Just ahead of what could be a record-breaking summer travel season, pilots from one of the nation's biggest airlines marched in picket lines at major airports on Friday as they push for higher pay.Related video above: See which airline ranks as the most reliableThe United Airlines pilots have been working without a raise for more than four years while negotiating with airline management over a new contract.The pilots are unlikely to strike anytime soon, however. Federal law makes it very difficult for unions to conduct strikes in the airline industry, and the last walkout at a U.S. carrier was more than a decade ago.The coast-to-coast protests by United pilots come on the heels of overwhelming strike-authorization votes by pilots at American Airlines and Southwest Airlines. United pilots could be the next to vote, according to union officials.Pilots at all three carriers are looking to match or beat the deal that Delta Air Lines reached with its pilots earlier this year, which raised pay rates by 34% over four years.United has proposed to match the Delta increase, but that might not be enough for a deal."We still have a long ways to go to resolve some of the issues at the table," said Garth Thompson, chair of the United wing of the Air Line Pilots Association.Thompson said discussion about wages has been held up while the two sides negotiate over scheduling, including the union's wish to limit United's ability to make pilots work on their days off.United spokesman Joshua Freed said, "We're continuing to work with the Air Line Pilots Association on the industry-leading deal we have put on the table for our world-class pilots."Even if the unions and companies fail to reach agreements quickly, strikes are unlikely in the next few months — when millions of Americans hope to fly over summer vacation. Under U.S. law, airline and railroad workers can't legally strike, and companies can't lock them out, until federal mediators determine that further negotiations are pointless.The National Mediation Board rarely declares a dead end to bargaining, and even if it does, there is a no-strikes "cooling-off" period during which the White House and Congress can block a walkout. That's what President Bill Clinton did minutes after pilots began striking against American in 1997. In December, President Joe Biden signed a bill that Congress passed to impose contract terms on freight railroad workers, ending a strike threat.The last strike at a U.S. carrier occurred at Spirit Airlines in 2010.Over the years, airline workers have conducted job actions that fell short of a strike but disrupted flights anyway. A federal judge fined the American Airlines pilots' union $45 million for a 1999 sickout that crippled the airline's operations, although the amount was later reduced.Related video above: FAA adds 169 new routes, making air travel more efficientArthur Wheaton, director of labor studies at Cornell University, said Congress would not permit an airline strike because of the economic harm it would cause, but unhappy pilots could still cause disruptions in other ways."They always have 'work to rule.' They could say, 'We're not working any overtime,'" Wheaton said. "I don't anticipate the pilots trying to screw up travel for everybody intentionally, but bargaining is about leverage and power ... having the ability to do that can be a negotiating tactic."Airlines are vulnerable to work-to-rule protests because they depend on finding pilots and flight attendants to pick up extra shifts during peak travel periods.Regardless of the legal hurdles to a walkout, unions believe that strike votes give them leverage during bargaining, and they have become more common. A shortage of pilots is also putting those unions in a particularly strong bargaining position.United has roughly 14,000 pilots, and the union expects at least 2,000 will picket Friday at 10 airports from Newark, New Jersey, to Los Angeles. The union is also distributing leaflets that highlight the pilots' desire for better work-life balance in their scheduling but make no mention of pay.