Last month, Spirit Airlines was charging 27 bucks to fly from here to Fort Lauderdale. But there was a catch: You had to join a club to qualify for the low fare. And “here” wasn’t BTV; it was PBG — code for Plattsburgh International Airport.

Less than four years after its conversion from a U.S. Strategic Air Command base to a civil aviation facility, Plattsburgh is starting up its engines. The airport is planning improvements that could attract many more Canadian travelers as well as Vermonters. That would mean real competition for BTV.

PBG is still dwarfed by its Vermont counterpart — 22 miles southeast as the plane flies. Only four airlines currently operate out of Plattsburgh, all of them on a charter basis, which means their schedules can change from day to day. The casual flyer has probably heard of only one of PBG’s carriers — US Airways Express — while BTV hosts five airlines whose names are writ large in American skies: Continental, Delta, JetBlue, United and US Airways. BTV also attracts seven times as many departing passengers as does PBG: 676,000 versus 95,000 in 2010.

That said, BTV — the only major airport between Albany and Montréal — is not exactly flying high these days. The airport has lost carriers, such as AirTran, as well as routes to Boston’s Logan Airport.

Its financial prospects have dimmed, too. In the last year alone, Moody’s Investors Service twice downgraded BTV’s credit rating, to the point where its bonds are now consigned to junk status. That dis was due in part to the airport’s 10 percent drop in passenger counts over the past two years. According to Moody’s, BTV’s creditworthiness has also been tarnished by its association with the city of Burlington, which in turn has had its bond rating whacked as a byproduct of the Burlington Telecom debacle.

PBG, by contrast, is muscling up. Boardings more than doubled last year and are projected to grow 50 percent this year, to a total of 150,000. The number will reach a quarter-million 20 years hence, according to calculations endorsed by the Federal Aviation Administration, although PBG manager Tom Long says, “We actually expect to do a lot better than that.”

The Plattsburgh airport has enormous potential for expansion. It sits on a former base that covers almost 3500 acres — nearly four times the size of the area encompassed by BTV. Built to handle heavy bombers, the Plattsburgh field includes a runway that’s more than two miles long and had been on a list of alternate landing sites for NASA’s space shuttle.

With a fireplace in its big-windowed atrium, PBG’s 33,000-square-foot terminal feels homey — and decidedly uncrowded at 8:30 a.m. on a recent weekday morning. The huge expanse of tarmac outside the terminal was devoid of airplanes. But with only one security checkpoint and a single Jetway, Long complains, it gets congested at certain times of day. “We’re going to be too crowded in here,” he predicts while leading a reporter on a tour of the terminal. “We’ve got to add space.”

PBG is hoping to get some FAA money to double the size of the terminal, a project that could get underway next year. The FAA provided $40 million to finance conversion to civilian use of the U.S. Air Force base that closed in 1995. The feds also subsidize US Airways’ PBG-Boston service to the tune of $1.4 million a year through a program that enables small airports around the country to connect with hubs. In the eyes of BTV’s former aviation director, Brian Searles, all that money flowing from Washington to PBG amounts to a form of unfair competition. He said as much in an interview last year.

But Plattsburgh isn’t apologizing. Instead, it’s ramping up its efforts to attract customers from metropolitan Montréal.

Québec and Ottawa account for 90 percent of the passengers departing from Plattsburgh, most of them en route to Florida on Allegiant Air, Direct Air or Spirit Airlines. They’re attracted by comparatively cheap fares to warm-weather destinations. The March price for Spirit’s round-trip flight to Fort Lauderdale is now $275, tax included — still a big savings over the $438 it would cost to leave from Montréal.

Canada heavily taxes air travel, which helps explain why Canadians make up 40 percent of BTV’s departing passengers; PBG is going after the same customer by billing itself as “Montréal’s U.S. Airport.”

The drive from Montréal to Plattsburgh’s airport is about 30 minutes shorter than the drive to BTV. Parking has also been free in PBG’s 1400-car outdoor lot, though drivers will soon be charged $5 a day for a space near the terminal and $3 for a spot further away. It costs $12 a day to park in BTV’s garage.

In addition to its flights to balmy resort towns, PBG offers thrice-daily service to Boston. BTV’s service to Logan ended three years ago — a victim, says Burlington interim director of aviation Bob McEwing, of the heightened security introduced following the 2001 terror attacks. “It got to be so much of a hassle that a lot of people just started driving to Boston rather than flying,” he comments.

Burlington-area residents bound for Boston still seem intent on driving rather than flying from Plattsburgh. PBG manager Long estimates that Vermonters account for only about .5 percent of passengers in Plattsburgh — which may not be surprising, given that it takes 75 minutes to drive to PBG from Burlington via the Grand Isle ferry, which costs $18 round-trip.

New Yorkers seem to make up most of the passengers on the US Airways Express 34-seat turboprops bound for Boston. Many of them, however, are actually heading to more distant destinations. And all five of those interviewed as they waited last week for a PBG-Boston flight said they would probably have flown from BTV to LaGuardia, Newark, JFK or another hub had Plattsburgh not been an option.

“I’ve flown many times from Burlington and always had a good experience,” said Drew Reither of Willsboro, N.Y., who nonetheless plans to fly from PBG to Myrtle Beach, S.C., in March. “It’s just more convenient to come here,” he explained. “There’s also the whole thing about keeping money in your local community.”

Winston Thomas, a Plattsburgh physician, said he uses PBG as a gateway to his native Dominica in the Caribbean, as well as to destinations around the United States. “I wasn’t too happy with my last time flying from Burlington,” Thomas said. “It was crowded and the flight was delayed. Here, things are much easier.”

Airport officials on both sides of the lake insist there’s no rivalry between them. Only in the “long, long term” would PBG pose a significant threat to BTV, predicts Gene Richards, a member of the Burlington airport commission. “We don’t have the same products. We serve different clientele,” he says.

Plattsburgh, Richards adds, is “more destination driven, while we’re hub driven.” PBG mostly flies to resorts; BTV takes passengers to larger airports where they can connect to anywhere, he observes.

Tom Long at PBG agrees there’s no poaching taking place. But he makes that point just minutes before leaving to visit to Heritage Aviation headquarters at BTV, with Plattsburgh development officials in tow. Heritage provides maintenance, repairs, charter flights and other aviation services. Long says the Plattsburgh team was going solely to look at Heritage’s award-winning green-design building at BTV, which, he suggests, could serve as a model for a similar facility at PBG.

Indeed, Laurentian Aerospace has announced plans to build a $175 million, 273,000-square-foot maintenance and repair facility at PBG that will be able to accommodate 747s. The project is expected to provide 200 jobs by 2013 and ultimately as many as 900.

BTV doesn’t have anything that big on its agenda, but Richards, McEwing and Searles are all bullish about its prospects. They attribute the recent decline in passenger numbers to runway work that limited the number of landings last year, as well as to the recession, which forced airlines to reduce flight frequencies.

Richards adds that BTV is “100 percent confident” that its favorable credit rating will be restored. “We’re working on each of the issues raised by Moody’s on a daily basis,” he says. “We’re not resting until we make it right.”