Airline reward programs are the only sure way to get flights for free. True, you might win a flight in a lottery, get a ticket gifted to you or even get your boss to foot the bill for your vacation flight, but how many people do you know who get that lucky?

On the other hand, anyone can join an airline rewards program and earn miles every time they fly. Airlines have even expanded the ways you can earn miles by offering miles when you spend with an airline credit card or shop at certain merchants.

But once you have those miles, how easy is it to get on the flight you want to using your miles?

The 2016 Switchfly Reward Seat Availability Survey was just released, ranking 25 airlines from around the world in terms of their overall award flight seat availability. After all, what good is an airline rewards program if you can't get a seat when you want one? The bad news for the domestic airline industry is that the best programs are offered predominantly by foreign carriers.

2 bright shining exceptions are Southwest Airlines, which tied for 1st overall with Air Berlin, thanks to both carriers' availability score of 100 percent, and JetBlue, which tied Lufthansa for 3rd place overall. The seat availability statistics cover the period of June through October 2016.

The survey found a general improvement in the overall availability of seats for members of "basic" (non-elite) frequent flyer programs. This year, 77 percent of all booking inquiries yielded 2 available seats, up from 74 percent in 2015 and 72 percent in 2014.

Here's how the 6 U.S. carriers' programs fared in the survey, in descending order of reward seat availability:

1. Southwest Rapid Rewards: Low-cost airline Southwest took a top spot in the survey. Its rewards program tied for the top overall rank among all 25 carriers thanks to 100 percent seat availability! Southwest does well in surveys because passengers rack up points based on fares paid rather than miles traveled. Passengers can pay for seats with points or cash. In 2015, award travel made up 12 percent of Southwest's total. The airline's reward payback (the reward value returned for every dollar spent on base fares) was 7.3 percent.

2. JetBlue TrueBlue: This value carrier tied for the 3rd spot. Like Southwest, it rewards passengers based on fares rather than miles. JetBlue had a seat booking availability of 93 percent in the survey, up 5.7 percentage points from last year. The airline had the highest reward payback among North American carriers, at 7.9 percent. As a TrueBlue member you earn points when you fly on JetBlue, shop through ShopBlue, or use JetBlue Plus Card.

3. Alaska Airlines Mileage Plan: Alaska Airlines did not perform great, but still beat larger US airlines. The airline's rewards program ranked 14th in the survey, with a seat availability of 72.9 percent, a 7.1 percentage point drop from 2015. It had the second highest reward payback, 7.8 percent. Alaska has an online mall (Mileage Plan™ shopping) to help you earn extra miles, and also issues a credit card.

4. United MileagePlus: United earned the 15th spot with 72.1 percent availability, down 2.9 percentage points from last year. United had a mediocre reward payback of 5.0 percent. The airline currently awards 5 miles for every dollar spent in the non-elite program.

5. Delta SkyMiles: Right behind United, Delta occupied 16th place, with an availability rate of 68.6 percent. That may be low, but it's up a handsome 10.7 percentage points from 2015. Delta has made seats available earlier than before, and booked more than 10 percent of sales on award tickets. Travelers can even buy drinks with miles at Delta's airport clubs. Sadly, Delta's reward payback was only 3.8 percent.

6. American Airlines AAdvantage: Pulling up the rear at the 21st spot out of 25 major airlines, American's 56.4 percent availability is down 10.7 percentage points since last year. American plans to reward flyers with miles based on fares rather than travel distances starting later this year. Awards accounted for 6.5 percent of all passenger miles in 2015 at American, which is low but still a 10 percent increase from the year earlier.