DALLAS - For years, Southwest was the most punctual of big U.S. airlines, so its tumble toward the bottom of government rankings for on-time arrivals was stunning.

Southwest officials needed to fix an ill-fated decision to squeeze more flights into the schedule. This summer, they backed off by allowing more time between flights. And they told employees that the first flight of the day on every route had to leave on time.

"If you're late out of the driveway in the morning, you're probably going to be a little bit late to work," said Steve Hozdulick, Southwest's senior director of operational performance. "You're going to hit the two traffic lights that you never hit."

In the airline world, delays build as the day continues. This summer, for example, airlines were on-time around 85 percent or better until midmorning. By midafternoon, the rate dropped into the low 70s, then plunged into the 60s by dinner time.

Delays are costly for airlines and their passengers. A 2010 study commissioned by the Federal Aviation Administration estimated that flight delays cost the airline industry $8 billion a year, much of it due to increased spending on crews, fuel and maintenance. Delays cost passengers even more - nearly $17 billion.

In the first nine months of the year, more than 1 million U.S. airline flights arrived late - about one in five. Tardiness creates other problems including missed connections, lost bags and short tempers among frustrated travelers.

On a freezing morning recently at Dallas Love Field, Southwest supervisors arrived at 4 a.m., two hours before the first flights. They'll assign two or more bag handlers to each flight.

The last bag should be on the plane and the bin doors closed five minutes before scheduled departure, said Dave Obeso, a Southwest ramp supervisor.

The pilots for Flight 454 to Phoenix inspected their Boeing 737 in the dark. Workers called "ops agents" calculated load weight and balance and completed paperwork. A fueler gassed up the jet. Inside the terminal, agents at Gate 12 started boarding the 136 passengers - they're supposed to close the aircraft door five minutes before scheduled departure.

And then, a snag. A broken communications radio. A replacement was ordered and installed, a tug pushed the plane back from the gate, and the pilots taxied into position for takeoff. But the damage had been done.

Flight 454 left 29 minutes behind schedule and arrived in Phoenix 34 minutes late. Southwest's control center relayed word of the delay to Phoenix employees, who "turned" the aircraft faster than normal before its next flight. Still, the plane remained nine to 28 minutes behind schedule for the remaining four flights of the day, according to the tracking service FlightAware.com.

As recently as 2009, Southwest led the big airlines in on-time arrivals, which the government defines as within 14 minutes of schedule. Southwest tumbled to last among the largest five carriers in 2013 and much of 2014 but topped American and United in October, according to figures released Wednesday.

Delta went from worst to first among the biggest airlines in 2011 and has stayed there ever since. Dave Holtz, senior vice president of operations, credited schedule changes, monthly bonuses of as much as $100 per employee for hitting on-time and other goals, and the rote, repeatable task of sticking to a minute-by-minute pre-departure checklist. Delays early in the day are particularly problematic.

"The key is to not let yourself fall behind," Holtz says. "Keep this thing churning on time for as long as possible because there's a deteriorating effect on your entire system once it starts to go."

Many travelers assume that airlines have plenty of spare planes. But Southwest uses about 600 planes on an average day and keeps just a dozen or so in reserve at big airports.

Airlines can boost their on-time grade by loosening or padding the schedule - allowing more time between takeoff and arrival. Southwest has done that, especially at bigger airports.

But padding has drawbacks. Planes don't fly as much, so they make less money. It drives up labor costs because crews are paid based on scheduled flight times. And it creates the inverse problem - flights that arrive so early that there's no gate available, forcing passengers to stay on the plane after they land.

Travelers may sympathize with airlines when the delay is caused by bad weather. They are less forgiving when they think the airline could have done better.

Paul Jarley, dean of the business school at the University of Central Florida, said his plane sat at the gate in San Diego because of bad weather in Las Vegas. What bothered him, however, was a second delay to replace a bad tire on the landing gear.

"The obvious question was, if the plane was sitting there for 90 minutes, why didn't anybody see the tire?" Jarley said.

Jarley, a frequent business flier who missed an appointment because of the delay, said he doesn't look at airlines' on-time ratings when booking a trip, "but if it happened to me a couple of times, that would make me change carriers."

Michael Baiada, a recently retired United captain whose company, ATH Group, calculates arrival times for Delta, thinks airlines could eliminate most delays themselves if they had more consistent procedures, left early if they knew there were headwinds or other delaying factors looming, and worked harder to catch up when they fall behind schedule.

"Airlines are comfortable with their product being late 30 to 40 percent of the time," Baiada says, using the strict definition of being even one minute late, not the 15-minute cushion. "Would you buy a cellphone that works 60 to 70 percent of the time?"