As frequent fliers, Kellogg's Jan Van Mieghem and Yuval Salant have seen the same scene unfold numerous times: Their plane touches down and the pilot announces over the loudspeaker that the flight arrived ahead of schedule. Upon hearing about this stroke of good luck, the passengers immediately perk up.
"Everybody is smiling," says Van Mieghem.
But are these smiles due to more than good luck? Van Mieghem, the Stuart professor of managerial economics and operations, and Salant, an associate professor of managerial economics and decision sciences, began to wonder whether airlines were strategically adjusting their schedules to make early arrivals more likely.
In a new study with Assistant Professor Dennis Zhang of Washington University in St. Louis, the researchers analyze two decades of data on 43 million domestic U.S. flights. They discover that published flight times--the flight duration that consumers see when they shop for plane tickets--increased 8.1 percent between 1997 and 2017, amounting to an additional 341 million passenger hours.
But when the researchers break down what caused that increase, they find that planes are not flying slower than they used to.
Rather, nearly half of the additional time comes from airlines strategically padding their schedules. Late planes are bad for business--so, by adding time to their predicted flight lengths, airlines can increase the odds that their planes will arrive on time. This schedule padding is especially common on flight routes with less competition, the researchers find.
In Search of Lost Time
The team looked at historical data from the Bureau of Transportation Statistics containing detailed data on 43 million flights over the last 21 years. They analyzed changes in flight duration for the same route operated by the same carrier year over year.
On paper, it appeared that the same flights now take significantly longer than they did in the '90s.
The researchers knew that there were several possible explanations for this. Planes may be spending more time circling in the air waiting for an open landing strip, or more time on the ground waiting for a gate to become available. Another possibility is that air travel has become less predictable--if so, airlines could be adding more buffer time to guard against increasingly frequent delays from bad weather or other unplanned events.
A final possibility was "strategic padding": airlines might extend their scheduled flight times not for any logistical reason, but as a matter of sheer business strategy.
"When flights arrive on time or ahead of schedule, you get happy customers," says Van Mieghem.
However, Salant notes, "there are costs associated with that." For one, if an airline pads too much, a competitor can undercut them by offering a shorter flight. Furthermore, flight crew members are often paid for the full scheduled flight duration, regardless of how long the flight actually takes.
The researchers created a mathematical model to tease out how much each factor--air time, ground time, unpredictability, and strategic padding--contributed to the increase in published flight times.
They found that planes spend roughly the same amount of time in the air as they did 21 years ago. And unpredictability did not seem to be playing much of a role in the increase, either.
Ground time was a different story. "Passengers were spending more time on the plane waiting for take-off, or waiting for a gate after landing, possibly because of increased air traffic," Salant says.
But that increase in ground time explained about half of the increase in published flight times, leaving more than 150 million passenger hours unaccounted for. The researchers concluded that this remaining time was the result of strategic padding.
"We find that the missing time is not due to physical constraints like air time and ground time or variability. It's due to strategic decision-making by airlines. They're padding their schedules, and they're doing it for a reason."
-- Jan Van Mieghem
Less Competition Means Longer Flight Times
Why has strategic padding become more common? The researchers suspected that part of the reason is dwindling competition.
"If you have plenty of competitors, logic dictates that you will seek to offer customers the most efficient route from A to B. You'll cut back your scheduled flight time," Van Mieghem explains.
But due to bankruptcies and mergers, there are fewer large U.S. airlines today than in 1997. So airlines today may feel less competitive pressure to offer shorter flight times.
To test this theory, the researchers looked at how published flight times changed after competitors either started or stopped offering flights on a particular route.
As predicted, when competition grew stiffer, airlines cut down travel time. Conversely, Van Mieghem says, "as the playing field thins out, less competition makes it easier for airlines to do the opposite.
A Silver Lining to Schedule Padding?
Walk around O'Hare airport, for example, and you are likely to come across a billboard advertising the "reliability" of United Airlines' flights.
"But it's not really about reliability," Van Mieghem says. "If airlines really wanted to improve reliability, they'd focus their efforts on enhancing their operations. What our study shows is that this is not the case. What they're going for instead is the easier--and cheaper--option of schedule padding."
Nonetheless, the researchers stress that schedule padding has its benefits. After all, a flight that usually takes 90 minutes may indeed take 120 on an unlucky day. By publishing a time closer to two hours, the researchers explain, airlines are simply erring more on the side of caution.
In fact, schedule padding may actually help travel go more smoothly. Passengers are not only more likely to experience the joy of an on-time arrival, but also get more buffer time to make a connecting flight.
And the paper's revelations suggest that there is an opportunity for passengers to be strategic, too, says Van Mieghem.
"We tend to allow a little extra time for delays or hold-ups when we're travelling," he notes. But if consumers know that their flight is likely to get in on time, they might save time by reducing that buffer in their schedule. "Of course," he adds, "this is a question of our individual tolerance for risk."