On December 10, 2010, at 1558 Pacific standard time, a Piper PA-28-140, N5800U, sustained substantial damage to the left wing during taxi after landing at Chino Airport, Chino, California. DuBois Aviation operated the airplane under the provisions of Title 14 Code of Federal Regulations Part 91 as an instructional flight. The student pilot and certificated flight instructor (CFI) were not injured. The local instructional flight departed Chino about 1430. Visual meteorological conditions prevailed, and no flight plan had been filed. The CFI reported that after performing a soft-field landing, the airplane was not taxing as expected. As they exited the runway, the airplane dropped onto its left wing, and spun to the left. Examination of the left main landing gear strut (part number 65319-3) by the NTSB investigator-in-charge revealed that the upper torque-link attachment lugs had detached from the landing gear strut. Additionally, the wheel assembly had become separated from the strut, coming to rest against the aft underside surface of the wing. The wing sustained wrinkles to the upper and lower inboard skins, and three collapsed wing ribs were observed. Both the CFI and the student pilot reported performing an uneventful preflight inspection prior to the accident flight, noting no preexisting structural damage to the airplane. The landing gear strut and lugs were sent to the NTSB Materials Laboratory for further examination. Metallurgical examination revealed that both lug separation surfaces exhibited curving boundary features, and areas of discoloration indicative of preexisting cracks. New Piper Aircraft, Inc., issued Service Bulletin (SB) 1131 on August 18, 2003, requiring visual and dye penetrant inspections of "cast" main landing gear strut cylinders at the next regularly scheduled maintenance event (or within 100 hours), and at 100-hour intervals thereafter until the cylinder was replaced with a new forged part. The subject cylinder was of the "cast" type, with a serial number that was found to be within the SB's affected range. Examination of the airplane's maintenance logbooks revealed that this Service Bulletin had not been performed.