Wsk Pzl Mielec M 18A N2283M

The experimental, restricted-category airplane impacted wooded terrain after a separation of the right wing during an aerial application flight. The right wing separated under normal loading conditions due to extensive pre-existing fatigue cracking in the right outboard wing forward spar lower fitting as the result of corrosion in the hole bores. In addition, the outboard half of the right aileron separated during the accident sequence due to fatigue cracking in the right outboard aileron bracket. Examination revealed evidence of moderate to severe corrosion throughout the airplane. Given the amount of fatigue cracking and the individual crack features of the spar fitting, it is likely that the cracking was present for an extended period of time before the final separation and also likely would have been visible during previous inspections. Maintenance records indicated that the airplane received an annual inspection about 6 months before the accident and inspection of the wing fittings in accordance with a Federal Aviation Administration airworthiness directive; however, these inspections failed to detect the pending failure of the right wing, indicating that they were performed inadequately or improperly. The maintenance records also indicated that the wing fittings on the accident airplane had been replaced on three occasions in the 15 years before the accident, indicating that proper cleaning and corrosion prevention procedures were not being performed. The airplane was operated over its certificated maximum gross weight on the accident flight and likely had been operated overweight for much of its lifetime. About 11 years before the accident, the airplane was outfitted with a larger hopper, increasing its capacity from 660 gallons to 800 gallons. While the increased hopper volume is beneficial for operation with lower-density solid or dry chemicals, it could easily be loaded with up to twice the certificated weight of liquid chemicals. Although the manufacturer published provisions for increasing the operating weight of the airplane, it required that the airframe service life limit of 10,000 hours be reduced by a factor of 1.35. Review of maintenance records indicated that not only did the operator not apply any service life limit factors to account for the overweight operation but that a reduction in overall airframe time was annotated several years before the accident; this reduction could not be reconciled based on the information presented in the records. Given the lack of information about its operating history, the actual service life of the airplane could not be determined; however, it is likely that the airplane was close to or had exceeded its service life, particularly given the overweight operation. Although the failed wing fittings had not accumulated the service life limit due to their replacement, the corrosion found throughout the airplane and its routine overweight operation would have reduced the true service life of the fittings. In addition to the increased airframe stress imposed by continuous overweight operation, the airplane was modified to install a turbine engine many years before the accident. The engine conversion likely caused more severe flight loads than accounted for in the design of the original radial piston engine-equipped airplane, further rendering the manufacturer-published service limits inadequate.

Quoted verbatim from the NTSB record.