Southwest Airlines Co. is scheduled to report fourth-quarter results Thursday. Here's a summary of key developments in the period.

OVERVIEW: Among the nation's six largest airlines, only Southwest expected to report that it made money in the fourth quarter.

But unless it posts a bigger profit than analysts expect, Southwest will post its first money-losing year since the early 1970s.

Southwest, the nation's largest discount airline, has been reporting increases in traffic for several months. CEO Gary Kelly says the company is taking customers away from other airlines because it doesn't charge for the first two checked bags.

But even though Dallas-based Southwest brags that "bags fly fee," it has begun charging fees and higher fares for other services; from letting a minor travel alone or bringing a small pet on board, to getting a guaranteed spot near the front of the boarding line.

Those extra charges, along with the increase in traffic, helped the company boost passenger revenue per available seat mile, a closely watched measure of financial performance in the airline industry, by more than 6 percent in the quarter. November and December were the strongest months.

"You look at the results for September, October, November, December, they are remarkably, remarkably better than our competitors," CEO Gary Kelly said last week. He said part of the explanation lies in baggage fees that are driving customers away from other airlines and to Southwest.

BY THE NUMBERS: As of Tuesday afternoon, analysts surveyed by Thomson Reuters expected Southwest to earn 6 cents per share, or about $45 million. Analysts don't count one-time gains and charges in their estimates, so the figure that Southwest reports using standard accounting rules could be different.

In the fourth quarter of 2008, Southwest lost $56 million, or 8 cents per share, under standard accounting rules. Excluding items, mostly write-downs of fuel-hedging contracts, Southwest said it would have earned $61 million, or 8 cents per share, a year ago.

Analysts expect Southwest to report revenue of $2.67 billion, down from $2.7 billion a year earlier.

Southwest has earned a profit for 36 straight years, by far the longest streak of any major U.S. airline. But the company could hit the fourth-quarter profit target set by analysts and still lose money in 2009. That's because using standard accounting, it lost $53 million through the first three quarters.

ANALYST TAKE: Jamie Baker of JPMorgan recently boosted his forecast for fourth-quarter profit on an "impressive" showing in the December revenue trend. But he expects Southwest will grow slower than other airlines in 2010 because it doesn't have international routes.

WHAT'S AHEAD: Southwest must boost revenue to remain profitable and cover the cost of pay raises it gave to union employees last year at the bottom of the airline industry slump.

The company is working on overhauls of its frequent-flier program and Web site to boost revenue. It will begin service this spring to northwest Florida, and it hopes to sell travel to Canada and Mexico by striking partnerships with airlines based in those countries.

STOCK PERFORMANCE: Southwest shares rose 19 percent in the fourth quarter, from $9.60 to $11.43, topping the 17.4 percent increase in the Amex index of 13 U.S. and foreign airlines over the same period.

In trading Tuesday, the shares rose 2 cents to close at $11.30. They have ranged from $4.95 to $11.79 in the past year.