Travelers planning to redeem airline reward miles when booking summer vacations might find it a bit easier to do this year, a study of 24 frequent-flier programs suggests.

Availability of rewards seats increased slightly overall, with 68.6 percent of queries resulting in available seats, according to a survey by consulting firm IdeaWorks. That's up from 66.1 percent last year.

Southwest Airlines came out on top for U.S. carriers, second overall, with award seats available for 99.3 percent of queries made.

US Airways, which has its biggest hub at Charlotte/Douglas International Airport, and Delta rounded out the bottom of the list. They held a 25.7 percent and 27.1 percent success rate, respectively. Both airlines improved since last year's study, however, with success of finding availability of award seats on those airlines more than doubling.

The study ranked airlines based on the availability of standard award seats in 20 of each airline's busiest markets. IdeaWorks made 6,720 queries on airline websites during March and early April of this year, asking for two award tickets at standard levels for travel dates spanning June through October.

Many airlines have raised prices this year to deal with rising fuel costs and other expenses. Baggage and other passenger fees also have increased.

Availability for awards seats to popular destinations can be difficult to come by, with added pressure as summer travel plans are booked earlier.

"If you're a dad looking to book summer travel for your family, it's still a difficult environment," IdeaWorks president Jay Sorensen said. "But I think frequent-flier programs offer tremendous value for those not looking to use it for family summer vacations."

US Airways representatives had not seen the study when interviewed but said looking at only the top markets for each airline might not accurately reflect the markets where customers redeem the most miles. Award seat availability changes regularly based on the season or particular events going on at popular destinations.

"If you look at different top markets, it's not necessarily the top markets for redemption, said Fernand Fernandez, managing director of customer loyalty and marketing programs for US Airways. "That's where you begin to see the disparity between carriers." Most reward miles for the carrier are redeemed in leisure markets such as Florida or Hawaii, he said.

The fact that some airlines offer customers the option to redeem miles at partner airlines could also determine availability of awards seats, Fernandez said. US Airways rewards customers have the opportunity to use miles with other Star Alliance airlines.

Randy Petersen, editor of InsideFlyer Magazine, said it's easier to find flights at off-peak times or to less popular destinations. The best time to use miles is Tuesday, Wednesday or Thursday, he said. Many travel during the weekends due to job schedules, so demand increases during these times.

He also suggests booking about six months in advance. By planning in advance, travelers also have the flexibility to change plans.