FTW99LA170

The pilot was applying a chemical to a field. After completing a pass, as he applied aft elevator for a straight-ahead pull-up, a loud 'boom' was heard, and the outboard section of the right wing separated. The airplane became 'uncontrollable' and impacted in an uncultivated field. Metallurgical examination revealed that the separation of the right wing forward spar lower attachment fitting was the result of fatigue cracking in the bracket lugs. The fatigue cracking initiated from areas of severe corrosion damage in the lugs. Evidence of extensive corrosion pitting was found in brackets from all other examined front spar attachment fittings, particularly on the internal surfaces of the lugs. The Service Manual and applicable service bulletins call for fluorescent penetrant or magnetic particle inspection of wing attachment fittings during the 3,000-hour inspection for extension of the airframe life to 6,000-hours. Although the aircraft records show that during the last annual inspection, which was completed approximately 596.5 hours prior to the accident, at an airframe time of 4,245.5 hours, all time life items were complied with, the metallurgical examination disclosed no trace of dye penetrant residue on any of the examined components.

On June 18, 1999, at 0750 central daylight time, a Wsk Pzl Mielec M-18A agricultural airplane, N7075D, registered to and operated by Basile Flying Service, Inc., of Basile, Louisiana, was substantially damaged while maneuvering near Basile, Louisiana. The commercial pilot, sole occupant of the airplane, sustained 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 local flight originated from the Basile Flying Service airstrip, at 0740.

The pilot reported that he was applying chemical to a field. After completing a west-east pass, as he applied aft elevator for a straight-ahead pull-up, a loud "boom" was heard, and the outboard section of the right wing separated from the fuselage. The airplane became "uncontrollable" and impacted in an uncultivated field.

Examination of the aircraft revealed that the right wing forward spar lower attachment fitting had failed. The right wing upper and lower attachment fittings along with the left wing lower attachment fitting (for comparison) were sent to the NTSB Materials Laboratory in Washington, D.C. for examination.

Examination of the separated bracket lugs from the right wing forward spar lower attachment fitting with the aid of a low power binocular microscope revealed that the fractures in the lower ligaments of the lugs, were flat, darkly discolored, and contained ratchet marks and crack arrest positions indicative of fatigue propagation. Rough irregular features on the fracture faces on the upper ligaments of the lugs, and the extent of plastic deformation associated with both of these fractures were typical of final overstress separations. Evidence of corrosion damage was noted in the bore of both lugs on the internal face of each lug, and on the fracture faces in the fatigue fracture regions. On the forward lug, severe pitting corrosion was noted on the bore surface along the back face of the lug; also noted were secondary cracks in the corrosion-damaged area of the bore. The most severe corrosion in the aft lug was noted in the area of the fatigue fracture origin.

The split bushing from the right wing forward spar lower attachment fitting had evidence of corrosion damage, secondary rotational scoring, fretting, and galling on the exterior surface. Mating of surface features on the bore of the forward and aft lugs with the features on the exterior surface of the bushing disclosed that areas of fatigue origins in the lugs coincided with a band of severe pitting corrosion in the bushing.

Examination of the right wing forward spar upper attachment fitting revealed severe pitting corrosion damage on the back (internal) face of the forward lug and moderate corrosion on the front (internal) face of the aft lug. No cracking was found in the bracket lugs of this fitting.

The fracture features and the extent of plastic deformation in all other separated components, including the right wing rear spar attachment fitting and the spacer, were consistent with overstress separations.

The left wing front spar lower attachment fitting was disassembled. Examination of the disassembled components with the aid of a binocular microscope revealed areas of extensive pitting corrosion on the exterior surface of the split bushing and on the bore and internal faces of both the forward and aft lugs in the bushing. The examination also revealed a crack in the forward lug.

A review of the aircraft's maintenance records indicated that an annual inspection was completed on March 1, 1999, at 4,245.5 hours total time, approximately 596.5 hours prior to the accident. The records showed that during this inspection, the right wing aft spar attachment fitting was replaced. No work related to any of the other wing attachment fittings was reported.

The Service Manual for the PZL M18 series airplanes and subsequent service bulletins, No. K/02.142/91 and No. E/02.152/94, call for a 3,000-hour inspection for extension of the airframe service life to 6,000 hours. According to these bulletins, all wing attachment fittings shall be examined by fluorescent penetrant or magnetic particle techniques during this inspection. The aircraft records indicated that during the March 1, 1999, annual inspection, all 6,000-hour time life items were complied with, including the special inspection requirements for the wing attachment fittings.

Examination at the NTSB Materials Laboratory of the upper and lower front spar attachment fittings for the right wing, as well as the lower attachment fitting for the left wing front spar showed that the bore surfaces of all lugs and corresponding surfaces of all bushings contained only patches of dried up lubrication grease. No evidence of sufficient lubrication was noted in any of the joints. Examination also revealed no evidence of fluorescent penetrant dye stain on the surface of any bracket or the mating fracture faces on the bracket lugs from the right wing forward spar lower fitting.