United took delivery of its first DC-10 in August 1971, and by the late 1980s had 55 of the massive, widebody aircraft. At various times in United's history, the DC-10 flew on the company's U.S. transcontinental routes, between its U.S. hubs and between the U.S. mainland and Hawaii. In 1997, United used four DC-10-30F aircraft to launch international freighter service. By mid-2000, United's DC-10 fleet had reached an average age of 20 years and was experiencing frequent minor maintenance problems. United decided that retiring the fleet would improve the company's overall reliability. The last passenger aircraft flight was Feb. 15, 2001, and the last freighter flight was Dec. 24, 2000.