Airlines typically claim that air traffic delays are due to such adverse factors as weather or airport congestion. Although such factors are predictable on average, airlines often fail to account for them in setting schedules. Using data on nearly 67 million flights between 1988 and 2000, we show that airlines schedule flight times well below expected travel time. Although much of the variation in travel time across seasons or years is due to deviations in average push-back delays—aircraft leaving their gates late—airlines do not account for push-back delays in setting their schedules. Instead an airline's scheduled travel time is almost exactly equal to the median time from pushback from the gate on departure to pulling up to the destination gate. If there is volatility in expected travel time, an airline typically schedules less time, not more. Similarly, airlines do not schedule longer layovers when the inbound flight is more likely to arrive late. Put together, our evidence suggests that airlines choose their schedules based on the performance of a flight on very good days, even if, on average, such good days are relatively rare. We find that the likeliest explanation is that airlines minimize labor costs at their passengers' expense, although there is also some support in favor of airlines trying to maintain greater aircraft utilization.