Q: I recently traveled from Philadelphia to Dublin, Ireland, on US Airways. On the return flight, we had a stopover in Frankfurt, Germany.

We boarded the plane on time and waited to push back and taxi to the runway. For four hours we sat doing nothing, save hearing occasional updates from the pilot.

Finally, the aircraft mechanics discovered the problem — a fault with a valve in one of the engines. They installed a new part and fired up the engines twice to test it. Both tests were successful, but we sat on the runway for two more hours waiting for a fuel truck.

Then US Airways canceled the flight and informed us that it wouldn’t be able to get us to our destination for two more days because it didn’t have enough planes or sufficient crew.

The airline representatives in Frankfurt were courteous and professional, and put us up in a nearby hotel. We were told that we were entitled to 600 euros in compensation for the delay. But when I returned to the United States, all the company would offer was a $200 voucher that had to be used on US Airways. How can this be possible?

– Michael Nelson, Orange, Conn.

A: My initial reaction, like yours, was: It can’t be possible. Surely you’re entitled to more than $200 in airline scrip for a two-day delay overseas.

Then I read the rules.

Under a traditional airline contract of carriage — the legal agreement between you and the airline — you are entitled to everything from phone cards to meal vouchers when your plane breaks down. What’s more, the airline has to put you on the next flight to your destination, even if it’s on a competitor’s airplane.

But US Airways seems to have broken with tradition. Its own terms of transportation are clear about the limits of its liability. “US Airways is not responsible or liable for making connections, or for failing to operate any flight according to schedule, or for changing the schedule of any flight,” it says.

The airline will cover only “limited expenses” during a mechanical delay and “may” attempt to rebook you on the next available flight of another airline “with which US Airways has an agreement allowing the acceptance of each other’s tickets,” say its terms.

The Frankfurt ground crew was right about the 600 euros, but wrong about the circumstances under which a check is cut. That compensation is offered only to customers who have involuntarily been denied boarding on their flight and arrive at their destination more than four hours later than the scheduled arrival time of the original flight.

According to US Airways, its regulations for flights originating in Europe let the carrier off the hook “if it can prove that the cancellation is caused by extraordinary circumstances.” Among the circumstances: “unexpected flight safety shortcomings.”

“We interpret ‘unexpected safety shortcomings’ to mean mechanical trouble,” said US Airways spokesman Carlo Bertolini.

US Airways offered passengers who complained about the delay a $200 “goodwill voucher” – but no more.

The ground crew who promised you 600 euros erred. But don’t go too hard on them. In a column several weeks ago, I confused Cheapseats.com and Cheaptickets.com. Easy mistake to make — and so was this one.

US Airways is right. But that worries me. I’m concerned that passengers’ rights are being eroded with each revision of an airline’s contract of carriage. I suspect US Airways is hardly alone in rewriting its responsibilities to better suit its bottom line. And I wonder if the day will come when the contract will allow an airline to wiggle out of its obligation to fly us anywhere at all.