 In 1991, to gain the ability to fly domestic cargo flights within Europe, UPS entered into a partnership with Danish airline Star Air (part of the Maersk conglomerate), leasing several 727 freighters to the airline (later replaced by 757 freighters).In the early 1990s, to add capacity to its network, UPS Airlines opened additional hubs, with primary hubs in Rockford, Illinois (Chicago) and Philadelphia; secondary hubs were opened in Dallas/Fort Worth; Columbia, South Carolina; and Ontario, California (Los Angeles).With most of its aircraft flying primarily on weeknights, the airline was eager to find other ways to produce income from its fleet.