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Consumers looking for a sweetheart deal in the latest round of airfare sales may come up empty-handed because discounts are rarely an across-the-board deal.

United Airlines, currently wrestling with a Chapter 11 bankruptcy reorganization, announced Thursday it was extending its winter fare sale of 40 percent off on non-business travel fares through May 18, matching the end date of the American Airlines discount program announced Wednesday. Southwest Airlines was already offering discounted fares, and industry watchers said it appears to be only a matter of time before other airlines bite the bullet and join the promotional parade.

But you’ll have to book the right flight at the right time to get the best deals.

“It’s not so much as a clearance or everything must go, but more like this rack is 25 percent off and this rack is 75 percent off,” said Terry Trippler, an airline consultant in Minneapolis. “Airlines can come and say they are having a sale and they will have these fares, but they don’t tell you how many seats are at that price.”

An airline could discount only a couple of seats on any given flight, and that number is constantly changing. On Thursday afternoon, for instance, Trippler said there were 41 different coach fares offered by American Airlines on a flight from Chicago to Los Angeles.

“They could say we are putting 20 seats on sale, but if none sell by the afternoon, they might change that to 10 seats and put another 10 sale seats on another flight,” he said.

Consumers are getting conditioned to never paying full price for anything thanks to aggressive retail promotions and the auto industry’s zero-percent financing. But the latest round of airfare discounts may have them doubting the financial precariousness of the airline industry.

“We’ve been scratching our head for a long time,” said Trippler. “You’re broke, you’re cutting back on capacity and now you’re having a sale? You’re going to the government, losing all this money and now you are giving away seats? It makes it difficult to generate a whole lot of sympathy” for the airline industry.

But it’s not unusual, analysts say, to see fare discounts this early in the year in an industry desperate for passengers. Business travelers once represented 40 percent of revenue for United Airlines. Now that figure has dropped to 20 percent, said Darryl Jenkins, director of the Aviation Institute at George Washington University in Washington, D.C.

“The first quarter is typically the lowest travel season,” Jenkins added. “The airlines need to stimulate traffic.”

It can be a tricky game: If they cut prices too deeply, they will sacrifice profit margins–but if they offer no incentive, planes will fly with empty seats. After all, a seat sold at a discount is better than an empty seat, and any bit of cash is helpful to offset a fixed expense.