When Flying For Peanuts, Use Caution

The offer may seem too good to be true: a round-trip ticket on Startup Airlines from New York to Los Angeles for only $49.95.

Before you Startup with your vacation plans, travel professionals advise you to be careful. New airlines have been known to vanish in thin air, so to speak.

To protect yourself from the possibility that Startup may go out of business before its only airplane can get you to Los Angeles, it is important to pay with a credit card. ''If something goes wrong, you won't have to pay the bill,'' says Rudy Maxa, a Washington-based reporter whose travel commentary, ''The Savvy Traveler'' is heard regularly on Marketplace, National Public Radio's evening business show.

If you do make a reservation on a discount airline, here are some things you might want to be aware of:

* There is a good chance the planes are going to be full. Mr. Maxa calls it the ''cattle-car atmosphere.'' Because there is no first class or business class on most of the discount planes, the aircraft can look like a continuous tube of seats. You may also end up on an airline, such as Southwest, that has no assigned seating. It is a first come, first-to-board situation.

* The airline may have some unusual methods to its cheapness. For example, many of the discount airlines don't issue tickets, only confirmation numbers. You will need a photo ID to get your boarding number or pass. You may also find that your travel agent won't issue you a ticket - there may not be enough profit. In addition, there is a good chance the airline is not connected to any of the main computer reservation systems.

* Some discounters don't have interline agreements, so you may have to lug your baggage back up to the check-in area if you are transferring flights. Also, don't expect any fancy meals. Peanuts and sodas might be offered.

* Don't expect Startup to credit your miles toward your major-airline frequent-flyer account. Some discounters have agreements with major airlines, but most do not. Instead, they may have their own program, offering a free ticket for every eight or 10 trip segments.

Despite these caveats, Maxa says, flying on a discount airline can be fun. The flight attendants at Southwest are known for their pranks, for example. He recalls a flight where the attendants asked everyone for a round of applause for a first time flyer - the captain. You can only hope Startup has the same sense of humor.