Airlines are pushing back against new rules that give fliers more rights.

They are threatening to cancel scores of flights in response to a new rule that would prohibit airlines from keeping passengers on the tarmac for more than three hours without giving travelers the opportunity to get off the plane. As of April 29, carriers that break the rule would face steep fines of up to $27,500 per passenger, or more than $4 million on a full Boeing 737 or Airbus A320.

Carriers say that to avoid those fines, they will aggressively cancel flights before and during storms?even if the bad weather never materializes. The threats could foreshadow significant changes in air travel, making it even less reliable for millions of road warriors and vacationers. By canceling flights, it could take days for all travelers to get home when storms strike.

Of course, the warnings from carriers could simply be posturing to pressure the government into leniency. Passenger-rights advocates say airlines are trying to scare fliers. And the Department of Transportation says carriers have a lot of other options to avoid fines.

Still, on Tuesday, Continental Airlines Inc. Chief ExecutiveJeff Smisek threw down the gauntlet, calling DOT’s rule “stupid.” Even though many passengers will risk long delays to get where they are going, “the government by God says, ‘We’re going to fine you $27,500,’” he said at an investor conference in New York. “Here’s what we’re going to do: We’re going to cancel the flight.”

Other airlines have also raised warning flags. Both jetBlue Airways Corp. and Delta Air Lines Inc. have asked the DOT for waivers from the new rules at New York’s Kennedy International Airport, where the longest runway is currently closed for resurfacing, adding to delays at one of the nation’s most-congested airports.

Airlines have already shown that they are willing to aggressively cancel flights. Amid record snow storms?and in anticipation of the new rules?airlines canceled 34,588 flights in February, nearly four times as many as were canceled in February 2009, according to FlightStats.com. That meant some travelers who might have been able to fly ended up stuck for several days before empty seats opened up for them on other flights.

“This is real,” said James May, chief executive of the Air Transport Association, an airline industry group that lobbied against the tarmac delay limit. “It’s hard to predict right now how significant it’s going to be, but I don’t think there is any question we’ll see significantly more flights canceled because no one wants to subject their company to those fines.”

The DOT won’t comment on airline motives, but in a statement, spokesman Bill Mosley said airlines have options other than resorting to large-scale flight cancellations.

“Carriers have it within their power to schedule their flights more realistically, to have spare aircraft and crews available to avoid cancellations, to ensure that their crews do not come up against flight and duty time limitations when tarmac delays occur, and to place passengers on other carriers’ flights when flights must be cancelled for whatever reason,” Mr. Mosley said. Travelers can always switch airlines if one carrier starts canceling too often, he adds.

Different airlines have always had different philosophies on handling severe weather, with some aggressively canceling all the time with stormy forecasts and others trying to operate every flight possible regardless of how late those trips might run.

Some carriers believe that canceling quickly offers customers more predictability, allowing them to stay home or hunker down in hotel rooms rather than wait (or sleep) at an airport. And those that try to wait out bad weather and operate as many flights as possible believe customers prefer airlines that make every effort to get you where you want to go, even if it means middle-of-the-night arrivals.

But each strategy has its risks. Delta, for example, was criticized for cancelling flights out of New Orleans early before Hurricane Katrina, leaving customers stranded at the airport for the storm. Delta still maintains early cancellations are preferred by customers, and the airline canceled flights aggressively ahead of this winter’s storms.

On the other hand, jetBlue, which used to avoid cancellations to an extreme, found itself in a mess in 2007 when a Valentine’s Day ice storm left planes piled up for hours and hours at JFK, passengers stuck in horrid conditions.

Last year, Continental had the lowest percentage of canceled flights among major airlines, at 0.5% of departures, according to FlightStats, a flight-tracking service. UAL Corp.’s United Airlines had the highest cancellation rate among major carriers at 1.6%?more than three times as high as Continental.

The three-hour limit doesn’t mean flights must be canceled at three hours or face fines, just that airlines and airports have to find a way to get people off of an airplane if they want off. That can mean returning to a gate to unload passengers or rolling up a staircase and busing passengers back to terminals. Pilots have to agree that it’s safe to unload people and air-traffic controllers have to agree that it wouldn’t be disruptive to operations-both major caveats likely to limit disruption from the new rule.

Getting people off flights can be a major disruption for airlines and airports. One major penalty: Air-traffic controllers take flights first-come, first-serve, forcing them into long lines and punishing them for leaving the line and returning to a terminal. And sometimes when flights return to a gate pilots become ineligible under federal duty rules to continue that trip.

Procedures like returning to gates or using portable stairs and buses are uncommon in the U.S. (Remote parking and busing passengers is far more common in Europe.) But airports and airlines are working on new procedures and options for flights that face long delays, including more buses, stairs and reserving gates for quickly unloading customers.

“People will make adjustments and it will settle down,” said James Crites, executive vice president at Dallas Fort-Worth International Airport. “In the end, it will be less than what you are hearing now. The industry is very adaptable. We will find a way to make it work.”

Mr. Crites notes that while some 900 flights were stuck sitting for more than three hours last year, it’s the rare exception of exceedingly long eight- and 10-hour ordeals that gets the most attention, with passengers seemingly locked up with little if any food, poor bathroom facilities and plenty of uncertainty.

A regional airline flight on behalf of Continental that was left sitting overnight last summer in Rochester, Minn., so outraged the DOT that it fined Continental, its partner ExpressJet Inc., and Delta, whose agent refused to open a gate for the stranded plane. That incident also moved the DOT to enact its new rule.

Mr. Crites says airports, like airlines, have been working to develop systems to track flights on the ground and raise alarms if they sit for long periods.

Kate Hanni became a passenger rights advocate, founded FlyersRights.org and pushed for the three-hour rule after being stranded for 10 hours on an American Airlines flight three years ago. She says travelers prefer cancellations to the uncertainty of being trapped on a stranded airplane or stranded at an airport halfway through their journey.

“We don’t get complaints when people are stuck in airports, only when they are stuck on airplanes. It’s better for airlines to pre-cancel,” she said. “They are trying to scare everybody that canceling is some kind of horrific travel nightmare and it’s not.”