As major airlines ground planes and eliminate flights to shrink the industry's losses, upgrading to a seat in first class is a long shot for travelers who aren't in the top tier of a frequent-flier program.

Or you could sneak your way to the front, as Julie Vrdoljak did on a December flight to Washington, D.C., from Los Angeles. The special-education teacher stumbled onto an obscure category of discounted coach fares that actually guaranteed her a first-class seat and frequent-flier credit on UAL Corp.'s United Airlines -- but cost hundreds of dollars less than a standard first-class ticket.

Ms. Vrdoljak bought a particular type of coach fare called "Q-up." Other airlines use codes such as "Y-up" and "Z." They represent little-known but surprisingly big bargains on thousands of first-class seats a day throughout the U.S. And often they can be much cheaper than last-minute coach seats.

Why would airlines encourage passengers to slip into first class -- and on the cheap? Most people sitting in first-class seats on domestic flights aren't paying first-class prices. Instead, many passengers are cashing in frequent-flier miles or buying upgrades, meaning those seats generate little extra revenue for the airlines. Q-up fares help carriers increase the number of paying passengers in first class.

Airlines won't disclose exactly how many Q-up, Y-up or Z fares are available. That is partly to avoid raising the ire of frequent fliers: The lure of upgrading from a cramped middle seat near the back of the plane to first class is one of the few perks these travelers still have following industry belt-tightening. The U.S. airline industry has lost more than $42 billion since the start of 2001, according to the Air Transport Association, an industry trade group.

Between New York and Miami, AMR Corp.'s American Airlines recently offered a one-way Y-up fare of $319 -- 66 percent lower than the $999 price of a regular first-class seat. The Y-up fare costs less than half the priciest walk-up seat in the coach section, though American also offered a restricted discounted round-trip coach fare for $333. Recently, it cost $284 for a one-way Q-up seat on Delta Air Lines flights to Washington, D.C., from Atlanta compared with $604 for the most-expensive one-way coach fare and $704 each way for a typical seat in first class. Delta also sells a restricted discount round-trip fare for $438 on the same route.

The Q-up and Y-up fares are usually the cheapest sure way to get into first class. That is because in recent years many airlines have changed their policies to restrict fliers who buy the least-expensive coach seats from jumping into first class by using miles or buying $50 or $75 upgrades. But, even when buyers of cheap seats aren't explicitly barred from making the jump, first class often fills up before they get a chance to buy an upgrade or use miles. Elite frequent fliers and buyers of higher coach fares are first in line to snare available upgrades.

Rick Seaney, chief executive of FareCompare LP, a Dallas firm that analyzes airfares, says airlines typically offer a few deep-discount first-class seats on U.S. flights, while making sure to save some upgrades for elite frequent fliers. The bargain fares are most appealing on routes heavily used by business travelers, since competition to use upgrades is fiercest on those flights. The number of bargain first-class seats varies by plane type, seat configuration and competition with other airlines.

Jim Whitehurst, chief operating officer at Delta, says airlines also use the fares to help their most loyal business travelers avoid roughing it in coach, where empty seats are scarce. Because these fares technically buy a coach-class seat, they allow travelers to maneuver around any corporate-travel department rules against first-class tickets.

"In other industries, they take their best product and sell it for the most money," says Scott Nason, vice president of revenue management for American Airlines. "We take our best product and use it as a lure to (get people to) buy on us, period."

These obscure fares have been available for the past few years but are attracting more buzz lately as the U.S. airline industry shrinks and planes become more crowded. Total available seat miles, a measure of industry capacity, declined 1.2 percent in March compared with a year earlier, the ATA says. Last month, American Airlines said it would ground 27 MD-80 planes by July 1, and Delta and Northwest Airlines are using bankruptcy protection to ditch unwanted aircraft.

Q-up, Y-up and Z fares aren't hard to find or buy, requiring just a few extra clicks of a computer mouse. Online travel Web sites such as FareCompare, Expedia Inc.'s Expedia.com, Sabre Holdings Corp.'s Travelocity and Cendant Corp.'s Orbitz let users search by specific fare type. While known as Q-up or Y-up fares, the codes airlines use to identify them sometimes contain additional airline-specific characters, as in one recent Delta fare labeled QUPBV.

At FareCompare, for example, bargain hunters can click "Trip Search," then enter the desired route and "business/first class." Using Expedia, select "Additional Options" and then check the box to search for seats in business class or first class. Both sites search for the lowest fare in the chosen seating cabin.

If a discounted first-class seat is available, "QUP" or "YUP" will pop up in the fare-class box shown by FareCompare. Otherwise, regular first-class codes such as "F," "A" and "P" will appear on the screen.

Expedia's Q-up fares can be seen by clicking on "Rules and Restrictions." Ms. Vrdoljak, the teacher in Los Angeles, says she hopes to turn the extra miles she earned from sneaking into first-class into a future ticket for one of her annual summer dream trips to Europe. Now that she knows how to look for Q-up fares, "I plan on buying them exclusively if I can, for the comfort of first class with a much lower price," she says.