BOSTON ( TheStreet) -- Low cost airlines love this place.

The largest mainline carrier at Boston Logan Airport is JetBlue ( JBLU), which serves 38 destinations with about 90 daily departures. Boston is a focus city for AirTran ( AAI), which has 21 daily departures to five cities. By October, Southwest ( LUV) will have 26 flights to six cities. And Virgin America operates five daily flights to California.

Of all the big-city Northeast airports, only Baltimore and New York Kennedy have a larger low-cost presence. But those two airports operate in a three-airport region and in a location considered to be distant from the region's center.
Boston Logan Airport

This means that Boston Logan is a low-fare nirvana, a close-in big city airport served by each of the Big Three low fare carriers. Moreover, five carriers each have double-digit market shares, so nobody has a stronghold.

Behind JetBlue comes Delta ( DAL), which holds 15.4% (that includes Northwest), American ( AMR) with 15.1%, US Airways ( LCC) with 13.5% and United ( UAUA) with 10%, according to full-year 2009 figures compiled for TheStreet by OAG Consulting. Flights operated by regional affiliates augment each of the network carriers' shares by a few points.

The airport's biggest transatlantic carriers, in order, are British Airways ( BA), Lufthansa and Air France, followed by American and Delta.

For airline geeks, Boston Logan's airport dynamics provide plenty of intrigue. For instance, on April 27, Southwest announced it will begin Boston-Phoenix service on Sept. 7, with a $99 introductory fare. On May 18, JetBlue said it will begin Boston-Phoenix service on Sept. 2, also with a $99 introductory fare. A third player is US Airways, which has three daily flights there flying to its Phoenix hub. (By the way, US Airways revenue managers are not dummies. They have already posted $99 fares for some Phoenix flights in September.)

Boston Logan was not always so welcoming to low fare carriers. People Express operated there briefly. It bought bankrupt Provincetown-Boston Airlines in 1986, but shut down the following year. When Southwest entered the Boston market, it did so by flying to Providence in 1996 and to Manchester, N.H., in 1998. At the time, Southwest sought to avoid congested airports with high operating costs and limited access. Now it seeks them out.

JetBlue, by contrast, got its start in 1999 at Kennedy Airport; it approaches big city airports with no fear. Arriving in Boston in 2004, it has added flights in every year since. New markets this year include Washington Reagan National and Sarasota, as well as Phoenix. JetBlue's share at Logan has grown steadily from 5% and seven destinations in 2004 to 19.5% in December, according to OAG. And JetBlue continues to grow.

"We plan to increase our Boston departures by approximately 30% year-over-year this summer, accounting for 4.5 points of our expected 2010 capacity growth," said CEO Dave Barger on an April earnings conference call.

JetBlue's 2010 capacity growth of 6% to 8% will likely represent the biggest expansion in the airline industry. JetBlue plans seven daily departures to National. Meanwhile, US Airways is shrinking in Boston.

Aviation experts say post-2001 downsizing by network carriers, combined with favorable airport polices, has enabled Boston Logan to change course.

" Earlier , a lot of the low-fare demand was sucked north to Manchester or south to Providence, and what was left was premium business," said Robert Mann. "Boston became a high fare town."

Then travel fell off and carriers like Delta and US Airways restructured in bankruptcy. Between 1999 and 2009, the combined Boston Logan share for Delta/Northwest slipped from 31% in 1999 to 15% in 2009, OAG said. That opened up some gates.

Additionally, the long-planned Terminal A opened in 2005. The $400 million project replaced a building once occupied by Eastern and added about a dozen gates. Subsequently, Delta moved into A, freeing space for JetBlue, and Northwest joined Delta, freeing space for Southwest. The airport also has a policy requiring intensive use of gates: "No gate hoarding allowed," said airport spokesman Matt Brelis. "We're really land-constrained on 1,700 acres. We have to maintain our resources carefully."

Boston Logan "has become an amazing market for low cost carriers," said consultant Jeffrey Breen, president of Cambridge Aviation Research in Cambridge, Ma. "Delta chose before 2001 to go into the new terminal, and I think they committed to more capacity than they ended up needing. When Delta downsized, JetBlue got a foothold."

Southwest arrived in 2009. "All along we have stated that while we have enjoyed success with our Manchester and Providence operations, we were not fully serving the Boston area," said spokesman Paul Flaningan. "People from the larger Central Boston area were not driving there to fly Southwest."

Consultant Mike Boyd said JetBlue forced Southwest's hand. "Southwest was living off Manchester and Providence, getting leakage out of Boston," he said. "But once JetBlue went there, the leakage stopped." Additionally, access to the airport eased in 2003 following completion of an early phase of the Big Dig construction project.

"Before, getting to the airport was like trying to get out of Saigon when it fell," said Boyd.

-- Written by Ted Reed in Charlotte, N.C.