By now, most of us are used to having Internet access wherever we go, but a majority of flights by the major U.S. airlines still don’t offer in-flight WiFi access to passengers. And your likelihood of finding WiFi on a plane all depends on which airline you’re flying on that day.

The folks at RouteHappy.com have put together a survey [PDF] of how widely each of the major domestic carriers have rolled out WiFi access.

Virgin America is the only airline in the survey offering WiFi on 100% of its flights. Of course, Virgin also has the fewest number of flights (173) in the survey, so it didn’t have to invest anywhere near as much time and money to reach full saturation. AirTran also offers WiFi on all of its planes, but that brand is set to be absorbed into the brand of parent company Southwest, which offers WiFi on about 74% of flights.

Of the largest carriers, Southwest has the highest percentage of flights with WiFi, with plans to roll out access to its entire fleet.

Delta has the most total flights with WiFi (3,443), but it is also the nation’s second-largest carrier, so that only represents about 65% of all its flights.

Meanwhile, the country’s biggest airline, United, is lagging far behind its competitors, with only 24 domestic flights offering WiFi access. It has plans to roll out WiFi for another 494 flights, but that still leaves 4,336 flights with no connectivity for passengers.

When American merges with U.S. Airways, it will add 1,293 WiFi-enabled flights, which is more than double what AA currently offers. But the combined airlines will still only have WiFi on around 1/3 of its flights.

Even when you have WiFi access, your laptop or tablet might not have enough battery power to last the entire flight without being plugged in. But only four carriers — Virgin, Delta, American, United — have flights that offer both WiFi and in-seat outlets. And not all flights on these carriers have outlets. For example, only 560 of Delta’s WiFi-enable flights allow the passenger to charge up in his seat, while 515 of American’s 541 WiFi-enabled flights offer outlets.

Availability of WiFi is also related to which cities you’re flying between. There are 31 non-stop flights between Los Angeles and San Francisco with WiFi access, the most of any two-city pairing in the survey, but that is still only around 66% of the daily flights on that route. Because of Delta’s large number of WiFi-enabled planes and its use of Atlanta as a hub, many flights in and out of Hartsfield International offer web connectivity to customers.

What the survey doesn’t say is that you can’t yet bank on actually having WiFi access even if you know for sure that your flight is WiFi-enabled. Sometimes the service fails, it’s often glitchy, and a handful of streaming video hogs could bog down the network for everyone else on the plane.

It almost always costs extra for in-flight WiFi access, so you may want to weigh the additional charge against what you’ll save by just reading a book. And services like iTunes and Amazon often allow users to download rented movies for offline viewing so that you can pass the time without having to worry about whether or not you’ll be able to pay for hiccup-y, expensive WiFi access.

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