On September 29, 2008, about 0853 Pacific daylight time, a Piper PA-31-350, N27996, landed hard at Fresno, California. Ameriflight, LLC, was operating the airplane as flight AMF 238 under the provisions of 14 Code of Federal Regulations (CFR) Part 135. The airline transport pilot was not injured; the airplane sustained substantial damage to the wings. Visual meteorological conditions prevailed, and an instrument flight rules flight plan had been filed. The cross-country flight originated from Oakland, California, about 0758. The pilot reported a firm landing and a blown right main tire, but indicated no other problems. Local contract maintenance replaced both tires. The airplane then continued in service for two flight sequences, flown by other pilots. Inspection by Ameriflight maintenance personnel in Oakland revealed a line of popped rivet heads on the upper surface of the left wing above the rear spar between the nacelle and the wing fillet fairing. A close inspection discovered popped rivet heads on the left side of the fuselage below the cabin window with a gap visible in the skin lap. Maintenance personnel conducted a detailed hard landing inspection with no further damage discovered. However, they reported that inspection of both main gear wheels and tires revealed cuts and impressions in the tire sidewalls indicative of a very hard landing. They replaced the damaged rivets, and repositioned the airplane, on a ferry permit, to the company's headquarters and major maintenance facility at Burbank, California, for further inspection. The senior training captain who ferried the airplane reported that performance and handling qualities were completely normal. At Burbank, maintenance personnel removed wing skins from the left nacelle and the leading edge of the left wing. They discovered buckling of both main spar webs between the fuselage and nacelles.