CHICAGO—In the year since United Airlines and Continental Airlines merged, the economy has weakened and the price of aviation fuel has soared. But business for United Continental Holdings Inc. has been surprisingly rosy.

The Chicago-based airline, now the world’s largest by traffic, is on course to turn a $1.4 billion profit this year. It is sitting on an $8.4 billion cash pile, its unit revenue gains are leading the industry and it is moving briskly to repaint its fleet and rebrand its airport terminals. Earlier this month, Fitch Ratings raised United’s credit rating by a notch, citing significant debt reduction and cash-flow generation.

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