recommended wing attachment fitting inspection had been conducted. The airplane underwent its last annual inspection on March 3, 2000, at an aircraft total time of 5,542.0 hours. At the time of the accident, the airplane had accumulated a total of 5,898.6 hours. The FAA Type Certificate Data Sheet (No. A47EU) for the M-18 series aircraft requires that operators comply with the aforementioned service manuals and bulletins to extend the aircraft's service life to 6,000 hours. The airplane was issued a special airworthiness certificate on May 30, 1989. The operator was the registered owner of the airplane since August 4, 1989. TESTS AND RESEARCH The right, left, and center wing sections were examined by the NTSB Investigator-in-charge and an FAA inspector on October 4, 2000. The right outboard wing section's forward lower attachment fitting was found fractured in two and heavily corroded. The lower fracture surface, which was orientated at the 6 o'clock position, was flat for approximately 7/8ths of the surface and displayed ratchet marks, indicative of fatigue cracking. There was a line of deep corrosion pits on the inner circumference of the lug near the fracture line. The upper fracture surface displayed 45-degree shear lips and plastic deformation. The remaining forward upper and lower wing attachment fittings were disassembled and examined. Each of the wing attachment fitting lugs displayed a line of heavy corrosion pitting on its inner circumference. It was noted on all of the fittings that the line of corrosion matched up with the expansion mandrel's gap position. The right upper, left upper, and left lower expansion mandrel gaps appeared to be oriented along the 4 o'clock, 6 o'clock, and 8 o'clock positions respectively. All 4 expansion mandrels and the conical ends of their bolts were examined. All of the mandrels and bolts displayed some degree of corrosion. The right side mandrels and their conical ends displayed the most corrosion. The corrosion had eroded away a significant amount of material on both the mandrel and the conical bolt ends. It was also noted that the right lower mandrel and bolt were totally void of grease or anticorrosive agents. The left side expansion mandrels and bolts displayed corrosion; however, it was not to the same extent as the right side.