US Airways officials went to the Paris Air Show last month and did something executives at most big U.S. carriers haven't done in years. They ordered planes: 92 new Airbuses at an estimated cost of $10 billion.

After a brutal half-decade in which the USA's airlines rang up $35 billion in losses, they're again profitable. But, US Airways' order notwithstanding, the little money they're making isn't nearly enough to cover what they'll need to rejuvenate and enlarge their fleets in the next two decades.

If projections by the plane-making giants, Airbus and Boeing, are to be believed, U.S. airlines could need as many as 9,000 new jets in the next 20 years. That could cost the industry nearly $1 trillion.

The high cost of fleet replacement and expansion will add to the financial pressures that already weigh heavily on the airlines, including the pressure to raise fares. But failure to launch the enormously expensive process soon could relegate the big U.S. carriers to flying antiquated aircraft for years while their foreign and low-cost domestic competitors fly newer, more fuel-efficient jets.

The planes flown by the USA's six largest conventional network carriers are now, on average, 13.3 years old, according to industry tracker Back Aviation Solutions. That's the oldest ever for this group of airlines.

"The situation with these very old planes is starting to get serious," says Michael Roach, a veteran consultant who co-founded America West Airlines in the 1980s. If the big U.S. carriers that haven't resumed buying planes don't do so soon, "they may never get out of the hole they're in."