For generations of air travelers, Boeing’s B747 jet, the four-engined behemoth with its distinctive hump, epitomized elegance in long-haul flying, harkening back to an era when travel was more refined, less exasperating. That era is long gone, and the B747 is disappearing from the skies as well.

Only two U.S. airlines, Delta and United, still operate the B747, and both are phasing them out, in favor of more technologically advanced, fuel-efficient jets.

Delta is on record as having set a deadline of the end of this year to replace its seven remaining B747s with new planes.

And in an internal memo circulated this week, United chief Scott Kirby announced that the airline would accelerate the retirement of its six remaining B747s, moving up the final flight date from the end of 2018 to sometime this year. “As deeply connected as we all are to this iconic aircraft, the time has come to retire our 747 fleet from scheduled service. It’s a bittersweet milestone—this jumbo jet with its unmistakable silhouette once represented the state-of-the-art in air travel.”

Whatever the B747 was and is, it’s definitely no longer state-of-the-art. The planes consume around one gallon of fuel per second, or five gallons per mile. Similarly sized alternatives like the newer-generation B777 and Airbus A350 use around 20 percent less fuel, and are more reliable.

United took possession of its first B747 in 1970, for flights between California and Hawaii, and has operated almost 90 of the planes in the years since. The six remaining B747s are used for flights between San Francisco and destinations in Asia and Europe.

What becomes of retired B747s? In United’s case, some planes will be stored until they’re sold. Just as likely, given their age, they’ll be dismantled for parts, an undignified end for a plane that figured so prominently in the history of commercial aviation.

Although the numbers are dwindling, there will be B747s in the fleets of other airlines for years to come. But if you want to fly a jumbo operated by a U.S. carrier, today you have only two choices; and the clock is ticking.

Reader Reality Check

I have many fond memories of flying the B747, to Asia and to Europe. You?

After 20 years working in the travel industry, and almost that long writing about it, Tim Winship knows a thing or two about travel. Follow him on Twitter @twinship.

This article first appeared on SmarterTravel.com, where Tim is Editor-at-Large.