FTW00LA149

The pilot completed a second spray run and had initiated a climb when the agricultural airplane's right wing separated. Subsequently, the airplane impacted the ground and came to rest against a tree line. Examination of the airplane revealed that the right wing's forward lower attachment fitting separated as a result of fatigue cracking. The fatigue cracking was a result of severe corrosion pitting in the bore of the central lug of the fitting.

History of flight

On May 22, 2000, at 0658 central daylight time, a Pzl-Mielec M-18A agricultural airplane, N2296C, was destroyed when its right wing separated in flight while maneuvering near Kilbourne, Louisiana. The airplane was owned and operated by Ward's Flying Service of Eudora, Arkansas. The commercial pilot, sole occupant, received serious 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 at 0650, from the operator's private airstrip in Eudora, Arkansas.

According to the pilot, he had completed his second spray run and was pulling up to clear the tree line when "the right wing started coming off causing loss of control." He added that the airplane impacted trees and the ground before sliding to a stop against another tree line.

According to the FAA inspector, who responded to the accident site, witnesses observed the airplane nose up to clear some trees after a spray run when the right wing "folded back and fell off." The FAA inspector reported that the right wing was found laying in a field approximately 50 yards from the main wreckage. He added that one of the right wing's attachment fittings had failed. According to the FAA inspector, it appeared that a "crack had progressed through approximately 7/8ths of the fitting's thickness before failing."

The airplane wreckage was transported to Air Salvage of Dallas in Lancaster, Texas, for further examination.

Aircraft information

The M-18A has three wing sections; the center wing section, and the two outboard wing sections (left and right). The three wing sections each contain a forward upper, a forward lower, and a single aft wing attachment fitting, which are mated together to secure the outboard wings to the center wing. The center wing attachment fittings consist of dual lugs, which straddle the singular lugs from 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 insert through the mandrel and are torqued causing the expansion mandrel 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. 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. According to the aircraft maintenance records, on August 1, 1999, at a total airframe time of 4,020.4 hours, the two aforementioned service bulletins were complied with. The maintenance entry for that date stated, in part, that mechanics "replaced wing to ctr [center] section attach[ment] bolts. Inspected attach[ment] fittings. Serviced & inspected A/C I.A.W. [aircraft in accordance with] airplane description and service manual PZL M-18 'Dromader' section 4 & section 8 as necessary." The airplane also underwent its last annual inspection on that same date. At the time of the accident, the airplane had accumulated a total of 5,166.8 hours.

The NTSB investigator-in-charge (IIC) conducted telephone interviews with the two mechanics who worked on the airplane during the August 1, 1999, annual. According to both mechanics, they conducted a flourescent penetrant inspection on the wing fittings and found no cracks. The mechanics stated that the fittings were cleaned with scotch-brite and examined for defects, corrosion, and elongation. According to one mechanic, the wing attachment fittings and mandrels were coated with an anti-corrosive material commonly referred to as "black-bear" (peracyltone, a manufacturer approved material), and the wings were then reassembled.

Tests and research

The right, left, and center wings were examined by the NTSB IIC and a representative from the FAA's Small Aircraft Certification office. The right wing's forward lower attachment fitting was found fractured in two and heavily corroded. The lower fracture surfaces, which were orientated at the 6 o'clock position, were flat for approximately 7/8ths of the surface. The upper fracture surfaces displayed 45-degree shear lips and plastic deformation. The right wing and right side center wing attachment fittings were removed and sent to the NTSB Materials Laboratory for further examination.

The remaining forward upper and lower wing attachment fittings were then examined. Each of the wing attachment fitting lugs displayed a line of heavy corrosion pitting on its inside 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 6 o'clock, 3 o'clock, and 7 o'clock positions respectively.

Visual examination of the right wing's forward lower attachment fitting, by the NTSB Materials Laboratory, revealed that the lower fracture surfaces were flat, darkly discolored, and contained ratchet marks indicative of fatigue cracking. The upper fracture surfaces were deformed and exhibited features typical of a tensile overstress separation. Examination of the lower fracture surfaces with the aid of a binocular microscope revealed that the fatigue cracking in the lug initiated at multiple origins located in the bore of the lug. The combined origin of the fatigue fracture extended linearly from the back face of the lug through about 95% of the lug thickness. From these origins the fatigue cracking propagated throughout approximately 90% of the lug cross-section. Further examination of the fatigue crack revealed that it originated from areas of "severe" pitting corrosion.