United Airlines will retire its final Boeing 747-400 by year-end — roughly a year sooner than planned — because the iconic aircraft is no longer cost effective to operate, the carrier said on Wednesday.

United still operates 20 of the hump-backed jets, using them on routes from San Francisco to Europe and Asia. It has flown the 747 — nicknamed the “Queen of the Skies” — since 1970, when it flew an earlier model from California to Hawaii, Scott Kirby, United’s president, told employees in a message.

“It’s a bittersweet milestone — this jumbo jet with its unmistakable silhouette once represented the state-of-the-art in air travel,” Kirby said. “Today, there are more fuel-efficient, cost-effective and reliable widebody aircraft that provide an updated inflight experience for our customers traveling on long-haul flights.”

As recently as two years ago, United had considered keeping some 747s until at least 2020. Early last year, it revised its plan, saying it would stop flying it by the end of 2018.

But United said Wednesday even that timeframe was too aggressive, telling employees the plane is no longer competitive. Though many of United’s 747s are less than 20 years old, the airline said the 747s require more attention from maintenance than other aircraft, and they receive lower customer satisfaction scores than other long-haul planes.

United also noted many other airlines have retired their 747s, making finding spare parts a challenge. Cathay Pacific recently retired its 747s, while KLM is reducing its fleet.

This year, United is taking delivery of 14 Boeing 777-300ER aircraft, which have a similar seat count to the 747. The 777s have two engines, rather than the 747’s four, but both aircraft can fly similar routes. And according to United, the 777-300ERs burn roughly 20 percent less fuel on a per-seat basis than the 747.

Delta Air Lines, the only other U.S. airlines with the Boeing 747-400, will also retire its feet later this year.

Tough Times for four-engine aircraft

The Boeing 747-400 is not the only double-decker aircraft in danger of fading away.

Since 2012, Boeing has been delivering a newer-generation aircraft called the 747-8, but with a few notable exceptions — Lufthansa, Korean Air and Air China have bought some — airlines have not been interested. Boeing is now only building six per year, and the company is mainly focused on cargo customers.

Airbus now builds its own iconic double-decker passenger jet, the Airbus A380. But though it is well-liked by passengers and recently has been more popular among airlines than the 747-8, it is not selling well, either.

John Leahy, an Airbus chief operating officer, said in a press briefing on Wednesday that the market for the Airbus A380 superjumbo “is getting softer,” but said the manufacturer will continue to produce the jet for the foreseeable future as it bets more airlines will want the aircraft for congested airports.

Airbus is building just one A380 each month in 2016, reducing productivity compared to 2016, when it delivered 28 aircraft.

“I’ll save you the questions,” Leahy told reporters. “Would I be pleased with more orders in ’16? Yes, I would have been pleased with more orders.”

Except for Emirates, the world’s biggest A380 operator, many carriers use the A380 as a niche aircraft, flying it only the most highly trafficked routes, such as London to Los Angeles, New York to Seoul and Shanghai to Frankfurt.

“Despite what I have to accept is a very slow commericial performance thus far, we have a future with this aircraft,” Fabrice Brégier, president of Airbus Commercial Aircraft, said during the briefing.

Airbus executives continue to say more airlines eventually will accept the A380, perhaps because they have no other choice. The manufacturer cites statistics suggesting the global number of air travelers is roughly doubling every 15 years. Eventually, the Airbus executives say, airlines will have no choice but to use larger planes, since many of the world’s most popular airports have limited capacity for new flights.

“They’re not going to be build another [London] Heathrow,” Leahy said Wednesday. “They’re not going to build another [Paris] Charles de Gaulle.”

Leahy also predicted airport congestion in Los Angeles, New York, and Singapore will persuade more airlines to consider the A380.

“The fact is, the A380’s day will come,” Leahy said. “I assure you of that.”

Photo Credit: United Airlines said Wednesday it will retire its Boeing 747 fleet by December, about a year earlier than planned. United Airlines