The demand by travelers looking for WiFi on planes is reaching new heights, but still a majority of flights by the major U.S. airlines still don’t offer the in-flight service.

Now, a new report from fare search website Routehappy reveals just how available it is on airlines.

According to the website's survey [PDF], flyers can expect to find WiFi on 38 percent of domestic flights overall. 

Virgin America and AirTran Airways have WiFi on 100 percent of their flights. But it should be noted that Virgin also has the fewest number of flights (173) in the survey, and AirTran (494) too has relatively fewer planes to outfit than the bigger carriers. 

Delta has the most total flights with WiFi (3,443), representing about 65 percent of all its flights. Southwest (2,320) and US Airways (1,293) come in second and third, respectively.  United had only 24 domestic flights offering WiFi access, although it has plans to roll out more WiFi-enabled flights.

More On This...

If you're looking to plug in while getting on the Internet, it may be even harder. Only four carriers — Virgin, Delta, American, United — have flights that offer both WiFi and in-seat outlets. But you'll have to check which flights actually have the outlets, because it's not available on all flights. 

The top three routes with the most WiFi-connected flights are Los Angeles-San Francisco, L.A.-New York Kennedy and Atlanta-Orlando. For example, there are 31 non-stop flights between Los Angeles and San Francisco with WiFi access, but that is still only around 66 percent of the daily flights on that route.

If you're flying overseas, WiFi becomes harder to get.  Only 279, or 6.5 percent, of flights from the U.S. have full-flight connectivity, but more airlines are adding the service to their international fleets.

Routehappy's survey is based on flights of at least 500 miles, offering at least an hour of online time.