When an airline cancels a flight, it owes ticketed passengers compensation and best efforts to get them booked on another plane. Many travelers know this and rightfully put up a fight whenever a carrier tries to strand them at the airport.

When an airline delays a flight and causes passengers to miss a connection, it very well may owe them compensation as well. This is especially true in the European Union where the law is strict on passengers' rights. Many travelers do not know this. The result, according one company, is a half a billion dollar tab owed to Americans alone that the industry still has not yet paid.

And it's growing. Per a recent report from Airhelp, a firm that specializes in European airfare compensation issues:

"Data shows more than 1 million passengers traveling from the United States in 2017 may be entitled to a total of 555 million USD for missing their connecting flights… Every passenger affected may be eligible for claims for alternative transportation and compensation up to $700."

Americans who travel abroad frequently don't know their rights, especially when it comes to issues of finance and compensation. Every year vacationers spend millions of dollars on VAT (what amounts to the local sales tax) in Europe without claiming their refunds upon going home. Others fall prey to scams that take advantage of a tourist's general ignorance of the city around them.

Then there's air travel.

Already a byzantine, obtuse system at home, for many travelers, flying can become an overwhelming process when they have to figure out all the new rules in a foreign language. And the rules can change dramatically. In the Greek islands, planes leave when they leave, often waiting for the passengers instead of the other way around. In Italy, ticketing is often a frantic scrum of harried gate agents and passengers perpetually about to miss their flights. Vacationers in France must plan around strikes that can shut down flights to entire regions of the country.

With the normal rules ordinarily turned on their heads many passengers can't tell the difference between when something has gone wrong and when a peculiar system is working as designed. When a flight gets delayed, they will simply roll with the punches. When that delay leads them to miss a connection onward they'll queue up (as one says across the pond) and get a new booking.

Sometimes, that rebooking is supposed to come with a check.

A European Union law known as EC 261/2004 entitles travelers to compensation for cancellations and any delay lasting more than three hours. The law applies to travelers within the European Union or from it, or anyone flying into the European Union on an EU-flagged airline. Compensation varies based on how long they made passengers wait, but it can go as high as 600 euros (approximately 740 USD at time of writing).

The law carves out a few exceptions, such as for bad weather, because regulators didn't want to encourage loss-averse pilots to risk health and safety. Nevertheless the European airline industry still pays hundreds of millions of dollars per year to passengers for delays.

Airlines are historically slow to pay EC 261 compensation, as it's known, with some of the budget carriers causing particular problems or creating impossibly high barriers for passengers who want to file a claim. As a result, a cottage industry of businesses exist specifically to help consumers get their checks, which generally keep a commission of around 25% of the collected fee.

Delay compensation is not something most American travelers tend to think about, as it took an act of government just to stop airlines from storing travelers on the tarmac for hours on end.

"Unfortunately, there are limited regulations on air passenger rights in the U.S.," said Airhelp CEO Henrik Zillmer. "These limited regulations allow airlines to put profit above passengers. When issues do arise, such as overbooked, delayed or cancelled flights, passengers deserve to be fairly compensated as disruptions may have huge consequences on travelers' deserved holidays or important business trips."

It may not be something an American would think about while flying overseas. As a result, airlines have kept hundreds of millions of dollars that should have gone into the pockets of stranded travelers.

The next time you're waiting at the gate on vacation, ask an attendant for more information about your rights. It may be worthwhile.

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