You’re paying to check your belongings, so why should other people’s excess baggage get a free ride?

That’s the question being asked by a growing number of travelers. As airlines look for new ways to boost revenue, fees for checked bags are on the rise; so is scrutiny of overweight customers whose baggage is built in.

It’s a touchy subject, Airfarewatchdog.com has found, and one that airlines have been happy to avoid discussing, where possible. As late as 2008, United Airlines wouldn’t even address the matter with us.

But an outcry among passengers, tired of their seatmates taking up more than their fair share of jealously-guarded seat space, is said to have played a role in the airlines’ new rules for transporting “customers of size.” Where a terse “we have no policy” was once the standard response, United adopted new regulations in 2009. Customers who were unable to confine themselves to one seat would be required to buy a second, should the crew be unable to reseat them.

It’s a policy that’s becoming increasingly commonplace.

To many, the idea seems simple enough – if you can’t fit into one seat, you should probably consider buying two.

It’s not simple at all. Canada’s government takes a dim view of the matter. In late 2008, the country’s Supreme Court upheld a ruling that prohibited airlines from charging the disabled or “obese” for a second seat, affecting Canadian flyers Air Canada and WestJet.  

Here in the United States, some airlines with upfront policies have spent their fair share of time in the courts. Southwest has long been famously transparent about its second seat rule, the one that United and many other airlines have emulated. The company has been sued more than once by disgruntled passengers.

“On the lawsuits, all have ruled on the side of Southwest,” spokesperson Whitney Eichinger points out.

Southwest’s policy is that those who cannot fit in one seat must buy two.  

“If the flight goes out with empty seats, Southwest will refund the cost of the additional seat,” Eichinger said.

Other airlines have had their share of legal trouble in this area.  

In the past, Air France warned passengers with what they referred to as “high body mass” not to expect to be seated if they have not purchased an extra seat. This is a warning that many airlines, even those who officially have tried to downplay any official policy, have long given to travelers.

Some travelers, however, don’t see the need. That, or the airline and the passenger disagree over what constitutes “need.” An Air France passenger traveling from New Delhi to Paris in 2006 sitting in a single seat was stopped by employees, who wrapped packing tape around him in public to prove that he was too fat. Citing humiliation, he sued, and won.

At the time, the airline had a program in place that offered passengers a second seat at a 25 percent discount, tax-free. It was a move that the airline had hoped would encourage customers to make arrangements in advance.

Recently, Air France made an update to the policy, bringing it more in line with Southwest’s policy, which has been around for decades. According to Air France spokesperson Karen Gillo, the second seat purchase is still optional. Now, however, the cost will be reimbursed if the flight is not fully booked.  

“It’s a way to encourage individuals to pre-plan to ensure their own comfort and safety; it allows them to travel with less stress,” she said.

Gillo stated that “for the mass majority of the cases, the flights aren’t fully booked” and passengers will be reimbursed.

Air France isn’t the only one making tweaks these days. JetBlue spokesman Mateo Lleras said the airline is currently working to refine its policy. And Spirit Airlines requires “customers of size” to buy two seats, period.

Currently, Lleras said, the airline does its best to accommodate customers free of charge. It will charge if it has to, but says that it approaches the matter on a “case by case basis.”

“We understand this is a sensitive issue,” he said. “Every time we can accommodate a customer we will.”