On five busy routes, average travel times in 2011 were shorter than they were five years ago, according to DOT stats. Getty Images

After baking extra minutes into flight schedules to boost "on time" performance, airlines are back in the kitchen, taking minutes out. This new recipe reduces the time passengers spend on planes and offers more reliable departure and arrival times.

American, Alaska, United and Southwest airlines have all reduced scheduled time for trips in at least 16 of the past 24 months, according to the Official Airline Guide data compiled by American. Together, the nine biggest airlines took an average of one minute per flight out of their schedules last year, according to American's schedule analysis. That may not sound like a lot, but minutes add up to days across thousands of flights. At just American, a one-minute change in schedules represents a total of 24.2 flight hours across its fleet each day.

Did Los Angeles move farther away from New York? Scott McCartney on The News Hub looks at how airlines are trying to make their schedules more realistic, and why some flights still take a lot longer now than 10 years ago. Photo: AP.

The actual time it takes to get from gate to gate is tracked by the Department of Transportation's Bureau of Transportation Statistics. Comparing five busy routes, average travel times in 2011 were shorter for all five routes compared with 2009, and shorter than five years ago, reflecting airline efforts to be more efficient as well as less congestion in the air-traffic system.

Airlines began aggressively padding schedules in 2008 and 2009 so that even late flights technically arrive on time. This can be frustrating to travelers: Fluctuating fight times mean planes often arrive well before gates are available, so passengers are left waiting on board.

The padded approach proved so inefficient and inaccurate, airlines felt the need to tighten schedules. They're doing this in different ways. Delta Air Lines, DAL +5.36% Delta Air Lines Inc. U.S.: NYSE $33.62 +1.71 +5.36% April 16, 2014 4:00 pm Volume (Delayed 15m) : 14.10M AFTER HOURS $33.51 -0.11 -0.33% April 16, 2014 7:51 pm Volume (Delayed 15m): 67,098 P/E Ratio 2.70 Market Cap $27.10 Billion Dividend Yield 0.71% Rev. per Employee $484,059 04/10/14 Italian Court Rules Against Em... 04/07/14 Why Small Airports Are in Big ... 04/07/14 Etihad Airways Seeks Deeper Eu... More quote details and news » for example, had the same flight schedules Monday through Friday. Now, it has shaved time off flights on Tuesdays, Wednesdays and Saturdays—days when there are fewer planes in the sky and thus fewer delays.

On Mondays, Delta operates 160 more flights than it does on Tuesdays. So on Mondays, Flight 812 from Atlanta to Chicago is scheduled for 2 hours, 10 minutes. On Tuesdays, Flight 812 is scheduled for 2 hours, 3 minutes.

"We've been tactically removing [time] to get us down to realistic flight times over the past couple of years," said David Holtz, Delta's vice president of operations control. "We work hard to make sure we're not over-padding."

US Airways Group Inc. has reduced its "block times"—the time it takes to get from gate to gate—with a major push to have planes depart precisely at the scheduled time, or earlier. Once a plane leaves late for one flight, it often runs late for the rest of its flights that day. And delays can grow exponentially—a flight off the gate late may find a long line of planes waiting to take off, or may find that its gate is no longer available at its destination. So it waits even longer.

"You can do all sorts of things to make up for poor performance," said Robert Isom, chief operating officer at US Airways, referring to padded schedules. "But you sacrifice efficiency, the passenger experience, the employee experience and profits."

American says it trimmed a minute or two off flight schedules in Dallas-Fort Worth, its largest hub, simply by making sure it had enough gates for arriving flights—something that had been a chronic problem.

"Even if you arrive on time, the goodwill is blown, and people think we are idiots," said Jonathan Snook, American's vice president of operations planning and performance.

American also sliced minutes off flights headed to spoke cities, then added minutes to flights headed into hubs. That way fewer planes sit at the gate waiting for connecting passengers.

What's unclear is whether the shorter times will stick. In the past couple of months, several carriers have added minutes back into schedules, and overall, schedules through June this year are back up an average 0.6 minute per flight. Airlines say some adjustments were made based on bad weather in early 2011.

Even with the new schedules, some airlines perform better than others on the same route. Both US Airways and United have flights from Newark to Charlotte leaving at about 6:30 p.m. US Airways schedules its flight for 2 hours, 2 minutes, while United's is 2 hours, 11 minutes. Despite the extra nine minutes, the United flight still runs late more than the US Airways flight. United's flight is on time only 45% of the time, according to FlightStats.com, with an average delay of 34 minutes. US Airways arrives within 14 minutes of its scheduled arrival—the DOT definition of on time—84% of the time.

United says it has added minutes to flights in the New York area, including Newark, because of congestion-related delays both in the air and on the ground. In addition, United cut flight times in 2011 after a big run-up in 2008 and 2009, but added time to former Continental Airlines flights. "Continental was relatively lower [in block time], and United was relatively higher,'' United spokesman Rahsaan Johnson said of the now-merged airlines. "For the most part, we are meeting in the middle." Together, the changes result in a net reduction, he said.

Of course, schedule padding hasn't come to an end. From Philadelphia to Dallas, both American and US Airways fly five flights a day. But American schedules its trips, on average, at seven minutes longer than US Airways.

Also, airline bankruptcy reorganizations have facilitated padding, since pilot pay is less tied to scheduled flight times. American is asking for similar changes in its ongoing bankruptcy reorganization.

Some routes do take a lot longer than they did 10 and 20 years ago. In both 1990 and 1995, the average trip from New York's Kennedy Airport to Los Angeles International was 352 minutes—5 hours, 52 minutes. Last year, the average was 14 minutes longer.

Airlines say on long routes, they've slowed down planes to conserve expensive fuel. "We'll speed up when we have to, and we'll slow down when we can, too," said Delta's Mr. Holtz.

Write to Scott McCartney at middleseat@wsj.com