Here is a piece from USA Today recently that discusses the padding by airlines of their flight times over a 20 year span.  One way to hide the fact that planes are often late is to manipulate the schedule so that the expectations are lowered.

For airline executives facing chronic delays on certain routes, the answer has been to pad those flights’ “block times” by lengthening the total number of minutes the aircraft is expected to operate, all the way from the gate at Airport A to the gate at Airport B.

As one airline operations manager said to me a few years ago: “The airlines are no more on-time than they used to be, but they’re better at covering it up.”

Think flight times are being padded? They are.  Bill McGee.  USA Today.  June 29, 2009.

A similar Wall Street Journal article from a couple years ago places the blame on increased congestion, which I’m sure is somewhat to blame, but increasing the time allotted for each flight gives the airline flexibility and allows it to be “on-time” when it otherwise wouldn’t be.  When the airlines themselves get to set the expectation, it becomes easier to meet (surprising, then, how many late flights there still are).

A check of two dozen flights from June airline schedules found that “block times” — the time airlines allot in their schedules for the trip — are about 10% higher than they were in June 1997. That kind of slowdown makes trips less productive for travelers with more time spent sitting and waiting. It can also frustrate travelers who arrive “early” on days when there aren’t slowdowns, only to wait for a gate to open at the scheduled arrival time.

The Middle Seat: Why Flights Are Getting Longer; Planes Are Faster and Navigation Better, but Airlines Are Padding Schedules Even More as Congestion Worsens. Scott McCartney. Wall Street Journal. (Eastern edition). New York, N.Y.: May 29, 2007. pg. D.1