The top dream in frequent-flier awards has become an illusion at U.S. airlines.

Landing a free first-class or business-class ticket on a long flight is the ideal reward for many travelers. They might spend years saving up enough miles for that special seat. But new data show that while some international airlines have generous availability of long-haul business-class award seats, U.S. airlines, particularly United, make it nearly impossible to find a seat at advertised lower mileage levels.

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