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Now departing from Kennedy International Airport: the Pan Am flying saucer.

Building Blocks How the city looks and feels — and why it got that way.

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With its elliptical four-acre parasol roof — cantilevered so far out that it almost seemed to be floating over the tarmac — Terminal 3 has long been a distinctive remnant of early jet travel and an emblem of Pan American World Airways, once considered the most glamorous American carrier (when “glamour” and “airline” could occupy the same sentence without irony). Pan Am called it the Worldport. Almost everyone else thought of it as a flying saucer.

By extending the roof 114 feet out from the terminal, through a cable system that made the top of the building look like an ensemble of small suspension bridges, Pan Am’s architects sought to protect passengers from the rain and snow. “It will eliminate the huddled dash through bad weather by extending the roof like a huge oblong umbrella over the aircraft parking space,” Richard Witkin wrote in The New York Times in 1957 as he described the plan.

Pan Am went under in 1991. Delta Air Lines then began using the Worldport. Last month, Delta decamped to greatly expanded space in Terminal 4 and closed Terminal 3 for good, after 53 years of service.

Seeing the handwriting on the wall, the National Trust for Historic Preservation placed the terminal on its annual roster of America’s 11 Most Endangered Places, released Wednesday, saying that the building “symbolizes America’s entry into the Jet Age.”

While the trust has no regulatory or legal power to impede demolition, its national imprimatur is often used by local preservationists to bolster their attempts at persuasion.

In the case of Terminal 3, however, “endangered” may be an understatement. “Doomed” is more like it.

Despite the existence of an impassioned grass-roots Save the Worldport campaign and an online petition that has attracted more than 3,000 supporters, there seems to be no cosmic scale on which the structure’s fate rests, capable of tipping one way or another.

Delta has every intention of demolishing Terminal 3. Workers are already removing asbestos and lead paint to prepare it for wrecking crews.

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By 2015, the airline plans to have turned the site into a parking zone for aircraft that cannot be accommodated or are not needed at the gates of Terminals 2 and 4, both of which are used by Delta. As it is, idle aircraft must be towed across the airport, said Leslie P. Scott, a spokeswoman for Delta. “This aircraft parking will drive a lot of operational efficiencies for us,” she said. “Planes will get to the gates quicker.”

The National Trust’s suggestion to incorporate the terminal into a connecting passageway between Terminals 2 and 4 has been rendered moot by Delta’s abandonment of the connector plan, Ms. Scott said.

The Port Authority of New York and New Jersey, which owns and runs Kennedy, could conceivably interfere with Delta’s plans. But it is a very willing partner in the redevelopment.

“The old Pan Am Worldport terminal at J.F.K. served this region for more than a half century, but is obsolete for 21st-century aviation purposes,” said Ron Marsico, a spokesman for the authority. “Unfortunately, J.F.K. is a land-constrained airport, and the space where Worldport is located cannot be set aside for preservation because it is needed for other aviation uses that will lead to job creation and economic growth.”

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In fact, the authority was persuaded more than a decade ago — sometimes kicking, if not screaming — to preserve and restore Eero Saarinen’s T.W.A. Flight Center, which is a landmark in every sense, including officially. Today, its low-slung, bird-winged profile is easily the most memorable work of architecture in the airport complex. However, it has not returned to full-time use. Mr. Marsico said the authority was negotiating with a hotelier.

The former National Airlines Sundrome, by I. M. Pei & Partners, was torn down in 2011. Protests were lodged, but no serious preservation effort was made. The building may have been too austere to engender the kind of affectionate embrace that Kalev Savi, Anthony Stramaglia and Lisa Turano Wojcik have thrown around Worldport in their Quixotic campaign to save it.

Mr. Savi’s mother worked for Pan Am, as he did part-time when he was in college. Mr. Stramaglia flew Pan Am frequently as a youth. And Ms. Wojcik’s father, Emanuel N. Turano of Ives, Turano & Gardner, was one of the architects of the Worldport, along with Walther Prokosch of Tippetts-Abbett-McCarthy-Stratton.

They professed encouragement on Wednesday.

“A year or so ago, Anthony and I felt like we were the only two people in the world who seemed to care that the flying saucer building was going to be demolished,” Mr. Savi wrote in an e-mail. “This announcement of having made the most endangered 11 sites in America list for 2013 is validation by the U.S.A.’s leading historical preservation society that the claims we have been making are true.”

Mr. Stramaglia said he hoped to inspire interest in a building that was designed to be a showcase. “Public apathy toward mundane things like air terminals these days, especially in today’s world,” he wrote, “make a trip to the airport more like a trip to the dentist.”

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An earlier version of this entry misspelled the given name of an architect of the Worldport. He was Walther Prokosch, not Walter.