There was a discussion of this in today's United system outage thread, but I wanted to start a new thread to avoid getting off topic there.

People were asking if it's possible for airlines to make up for lost time. The answer is yes, but they won't always do it.

Airlines usually run the planes below maximum speed to save on fuel costs. This is the same principle as in your car. Safety issues aside - suppose your car has a maximum speed of 120mph. You could drive 120 mph to get to your destination as fast as possible, but driving so fast causes more air resistance, and is below the maximum efficiency of your engine. If you drive at 60mph though, your engine is running more efficiently and you have less air resistance, giving more fuel economy. Same principle applies for planes.

However, if a flight is delayed in departure, the airline will sometimes fly the plane faster to get it to the destination on time.

When do they do this? Fortunately, we have a lot of data to look at this, and Nate Sliver at 538 has done just such an analysis here:

http://fivethirtyeight.com/features/...me-in-the-air/

Airlines try to get the plane to arrive close to the scheduled arrival time if possible, since on time arrivals are tracked and are an important performance metric. They also usually include a bit of extra time in the schedule for this event. So for a 6 hour flight delays of <30 minutes the flight runs as normal. For delays between 30 and 60 minutes, the plane runs faster than usual to make up the time. For delays more than 60 minutes, the are not going to make an on time arrival anyway, so they go back to running the planes at the slower, more efficient speeds.