As airlines sacrifice seat numbers for passenger comfort on Airbus' new A380, airports may not see quite the strategic benefits they had hoped for from the double-decker jetliner.

The A380 was initially sold as a plane able to seat 555 passengers on average, but early customers like Singapore Airlines and Qantas Airways are planning to carry some way below this level as they look to maximise space for big-spending premium-class passengers.

That's not ideal for hub airports like London's Heathrow and New York's John F. Kennedy. They and others have invested huge sums into accommodating the A380 based on the assumption it will help them expand passenger traffic without adding to congestion or breaching limits on aircraft movements.

Initial A380 commercial flights - which are due to start late this year - won't be carrying the 35 percent to 40 percent more passengers than Boeing Co’s 747 aircraft, as Airbus has envisaged. That's important because the charges airports levy on airlines are normally based on the number of passengers being flown in or out on each airplane. And, the more people there are waiting for flights, the more potential purchasers there are of tax-free goods, food and drinks.

Airport operations may be slowed down slightly each time an A380 lands owing to regulations designed to protect following airplanes from getting caught in hazardous turbulence known as wake vortex. Large jets will need a separation of an additional two nautical miles when trailing an A380 compared with rules for existing large planes like the 747. This rises to an additional four nautical mile spacing for light airplanes. This means airports won't be able to squeeze in as many flights in the minutes after an A380 lands as they do currently with a 747.

Airbus recently cut its formal design seat count for the A380 to 525 from 555, based on three cabin classes. This change was designed to reflect more realistically the layouts being adopted by airplane buyers, according to a company spokesman. There have long been expectations that some carriers might look to squeeze in as many as 800 economy-class passengers on shorter high-density routes, particularly in Asia.

Although having more seats cuts an airline's overall operating costs for each passenger per flight, carriers are increasingly looking to accommodate as many first or business-class passengers as possible.

These premium fliers generate much of the profits for big carriers like British Airways, so A380 operators are offering a swathe of products like private suites, flat beds, armchairs, self-service bars, sofas and entertainment systems to entice them.

Singapore Airlines plans to have 471 seats on its A380s, with the first service due to start late this year between Sydney and Singapore and a London service beginning next spring. Qantas, meanwhile, expects to have 450 seats on the A380s that are due for delivery from August 2008, including 14 seats in first class, 72 in business class, 32 in premium economy and 332 in economy. Dubai's Emirates Airline, the biggest A380 buyer with 47 on order, plans to have 489 seats on its low-density, long-range, three-class A380s, rising to 517 for medium-range routes and a higher-density, two-class, 644-seat version, also for medium-range routes.

Flag carriers typically carry 370 to 380 people on Boeing's current 747-400 models, although it's sold with a baseline seat count of 416 for three classes, according to a Boeing spokesman. Boeing's planned new version of the 747 - known as the 747-8 Intercontinental - features a longer fuselage and will be able to carry about 50 additional seats.

Aside from terminal stands and walkways, airports have also invested in runway and taxiway resurfacing and widening to accommodate the A380. The Airbus jet has a wingspan of about 80m and a maximum takeoff weight of 560 metric tonnes compared with 64.4m and 397 tonnes for Boeing's 747-400 model.

Meanwhile, Singapore Airlines, the first carrier in the world to fly the new superjumbo Airbus A380, has sold a pair of premium seats on the jet's inaugural flight for more than US$100,000.

Singapore Airlines is auctioning all seats on the A380's October 25 flight from Singapore to Sydney on eBay and donating the proceeds to charity.

Airbus' first delivery of the jet, with 471 seats in three classes, has been set for October 15.

The highest bid so far was by an Australian businessman who paid $100,380 for a pair of first-class seats.

The auction, which started last week, has raised about $400,000 in total so far. The first one-way economy-class seat sold for $5,000, and the first one-way business-class seat was purchased for $15,000.

Seats on the Sydney-Singapore return flight were to be auctioned yesterday.