According to a USA TODAY survey of the check-in policies of 18 U.S. airlines, fewer than half guarantee that a late-arriving passenger who is denied boarding will be booked on the next available flight for free. Many require the passenger to pay a ticket-change fee, the fare difference or both.

But the added costs are at the discretion of airport agents who often waive them.

"They're absolutely getting stricter," says Terry Trippler, an air travel expert at Cheapseats.com. Airline agents, he says, are more often enforcing check-in times "to the letter." Airlines typically have policies for check-in times both at the airport counter and at the departure gate.

Agents have become stricter since the Sept. 11 attacks, says Midwest Airlines spokeswoman Cathy Unruh. Longer times required for security screening make strict enforcement of check-in times more important. Frequent flier Richard Catalano, of Cleveland, says "too many times" he's arrived too close to take-off and not allowed to board. "It's so annoying."

Schedule cited

Airlines say they're just trying to keep on schedule. "It is very important to customer satisfaction that flights are on time, and the best way to keep arrivals on time is to depart on time," says American Airlines spokesman Tim Wagner.

If a passenger who hasn't met a check-in time is denied boarding, no compensation is due, and the seat can be assigned to someone else, says Tom Parsons, CEO of BestFares.com.

Airlines insist they bend over backward to accommodate late-arriving fliers. Decisions are made on the spot by airline agents at each airport, they say, and there is no attempt to squeeze more people aboard or to skirt rules requiring that airlines compensate passengers who are bumped from an oversold flight.

"If a person is late, that person is late," says Wagner. "It has no connection with oversold flights." Most oversold flights are resolved by passengers volunteering to stay behind in return for compensation, he says.

Many last-minute fliers today learn before they reach the gate that they cannot board. A ticket counter agents tells them, or a ticketing kiosk won't issue a boarding pass.

To avoid getting shut out at the ticket counter, frequent-flier Charles Franklin, of Los Angeles, recommends going to an airline's website before arriving at the airport and printing a boarding pass.

Some airlines allow fliers to print their own boarding passes beginning a day before and up to 90 minutes prior to departure. The passes enable fliers to go directly to the gate without having to meet the ticket counter check-in time.

Franklin, a travel manager for an auto manufacturer, says he was denied boarding before two recent flights from Los Angeles International Airport. In one case, he got to the ticket counter 29 minutes before flight time and was told it was too late for a boarding pass. Another time, one agent said it was too late. He appealed to a second agent and was allowed to board.

Watching the clock

It's not easy to keep track of airline check-in times.

Delta Air Lines, for example, requires fliers to check in at the ticket counter 15 minutes before shuttle flights or most other domestic flights. With a checked bag, it's 30 minutes on those flights.

It requires check-in 45 minutes before international flights, or for departures from six U.S. cities. With a checked bag on those flights, it's 60 minutes. But the carrier has exceptions to those rules, and a separate set of rules for gate check-in.

U.S. Department of Transportation spokesman Bill Mosley suggests consumers become familiar with check-in times by calling the carrier or going to its website. Such information can be found in the "contract of carriage" or another screen on the site.

Flexibility, if needed

Several airlines that charge fees for late check-in, though, say their policies are flexible. Continental Airlines' policy, for example, says passengers checking in less than 30 minutes before a domestic flight — or fewer than 15 at the gate — might be denied boarding and charged a change fee and the fare difference for a subsequent flight. But customers are generally accommodated on the next available flight without charge, says spokeswoman Courtney Wilcox.

Catalano and others who have missed recent flights say ticket and gate agents have put them on the next flight without an additional charge. Paul Villaret, a pharmacist in Las Vegas, wasn't so fortunate. He says he arrived at a United Airlines ticket counter 30 minutes before a flight and was told it was too late to check his baggage. He says he was told that his late arrival nullified his ticket, and he would have to buy a full-fare ticket for another airline's flight.

United spokeswoman Robin Urbanski says Villaret's ticket would have been honored free of charge on the next United flight. But United will not book late-arriving passengers for free on other airlines' flights, she says.

Catalano, the Cleveland traveler, and other road warriors say they adamantly oppose an extra charge when there isn't a change of airline. "These nickel-and-dime charges are out of hand," says Catalano, a vice president of a supermarket chain. "I hate hidden fees, and the people who most often have to pay them are the frequent travelers."

Charlie Stortz, a vice president of a software company in Missoula, Mont., says airlines need to understand the twists and turns in passengers' lives. "Sometimes, there are crashes on freeways and other things that come up to make life a bit more difficult," he says.

Airlines say their airport agents consider each passenger's reason for late arrival and might waive extra charges for flying on a subsequent flight.

"We try very much to accommodate passengers any way we can to ensure they have the best flight experience," says Continental's Wilcox. "But there has to be some standard how to address someone who doesn't arrive on time."