Overbooking is not illegal, and most airlines overbook their scheduled flights to a certain extent in order to compensate for "no-shows." Passengers are sometimes left behind or "bumped" as a result. When an oversale occurs, the Department of Transportation (DOT) requires airlines to ask people who aren't in a hurry to give up their seats voluntarily, in exchange for compensation. Those passengers bumped against their will are, with a few exceptions, entitled to compensation.

Voluntary bumping

Almost any group of airline passengers includes some people with urgent travel needs and others who may be more concerned about the cost of their tickets than about getting to their destination on time. Our rules require airlines to seek out people who are willing to give up their seats for some compensation before bumping anyone in- voluntarily. Here's how this works. At the check-in or boarding area, airline employees will look for volunteers when it appears that the flight has been oversold. If you're not in a rush to arrive at your next destination, you can give your reservation back to the airline in exchange for compensation and a later flight. But before you do this, you may want to get answers to these important questions:

* When is the next flight on which the airline can confirm your seat? The alternate flight may be just as acceptable to you. On the other hand, if they offer to put you on standby on another flight that's full, you could be stranded.

* Will the airline provide other amenities such as free meals, a hotel room, phone calls, or ground transportation? If not, you might have to spend the money they offer you on food or lodging while you wait for the next flight.

DOT has not said how much the airline has to give volunteers. This means carriers may negotiate with their passengers for a mutually acceptable amount of money-or maybe a free trip or other benefits. Airlines give employees guidelines for bargaining with passengers, and they may select those volunteers willing to sell back their reservations for the lowest price. If the airline offers you a free ticket, ask about restrictions. How long is the ticket good for? Is it "blacked out" during holiday periods when you might want to use it? Can it be used for international flights? Most importantly, can you make a reservation, and if so, how far before departure are you permitted to make it?

Involuntary bumping

DOT requires each airline to give all passengers who are bumped involuntarily a written statement describing their rights and explaining how the carrier decides who gets on an oversold flight and who doesn't. Those travelers who don't get to fly are frequently entitled to an on-the-spot payment of denied boarding compensation. The amount depends on the price of their ticket and the length of the delay:

* If you are bumped involuntarily and the airline arranges substitute transportation that is scheduled to get you to your final destination (including later connections) within one hour of your original scheduled arrival time, there is no compensation.

* If the airline arranges substitute transportation that is scheduled to arrive at your destination between one and two hours after your original arrival time (between one and four hours on international flights), the airline must pay you an amount equal to your one-way fare to your final destination, with a $400 maximum.

* If the substitute transportation is scheduled to get you to your destination more than two hours later (four hours internationally), or if the airline does not make any substitute travel arrangements for you, the compensation doubles (200% of your fare, $800 maximum).

* You always get to keep your original ticket and use it on another flight. If you choose to make your own arrangements, you can request an "involuntary refund" for the ticket for the flight you were bumped from. The denied boarding compensation is essentially a payment for your inconvenience.

Like all rules, however, there are a few conditions and exceptions:

* To be eligible for compensation, you must have a confirmed reservation. A written confirmation issued by the airline or an authorized agent or reservation service qualifies you in this regard even if the airline can't find your reservation in the computer, as long as you didn't cancel your reservation or miss a reconfirmation deadline.

* You must meet the airline's deadline for buying your ticket. Discount tickets must usually be purchased within a certain number of days after the reservation was made. Other tickets normally have to be picked up no later than 30 minutes before the flight.

In addition to the ticketing deadline, each airline has a check-in deadline, which is the amount of time before scheduled departure that you must present yourself to the airline at the airport. For domestic flights most carriers require you to be at the departure gate between 10 minutes and 30 minutes before scheduled departure, but some deadlines can be an hour or longer. Check-in deadlines on international flights can be as much as three hours before scheduled departure time. Some airlines may simply require you to be at the ticket/baggage counter by this time; most, however, require that you get all the way to the boarding area. If you miss the ticketing or check-in deadline, you may have lost your reservation and your right to compensation if the flight is oversold.

* As noted above, no compensation is due if the airline arranges substitute transportation which is scheduled to arrive at your destination within one hour of your originally scheduled arrival time.

* If the airline must substitute a smaller plane for the one it originally planned to use, the carrier isn't required to pay people who are bumped as a result.In addition, on flights using aircraft with 30 through 60 passenger seats, compensation is not required if you were bumped due to safety-related aircraft weight or balance constraints.

* The rules do not apply to charter flights, or to scheduled flights operated with planes that hold fewer than 30 passengers. They don't apply to international flights inbound to the United States, although some airlines on these routes may follow them voluntarily. Also, if you are flying between two foreign cities-from Paris to Rome, for example-these rules will not apply. The European Community has a rule on bumpings that occur in an EC country; ask the airline for details, or contact DOT.