On October 7, 1993, about 1940, Atlantic standard time, a Douglas DC-3C, N781T, registered to and operated by Tol Air Services, Inc., under 14 CFR Part 135, flight No. 522, as a nonscheduled, international, cargo flight, was substantially damaged on landing at the Luis Munoz Marin International Airport, San Juan, Puerto Rico. Visual meteorological conditions prevailed at the time and an IFR flight plan was filed for the flight from Santiago, Dominican Republic. The airline transport-rated captain and commercial-rated copilot were not injured. The flight originated about 2 hours 2 minutes earlier. The pilot-in-command stated that the second pilot had landed the airplane and while taxiing, he observed that an outer section of the left wing was bent up. Examination of the airplane revealed that Rib No. 19 of the left wing failed. The failed rib was removed and sent to Douglas Aircraft Company for analysis. Analysis of the rib revealed that it failed chordwise due to exfoliation corrosion midpoint between the rivet line and the nutplate line on the lower horizontal section. According to McDonnell Douglas personnel, the location of the corrosion could not be observed by a flightcrew when performing a walk around inspection but can be observed during the FAA approved 50-hour inspection. Review of the FAA approved 50-hour inspection checklist used for the inspection specifically states to inspect all interior structures for corrosion, damage condition and security. Additionally, the outer wing attach angles and bolts are required to be inspected.