NEW YORK—Domestic airlines improved their on-time arrival rates in May, although more than one in five flights still failed to get passengers to their destination as scheduled, according to government data released Monday.

A total of 21 percent of commercial flights in the U.S. arrived at least 15 minutes late, were canceled or diverted in May, according to the Transportation Department’s Bureau of Transportation Statistics.

That is down from more than 22 percent of late flights in the same month last year and in April 2008. The previous month’s figure was higher in part because AMR Corp.’s American Airlines, the nation’s largest carrier, was forced to ground thousands of flights amid tighter government scrutiny of maintenance issues.

For the third month in a row, American ranked last in on-time service. Passengers on just over two-thirds of the Fort Worth, Texas-based carrier’s flights — 67.3 percent — got to their destinations on time in May.

“Unfortunately, the main reasons include weather and significant (air-traffic control) delays at three of our main network operating areas” in Chicago, New York, and Dallas-Fort Worth, as well as in Miami, American spokesman Tim Smith said in an e-mailed statement.

UAL Corp.’s United Airlines, the second-largest carrier, which reported 72.4 percent of arrivals were on time, and Continental Airlines Inc., with 75.4 percent, rounded out the bottom of the on-time list.

Hawaiian Airlines was able to stick closest to its timetable, delivering passengers as scheduled 88.9 percent of the time. The carrier, a division of Hawaiian Holdings Inc., avoids many of the nation’s most delay-prone airports, however, reporting flights to just 14 destinations.

The New York area continues to account for some of the worst delays. New York’s LaGuardia and Newark Liberty International in New Jersey swapped places to rank second to last and last among major airports.

Fewer than two-thirds of passengers at either airport arrived on time in May.

Weather was by far the biggest cause of late flights nationwide in May, accounting for just over 44 percent of all delays. Only about 39 percent of flights were late because of weather a year earlier.

U.S. carriers also improved their baggage handling in May. About 4.6 passengers out of every 1,000 reported a mishandled bag during the month, compared with nearly six per 1,000 a year earlier and five per 1,000 in April.

Reported passenger complaints fell to 885, compared with 930 a year earlier and 1,113 in April.

The U.S. airline industry is straining under record-high fuel prices, which have prompted many carriers to raise fares, announce sweeping flight cutbacks and begin charging for amenities such as checked bags and extra legroom.

Oil prices fell sharply Monday but nonetheless remain within range of last week’s all-time highs.

Light, sweet crude for August delivery fell $3.92, or about 2.7 percent, to settle at $141.37 on the New York Mercantile Exchange.