The daily gain, which made United the top performer in the S&P 500, came after the airline reported a better-than-expected quarterly profit and said a glut of cheap flights had subsided. United also said it would buy back $1.5 billion in shares. Delta Air Lines, which issued its quarterly report last week, and American Airlines, due to report next week, joined the rally, climbing 6.8% and 7.1% respectively.