PARIS (AP) -- Airbus, which has delivered more airplanes than Boeing for the second year in a row, is about to unveil another No. 1: the world's largest passenger jet.

The A380, a four-aisle, four-engine, double-decker "superjumbo," will roll onto the tarmac Tuesday at Airbus headquarters in southern France, in a lavish ceremony attended by EU leaders and thousands of guests.

Sales have beat expectations so far, and most of the technical problems that have dogged the program have been resolved, at a price.

But the real sighs of relief won't be heard in Toulouse until later -- sometime before March 31, Airbus says _ when the A380 hauls its 280-metric ton (308-ton) frame aloft.

That's when the plane's engineers will begin to find out whether their gargantuan offspring lives up to the performance promises, as the first test-flight data streams in.

In a standard three-class cabin configuration, the A380 will carry 555 passengers _ one-third more than the plane it is designed to displace, the Boeing 747.

On a full tank, it will also carry them 5 percent further than Boeing's longest-range jumbo, Airbus claims, producing costs per passenger that are up to one-fifth below its rival's.

Meeting these targets has been "no picnic," Airbus CEO Noel Forgeard acknowledged Wednesday, when he also confirmed that the A380 is both over budget and slightly overweight.

Forgeard said the plane will weigh in about 1 percent heavier than its target of 277 metric tons (305 tons) but stressed it will still deliver on promised fuel efficiency and other guarantees, since the internal benchmark was deliberately overambitious.

He said the program's $1.9 billion overspend -- 18 percent of its $10.7 billion overall budget at current exchange rates -- would likely be trimmed by a renewed cost-cutting drive.

The struggle to meet weight targets accounts for much of the overspending, Airbus officials say. Jean-Claude Schoepf, head of the A380 final assembly line, said the problem became a headache early on.

"We found there was too much mass," Schoepf said. "We had to work pretty hard to get back to the specifications we'd committed ourselves to with our clients."

Parts went back to the drawing board to be meticulously pared down, without sacrificing strength. More carbon composites were introduced _ for example, in the horizontal struts that support the two cabin floors and hold the fuselage in shape.

By using chromate-free paint, engineers got the outer paintwork down to about 350 kilograms (770 pounds), Schoepf said. ''That's compared to 550 kilograms (1,210 pounds) for a plane of this size using other paints."

At the giant hangar where Schoepf and his 1,500 engineers and support staff work, wings, nose cones and fuselage sections arrive by road convoys after being transported by barges from Bordeaux, western France, where they come in from Airbus facilities in Spain, Britain, Germany and elsewhere in France.

By 2008, Schoepf plans to hire another 1,000 staff to boost the production rate to one A380 per week.

Airbus has 139 firm A380 orders from 13 airlines and freight companies, worth $39 billion before any discounts on the plane's $280 million list price. A new 747 costs up to $211 million before discounts.

The backlog will rise when UPS Inc. finalizes a deal to acquire 10 of the A380's freighter versions, with options on 10 more.

The European Aeronautic Defence and Space Co., which owns 80 percent of Airbus, says the A380 program will break even at about 250 sales.

Over the next 20 years, Airbus sees global demand for 1,250 A380-size behemoths to shuttle passengers between the world's largest airports, which serve as connecting hubs for flights to less busy destinations.

More than half the new superjumbos will fly between just 10 major airports, Airbus forecasts, mainly in Asia. Singapore Airlines Ltd. is scheduled to become the first carrier to operate the A380, in the second half of 2006.

Chicago-based Boeing Co., like Airbus, expects overall air passenger traffic to increase threefold over the next two decades. But Boeing forecasts only ''a few hundred'' sales of very large planes, as travelers reject stopovers in favor of direct service aboard smaller long-range jets -- like its fuel-efficient 7E7 Dreamliner, due to enter service in 2008.

"The data shows unquestionably that passengers, when they can, want to fly from wherever they are to wherever they're going, without having to connect in a hub,'' said Boeing spokesman Todd Blecher. "The A380 is flying into the headwind of reality."

But Boeing, which delivered 285 planes in 2004 to Airbus' 320, is hedging its bets. It announced plans last year for a larger, 450-seat 747, despite having dismissed the case for a bigger plane since Airbus began discussing the concept in 1991. A launch decision is expected in mid-2005.

Whichever way the wind blows in Toulouse on Tuesday, the A380 seems certain to become a milestone in civil aviation history alongside the 747 and Concorde. Unlike the supersonic Concorde, however, whose claim to fame was how fast it crossed the Atlantic, this latest fruit of European aerospace cooperation will ultimately be judged on how fast it makes money.