While union membership has plummeted in most American workplaces, unions that represent transportation workers are generally holding steady.

        Memberships in transportation unions in general and the airline pilots union specifically are stable for a simple reason, said Leo Troy, who teaches at Rutgers University in Newark, N.J.

        “The planes don't fly without them,” said Mr. Troy, a professor of economics, labor economics and industrial relations.

        “It's a basic economic principle. If you have a worker in a factory who is somewhat skilled (and strikes), you can always find another. But how many airline pilots can you find to substitute for a pilot?”

        The U.S. Department of Labor reported in January that 13.5 percent of American workers in the public and private sectors were union members in 2000, the lowest level since 1983.

        Union membership in the transportation sector is at 26 percent, according to the Bureau of Labor Statistics. That's a rate almost three times as high as the 9 percent union membership at private companies in the United States.

        Although that figure is double the national average for public and private sector union membership, some transportation-related union rolls continue to slide.

        The reason for the dip is that some individuals, particularly in the South, buy tractor-trailers and create nonunionized small businesses that often have only one employee: the truck owner.

        “For interstate trucking, the Teamsters once had 2 million members,” Mr. Troy said. “That union has lost about a million members.”

        Kate Bronfenbrenner, director of labor education research at Cornell University, in Ithaca, N.Y., and a union expert for 15 years, said airline union membership remains high because they never stopped organizing.

        “Airline unions have been strategic, effective and aggressive,” she said. “They could have stuck their head in the sand in the 1960s and 1970s like many others, but they went after every new airline and tried to organize the workers.”

        The size of the potential bargaining unit at an airline made it attractive for organizers years ago.

        “What's so special about airlines is that if you win, you win big. If you're trying to organize a hotel company, there are hundreds and hundreds of hotels in the chain across the country,” she said.

        “To have an impact, you have to shut down a lot of hotels. When you organize pilots at a company, it's one big unit across the whole country. That's a great deal of power.”

        Because skilled employees such as pilots are difficult to replace, that gives airline pilot union representatives a strong hand when it comes to negotiations that raise wages and cut the workweek, he said.

        “The same is true for mechanics. It is somewhat easier to find a substitute, but it is far from easy,” he said.

        Experts say a handful of broad trends have stabilized membership rosters for transportation workers or at least prevented the erosion seen in other sectors:

        • For the airline industry, labor relations falls under the federal Railway Labor Act, which has no procedure to decertify or eliminate a union at a company, said David J. Walsh, associate professor department of management at Miami University.

        “The National Mediation Board is the entity under the Railway Labor Act for administering statutes, and that board tends to promote collective bargaining in an industry,” Mr. Walsh said.

        • Airline, railroad and trucking companies cannot respond to unions by shifting jobs offshore or to a third world country such as Mexico, as is the case with manufacturing, where unions are losing ground each time a plant closes.

        “Transportation is a labor-intensive industry and it has to operate in this country,” Mr. Walsh said.

        • Pilots have a strong occupational identity and historically have supported the union efforts of flight attendants and mechanics.

        “Baggage handling is not a very skilled occupation,” Mr. Troy said. “While the mechanics have strength as highly skilled workers that maintain the aircraft, they are affiliated with baggage handlers, so when mechanics go on strike, the baggage handlers get a free ride.

        “Flight attendants also get carried along depending upon the outlook of airline pilots (union).”

        Ms. Bronfenbrenner predicted a resurgence in labor organizing in the years to come in the American workplace. It's already under way at the airlines, she said.

        “Airline unions did that ahead of many unions,” she said. “They also faced deregulation before telecommunications and before trucking.”

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