John Koehn planned his cross-country trip from Washington to Medford, Ore., with his wife and three-month-old daughter carefully. He booked their flight a year in advance to make sure they could sit together.

And then, just a few months before the trip, everything fell apart.

When Koehn logged on to the United Airlines website to check his reservation, he discovered his flights had been changed, along with his seat assignments.

“So now the airlines can change flights without even notifying customers,” he says. “Could it possibly get worse?”

Well, yes. Airlines don’t just switch out flights to suit their purposes — a so-called “equipment change” — without compensating their passengers. Sometimes, they try to monetize the switches.

Consider what happened to Andy Eckel when he booked a Delta Air Lines flight for his elderly parents.

“We specifically purchased tickets — and paid more than we would have otherwise — that departed at around 9 a.m. due to concerns about my parents arriving home at night and possible delays that are common,” he told me.

But two months before his parents’ trip, Delta changed their flight to a new one that left six hours later.

“We resigned ourselves to the situation, assuming that the earlier flight was no longer available,” he says.

But on the day of the flight, he was surprised to learn the original flight was available, but it would cost $1,400 extra to have his parents moved back.

“I called the airline’s customer service line and asked if they could get me on the original flight for the original price, and was told, “no,” he says.

After he complained, Delta sent his parents a form letter and a $125 voucher on a future flight. “This is a bit insulting, since the airline themselves put the value of the change at $1,400,” he told me.

Delta’s explanation, which I’ve seen time and again from other airlines, is that it needs to be flexible.

“In the process of providing air service over many different routes each day, we sometimes encounter mechanical problems, adverse weather, and other unavoidable interruptions,” it said, in explaining Eckel’s switch. “These are situations faced by all airlines and no carrier can guarantee that all flights will depart and arrive as planned.”

Michael Miller, a vice president at the American Aviation Institute, a Washington think tank, says airlines change engines, avionics and other equipment as needed.

“Sometimes they can plan for it, sometimes they can’t,” he adds. Airlines don’t want to make these switches, and they don’t think about profiting from them.

“It’s a pain for them,” he explained. “Imaging re-seating 250 people and getting a different set of pilots for a different plane just hours before departure. Most airlines have spare aircraft of the same type, especially at their hubs, but that not always the case. You don’t want to have too many $100 million aircraft sitting around.”

But to passengers, it does seem as if the airline industry has a set of double standards. On the one hand, it allows itself to reschedule flights and arbitrarily change your seat assignments. On the other hand, if you decide to change your plans, it charges you a fee.

It’s not as bad as it seems. Under most airline contracts of carriage — the legal agreements between you and the airline — if a flight is canceled and rescheduled for later, you’re entitled to a full refund. Normally, that’s a meaningless gesture, since a replacement flight on a different carrier would cost far more. Still, it’s nice to know the airline is required to refund your ticket.

But when it comes to seat assignments, airlines are under no similar obligation. If you accept the new flight, you’re doing it on its terms. It’s only required to offer you a seat in the same class of service.

Is that enough? Passengers think the game of musical chairs has gone too far when an airline doesn’t bother telling passengers about the change until they get to the airport, and especially when they appear to be trying to profit from an equipment change.

“I can only imagine a future where the airline changes your flight and then charges you for the difference,” says Koehn. “But hey, at least that would require them to notify you of the change.”