Chris Sclafani did everything by the book ahead of his flight from Philadelphia to Denver last Sunday.

He bought the ticket weeks in advance, checked in at the airport two hours early and arrived at the gate with plenty of time to spare before departure.

But just 30 minutes before the plane was scheduled to leave, a US Airways agent told Sclafani and several other passengers they would be bumped to another flight the next day.

The reason: the airline oversold their flight.

"I couldn't believe it," said Sclafani, who needed to get to Fort Collins on Monday morning for the start of classes at Colorado State University. "That's never happened to me before, and I fly pretty often."

In the end, US Airways was able to get Sclafani and the others on, in part because several passengers volunteered to give up their seats for free tickets.

An increasing number of passengers are finding themselves in similar situations, as airlines try to cram as many people as possible onto their planes.

Nationwide, nearly 37,700 passengers were "involuntarily denied boarding" this year through June, a 13 percent spike compared with the first six months of 2006, according to data from the U.S. Department of Transportation.

So far, the total this year already has surpassed the amount for all of 2002. Denied boardings are on pace to hit their highest level ever, both in number and in average per 10,000 passengers.

While the numbers are still minuscule, compared with the millions of passengers airlines carry each year, getting bumped is yet another growing hassle travelers face during what's turning out to be a congested, cramped and frustrating summer for air travel.

"When it does happen, it's a terrible thing," said Joe Brancatelli, an industry expert who runs a Web site for business travelers. "There's nothing worse than when you show up on time, when you've done everything right, and then the airline leaves without you."

Overbooking is a common practice airlines employ to offset no-shows and last-minute cancellations.

Carriers use historical data on each route and flight to estimate how many people likely won't show up. They then oversell accordingly - sometimes by dozens of seats.

But many airlines cut back on domestic capacity in recent years, meaning there are fewer open seats and less wiggle room in general. Load factors, which measure how full a plane is, are running at record levels this year, hitting upward of 90 percent on many flights this summer.

"The industry, as a whole, is overbooking at what used to be manageable levels, but people are flying at much greater levels, and airlines are trying to stuff more people into the same space," said Stuart Klaskin, an aviation consultant in Florida.

It's also becoming more difficult to persuade passengers to give up their seats voluntarily. In many cases, the next flights are overbooked as well, meaning passengers sometimes must wait 24 hours or longer to get a confirmed seat.

"In the past, they could get you out on a flight in three or four hours, but it often takes much longer now," said industry expert Terry Trippler. "I think people are saying, 'Forget your travel vouchers. I'm getting out of town while the getting is good.' "

Federal data back that up. The number of passengers who volunteered to give up their seats dropped slightly through the first six months of the year, according to the DOT.

Airlines say they're under increasing pressure to boost revenue, as ticket prices have been at historically low levels. So they're trying to make sure each plane is as full as can be.

"It's all about maximizing revenue so you don't have any empty seats going out because we can't make it up on the fare side," said Frontier spokesman Joe Hodas.

Passengers who are bumped are entitled to as much as $400, depending on when they actually get to their destination and how much their ticket cost. But federal lawmakers are considering changing the rules, possibly raising the maximum compensation or abolishing the cap on how much passengers can receive.

The agency is taking public comments on the matter and could make a decision later this year. It has received more than 1,100 responses to date, including two dozen from fliers in Colorado.

Not surprisingly, most argue for a significant bump in compensation. Others say the compensation rules should be more equitable, providing higher rewards for higher-priced tickets.

So just how do airlines decide whom to boot?

Most say they bump the last people to check in or arrive at the gate, while others seem to target the passengers who paid less for their tickets. Some use automated systems that incorporate several factors.

US Airways says it bumps the last passengers to show up at the boarding gate, although it gives priority to preferred members of its frequent-flier program.

The carrier's policy, though, comes as a surprise to Robert Laureti. The Denver businessman was one of the few who were initially bumped from the US Airways flight from Philadelphia to Denver last Sunday - even though he checked in about three hours before departure.

"They just steadfastly did not answer my question as to why I was chosen," said Laureti, who was eventually able to board the flight. "Certainly, there had to be plenty of people who checked in after me. It was one of the most stressful things I've ever been through."

Bumping basics

Rules governing denied boardings:

• If a flight is oversold and too many people show up, airlines first must ask for volunteers to give up their seats, typically in exchange for some form of compensation, such as a travel voucher.

• If the airline can't persuade enough passengers to volunteer, those bumped involuntarily are entitled to as much as $200 if they're routed to their destination between one and two hours of the originally scheduled arrival time. If it takes longer, passengers get up to $400.

• Involuntarily bumped passengers can demand immediate compensation in the form of a check.

• Airlines must give all involuntarily bumped passengers a written statement outlining their rights and explaining how the carrier decides who gets on an oversold flight and who doesn't.

There are a few exceptions:

• You must have a confirmed reservation and meet the airline's check-in deadline.

• You're not entitled to compensation if the airline has to substitute a smaller plane or if the original flight is on an aircraft with 60 or fewer seats.

• The airline is not required to provide compensation if it gets you to your destination within an hour of the originally scheduled arrival time.

Want to give up your seat in exchange for a ticket? Some things to know:

• Make sure you ask when your next flight will be and whether you'll have a confirmed seat or will have to fly standby. If you have to fly standby, you could get stuck at the airport much longer than you anticipated, especially if overbookings cascade throughout the day.

• Ask the airline if it provides meal vouchers, a hotel room and ground transportation, especially if your new flight doesn't leave until the next day.

• Ask about the restrictions on any travel voucher you take, as they can be difficult to use because of blackout dates or other rules.

How to avoid it

Each airline has different policies for determining who gets bumped, but typically they target those who check in last. Sometimes, they also choose passengers who paid less for their tickets. To lessen your chances of getting shoved off a flight:

• Arrive at the airport early.

• Print out your boarding pass at home.

• Allow extra time to handle unexpected hassles such as traffic, full parking lots and lines.

• Get a confirmed seat when you check in. If not, get to the gate quickly and try to get one.Sources: U.S. Department Of Transportation, Joesentme.Com, Terry Trippler, Consumer Travel Rights Center

walshc@RockyMountainNews.com or 303-954-2744

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