American Airlines changed the terms on its tickets last month to say that its “sole obligation” for flight cancellations or missed connections is to refund the remaining portion of your ticket.

That sounded like the airline was going to leave a whole lot of people stranded.

So American changed its contract of carriage again on Sept. 24 to...

American Airlines changed the terms on its tickets last month to say that its “sole obligation” for flight cancellations or missed connections is to refund the remaining portion of your ticket.

That sounded like the airline was going to leave a whole lot of people stranded.

So American changed its contract of carriage again on Sept. 24 to add that a refund remains its sole obligation “subject to our policy for rebooking your delayed / canceled flight.” In other words, the airline still has to get you to your destination, even though it implies it can just hand you your money back.

Airlines change the rules on their tickets all the time, with customers or regulators often hardly noticing. Frontier Airlines says it has revised its contract of carriage 77 times since 2002. JetBlue says it revises its rules several times a year to reflect what it calls its “evolving operational and commercial needs.”

In September, American, Delta, JetBlue and Alaska all revised their contracts of carriage, which are voluminous. JetBlue’s contract of carriage runs 59 pages; Alaska’s is 58 pages; United and Southwest both take 47 pages to print. Each of the eight largest U.S. airlines have all revised terms since April. (You can often find links to these contracts at the bottom of airline home pages.)

Some are simple changes, like inflation adjustments to the limits on payments for bumping passengers or losing luggage. Others are more significant and erode customer rights. Many travelers still believe airlines will book you on a competitor if they have to cancel a flight. That was written out of the rules for most airlines years ago.

In legal terms, it’s a contract of adhesion—take it or leave it. Many states have laws or court precedents requiring some fairness for buyers in contracts of adhesion. But airlines aren’t subject to state consumer law.

A federal statute says the Transportation Department is supposed to police “unfair or deceptive” practices by airlines. Consumer groups have been pressing DOT for more action on contracts of carriage.

“They don’t really supervise much on these contracts,” says Paul Hudson, president of FlyersRights.org. “Anything goes. You get this constant revision to try and make them more and more in favor of airlines.”

He points to force majeure provisions that typically absolve airlines of responsibility for canceling flights after terrorist attacks or major storms. The contracts don’t relieve passengers of responsibility, so in some cases passengers don’t get a refund, but a voucher or credit and the airline keeps the cash. During the pandemic, airlines hung on to tens of billions of dollars from customers who couldn’t travel because of border closings or health concerns, even when flights were canceled.

“It should be evenhanded. If the contract in those situations is void, and you didn’t get the service you paid for, you should get your money back,” Mr. Hudson says.

Frontier Airlines has revised its contract of carriage—the fine print you agree to when buying a ticket—77 times since 2002.

Photo: Steve Pfost/Newsday/Getty Images

DOT rules do say that the rules in place when you buy a ticket have to apply to that ticket. Airlines can’t retroactively change the game on you.

A DOT spokesperson said in a written statement that the agency “is committed to ensuring that airline passengers are treated fairly” and that it does review airline contracts of carriage “as appropriate.” The DOT said that most recently it investigated pandemic refund policies at Air Canada and United and “directed these airlines to revise their contract of carriage to make clear that they will provide refunds to passengers if the airline cancels or makes a significant schedule to a flight.”

Airlines say many of their recent revisions have been attempts to add transparency about their policies so customers don’t have false expectations.

Some are small issues. In August, Southwest tightened the terms on voluntary standby—when someone wants to move to a different flight at the airport on the day of travel. The new rule clarifies that any difference in fare, taxes and fees may be collected per flight segment instead of just per trip.

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In April, Frontier updated its policy to add flights that return to the gate among the circumstances where passengers with a medical condition or disorderly conduct will have to reimburse the airline for costs incurred, including the cost to accommodate other passengers. Previously, Frontier listed only unscheduled landings. Arguments over masks and other disruptive or drunk behavior have resulted in some flights going back to the gate before takeoff to remove passengers.

In American’s case, the airline was being sued by some customers who were stuck after an American cancellation and bought tickets on other airlines. The customers wanted American to reimburse them for the flights, because American’s contract of carriage said it would rebook customers on the next available flight.

The rules didn’t specify that American would rebook customers only on flights operated by American or its partners, even though that has been the airline’s practice for years. So American’s legal department tightened the contract of carriage. And it inserted the “sole obligation” language, first spotted by travel blogger JT Genter.

American spokeswoman Andrea Koos says the company was trying to “more accurately reflect” its practices. “We always strive to provide our customers with the best service possible and support them when travel does not go as smoothly as intended,” she says.

Asked if “sole obligation” didn’t mean the only obligation—a refund and not an obligation to actually fly passengers to their destination—American took a few days to respond and then said it was changing its contract of carriage again.

The revision to the revision, made Sept. 24, was “to clear up any confusion regarding our obligation to rebook passengers or provide a refund when delays, cancellations or diversions occur,” Ms. Koos says.

Write to Scott McCartney at middleseat@wsj.com