It’s incredibly easy to pile up airline miles. I think I just earned 400 miles for merely mentioning this fact. But as you may have already discovered, it’s not always so simple to actually redeem those miles. A new survey looked at dozens of domestic and international carriers to find which ones were more likely to have seats available for rewards travel.

The folks at IdeaWorks queried the availability of rewards seats on a range of dates from this coming June through October.

Southwest Airlines’ Rapid Rewards program was the best of any U.S. airline (and tied with Air Berlin’s Topbonus for best overall), with 100% of travel dates having flights with at least two rewards seats available.

The next-best U.S. carrier was AirTran, which is now owned by Southwest. IdeaWorks had a success rate of 87.1% when searching for rewards travel on the discount airline. This tied AirTran with United Airlines — the highest-rank major domestic carrier. Both AirTran and United saw remarkable upticks in rewards seat availability (40% and 15.7%, respectively) over last year’s survey.

These two were followed immediately by JetBlue, with an 86.4% availability rate. The nearest U.S. carrier on the list after that was American Airlines, with only a 45.7% rate — a drop of 17.2% from last year.

But even that wasn’t as sad as the numbers for U.S. Airways (33.6%) or the survey’s worst-ranked airline, Delta, which remained unchanged at 27.1%.

You can check out the entire report, which includes several international carriers, HERE [PDF].

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