United Continental Holdings

Boom times for big US airlines: $3.9 billion in 2Q profits

Compared with their checkered track record, major airlines are enjoying boom times.

Planes are full, and jet fuel is still much cheaper than it was last year. The four biggest U.S. carriers just reported a collective second-quarter profit of $3.9 billion.

And yet investors seem to be looking past the bottom line. They have become obsessed with fare prices — falling now for more than a year — that may foreshadow thinner profits in the future.

When the airlines held calls this month to discuss their second-quarter operations, investors pressed them to reduce flights and the supply of seats in order to drive up fares. They cheered when Delta Air Lines Inc....