History of Flight
On July 12, 2021, at 0805 central daylight time, a PZL Mielec M-18A Dromader, registration N9310R, was destroyed in an accident near Seiling, Oklahoma. The pilot was fatally injured. The airplane was operating as a Title 14 Code of Federal Regulations Part 137 aerial application flight. The pilot departed Fairview Municipal Airport (6K4), Fairview, Oklahoma, at 0744. He proceeded to an agricultural field about 19 miles southwest of the departure airport. GPS data indicated the pilot completed 11 north-south oriented application passes over the field. The final data point was recorded at 0805:42 at an approximate altitude of 1,787 ft mean sea level (about 35 ft above ground level) at the north end of the field at the end of an application pass, aligned with where the pilot had previously initiated a turn to reverse course. The airplane impacted a pasture about 0.34 mile northwest of the final data point.
Personnel Information
The pilot held a Federal Aviation Administration (FAA) mechanic certificate with airframe and powerplant ratings. He did not hold an inspection authorization associated with that certificate.
Aircraft Information
The airframe featured a fuselage-mounted center wing section, with left and right outboard wing sections attached via upper and lower fittings on the main wing spar and one fitting on the rear spar. The fittings were secured by an expansion mandrel assembly consisting of a split mandrel, tapered plug, retention bolt, washer, and castellated nut with cotter pin. In August 2000, the manufacturer issued service bulletin (SB) E/02.170/2000 requiring recurring inspections of the wing attachment fittings due to two in-flight wing separations. The FAA issued airworthiness directive (AD) 2000-18-12 mandating compliance. The AD/SB required inspection of the center-to-outboard wing attachment joints for corrosion, cracks, and ovalization of through-holes. Corrosion necessitated magnetic particle inspection; cracks required fitting replacement. Ovalization within limits could be repaired by reaming and installing a repair mandrel. After inspection, wing fittings and mandrel hardware were to be coated for corrosion protection.
Maintenance History
Maintenance records showed the pilot performed the most recent annual inspection, which included AD compliance. An FAA-certificated mechanic with airframe and powerplant ratings and an inspection authorization (A&P/IA) subsequently certified compliance. Records indicated AD compliance for each annual inspection from 2003 to 2021, though only the 2006 inspection specifically noted magnetic particle inspection. There was no record of the failed fitting being replaced or the lug bore being machined. The A&P/IA stated that the pilot/mechanic removed wing attachment bolts and mandrels to check for corrosion and dimensionally inspect the lugs, but he was unsure if magnetic particle inspection was performed. The A&P/IA believed the AD was complied with by inspection and that the wing lug fittings were not replaced at the last annual. The pilot reportedly flew at reduced weights to minimize airframe stress. In 2016, a Honeywell TPE331 turboprop engine was installed.
Wreckage and Impact
The main wreckage comprised the fuselage, engine, right wing, and empennage. A ground impact depression oriented northwest was adjacent to the main wreckage. The left wing had separated and was located about 120 ft northeast of the main wreckage. Flight controls remained attached except for the right aileron and the outboard portion of the left elevator, both found at the site. Minor debris from the left wing was located about 190 yards southeast. Examination revealed that the left outboard wing main and rear spars had separated from the center wing structure. The lower lug of the left outboard wing main spar lower attachment exhibited fatigue fracture through approximately 90% of its cross section, with the remaining area fracturing in overstress. The separated upper lug showed features consistent with overstress fracture.
Tests and Research
Metallurgical examination determined that the fatigue crack originated on the lug bore surface about 0.12 inch from the forward face of the lower lug. The crack origin was associated with a corrosion pit. The crack propagated about 0.80 inch before the remaining ligament failed in overstress. Fatigue crack surface deposits were consistent with corrosion products. Two additional corrosion pits were near the origin. The bore surface exhibited significant corrosion pitting, and corrosion deposits were prevalent on the expansion mandrel assembly in contact with the outboard lower main attachment fitting bore. Examination of the opposing upper lug revealed fracture features consistent with upward bending of the inboard portion of the lug. Surface deposit samples from the retention bolt, tapered plug, and split mandrel showed spectra consistent with degraded or oxidized corrosion inhibiting compound. Samples from the lugs and bore did not match the corrosion inhibiting compound spectrum. The right outboard wing to center wing main spar lower attachment fitting was intact with no cracks but exhibited mechanical pits and no positive indications of fluorescent deposits consistent with previous magnetic particle inspection. Its expansion mandrel components had a brittle, crumbling brown coating consistent with an aged or degraded corrosion inhibitor coating.