Airbus may have taken the wraps off the world's biggest jetliner, but the first airline to operate the A380 remains tight-lipped about how it will use all that space.

Singapore Airlines built a mockup at its training center in Singapore to develop and test new cabin features for the A380 -- it gets the first four planes delivered by Airbus in 2006.

But the few allowed inside the mock-up must sign a non-disclosure agreement. And that includes the carrier's elite frequent fliers who have been brought in to offer their opinions about the A380 cabin innovations.

Airbus has advertised its new double-decker flagship as a cruise ship of the skies, even a flying hotel with duty-free shops, boasting that the design offers nearly 50 percent more passenger space than Boeing's aging 747.

But airlines that have ordered the A380 -- other than possibly Virgin Atlantic -- are not expected to offer pie-in-the-sky luxury that would sap revenue. The airline industry has been there before. Thirty-five years ago, a couple of airlines put piano bars in their new 747s. But later, in an era of deregulation and increased competition, they were replaced with money-making seats.

"This was a battle we fought very hard here at SIA," Yeoh Teng Kwong, senior manager of cabin innovation for Singapore Airlines, said of an internal debate about seat count vs. luxury features on the carrier's A380s. "We have struck a good balance."

Recognized as one of the world's leading airlines for in-flight comfort and amenities, Singapore Airlines will have 490 seats on its A380s. That's 65 fewer than the 555 passengers Airbus says can be accommodated in a typical three-class cabin layout.

"Our A380 product will be a very pleasant surprise to the market," Kwong said, without giving away any secrets.

Emirates will be the second A380 operator next year.

Singapore and Emirates are competitors, so no one at either carrier wants to reveal too much about their respective cabin innovations for the superjumbo.

"We can't play around with revenue-earning capability," Emirates President Tim Clark said in an interview last year. "We bought the aircraft on a seat count that saw most of the space occupied. We can't mess around with that."

But he said, without elaboration, that the A380s will have "a lot of additional features that will be a first for the civil aviation industry."

Emirates, which has placed 41 firm orders for the A380 -- by far the most of any customer -- is expected to configure its initial A380s with 480 to 490 seats in three classes, and about 650 seats in two classes.

Emirates, Singapore and other A380 operators will use the A380 upper deck for business and premium seating, with the main deck for economy.

First class on the Emirates A380s, Clark said, will be "more of the same, only better."

On its new long-haul Airbus A340-500s, Emirates provides first-class accommodations that resemble private rooms. "We created cabins within the cabin," Clark said. "You can close the door and sleep in private." First-class passengers phone the galley for meals.

"I thought of that," Clark said. "That's what I do in a hotel when I'm hungry. I ring room service."

Qantas, the next airline to receive an A380 after Emirates, revealed some cabin details last week on its Web site. The big Qantas jets, which will operate initially between Los Angeles and Melbourne, Australia, will have 501 seats. Cabin features will include special lounge areas in first, business and economy classes; facilities for business meetings and business presentations; and video on demand, with larger entertainment screens in all seats.

First-class A380 passengers, Qantas said, will have "greater space, comfort and privacy."

Virgin Atlantic was to have been the second airline after Singapore to take delivery of an A380. But it has delayed taking its first plane until 2008.

Last week, when Airbus unveiled the first A380 in a ceremony in Toulouse, France, Virgin's colorful chairman, Richard Branson, told reporters his 380s will have a gym where passengers can work out, as well as a beauty parlor and casino.

That remains to be seen.

Given the danger of unexpected in-flight turbulence, the Federal Aviation Administration could have something to say about gyms and such on A380s that fly into the United States.

And there is also the extra weight.

Virgin delayed taking its A380s in part to give suppliers more time to develop lighter cabin furnishings.

John Leahy, chief commercial officer for Airbus, who has never been shy about extolling the virtues of the A380, acknowledged the weighty issue of too much luxury.

"What annoys me in this industry, and Boeing has the same problem," he said, "is that you fight tooth and nail for every kilo of weight in the airframe, and you do the same thing for every kilo of weight on an engine, and then you congratulate yourself that you got a couple hundred pounds out. Then you see some of these new first-class seats that weigh hundreds of pounds."

Joe Sutter, Boeing's famous engineer who led the design team in the 1960s for Boeing's 747, recalled that Juan Trippe, the chairman of Pan American Airlines, launch customer for the 747, wanted a window in the nose so first-class passengers could see where the plane was going.

Sutter made some calculations and informed Trippe the extra weight would be the equivalent of eliminating 2 1/2 passengers.

There is no nose window on the 747. Nor is there a Tiger Lounge.

Boeing built a full-size mockup of that 747 lounge in the early 1970s, with the furniture in tiger skin, to show customers the possibilities of passenger comfort on its new jumbojet.

Early Boeing ads for the 747 talked about onboard movie theaters, a library, staterooms for passengers and conference rooms for business travelers. Even private lavatories.

Most of that never happened, with the exception of a couple airlines that for a short time had piano bars in the 747's signature "hump" that serves as an upper deck.

Today, the 747-400, the latest version of the plane, can carry about 416 passengers in three classes. Singapore Airlines has configured its 747-400s to seat 378 passengers.

Boeing is considering a 747 Advanced, which would have a longer fuselage and use the more fuel-efficient engines under development for its new midsize jet, the 7E7.

The 747 Advanced would carry about 35 more passengers than the 747-400. It would have room above the main cabin for innovative "sky suites," said Jeff Peace, Boeing's vice president of 747 derivative programs. The suites could be private bunks for sleeping or meeting rooms.

While many international airlines are providing passengers more luxury these days in first and business classes, some are beginning to offer more for economy passengers, too.

Japan's All Nippon Airways, the launch customer for Boeing's 7E7, has configured its new extended range Boeing 777-300ER jets with a four-class cabin. There are 174 seats in economy, and 24 in "super" economy. The supereconomy section has eight-abreast seating, one fewer seat per row than in the standard economy cabin. Passengers pay more for the wider seats and additional legroom, although not as much as a business-class seat would cost.

Kwong, the Singapore Airlines executive for product innovation, said the airline might offer more than one type of economy class on its A380s.

Klaus Brauer, Boeing's highly regarded interior specialist, likes this new focus on the economy cabin.

"Airlines are doing a lot with premium concepts," he said. "But the big breakthrough has to be in economy class. ... I sincerely believe the industry has to go that way."

Tourists who fly economy class generally prefer cheaper fares over comfort. But business travelers in economy want wider seats and more leg room.

"Right now, most business travelers in economy class don't get anything more than those who pay the discount fares," Brauer said.

He is helping lead the Boeing effort to give passengers a new kind of flying experience on the 7E7, due to enter service with All Nippon in 2008.

The 7E7 windows will be about 40 percent bigger than those on passengers jets today. Instead of pull down shades, the windows can be darkened electronically.

Fluorescent lighting will be gone. The 7E7 cabin will have light-emitting diodes, or LEDS. It will be more pleasing and natural and can be changed to provide different "moods."

Emirates has mood lighting on its A340-500s jets. Virgin has this feature in its 747 premium cabins.

At the opposite end of the luxury spectrum is what All Nippon and Japan Airlines have done to a few of their 747-400s for high-density domestic routes in Japan.

They carry nearly 600 passengers in one class.

The A380 could seat up to 880 passengers in one class, according to Airbus.

So far, no airline that has ordered the jet has indicated it would consider this kind of cattle-car layout.

But Emirates and Virgin are pressing Airbus to develop a stretched version of the A380 that would seat about 140 more passengers than the planes they have ordered.