It was the best of times for Boeing Co. (BA) and Airbus, which announced record plane orders. But 2005 was close to the worst of times for many of the world's airlines, which lost an estimated $6 billion.

Booming growth in the Middle East and Asia, where airlines were profitable overall, helps explain the disconnect.

But some analysts are betting that the poor health of U.S. carriers and still fragile finances of airlines elsewhere will catch up with the plane makers soon, puncturing the order boom that sent Boeing shares soaring 35.7 percent last year.

So far this year, Boeing is up 5.5 percent, lagging the sector's Amex Defense index , which has gained 11 percent. Its stock, trading at nearly 22 times last year's earnings, has the second-richest valuation among U.S. aerospace and defense stocks, according to Reuters Analytics. Stewart & Stevenson Services Inc. , which makes army vehicles as well as equipment used in the power generation business, has the highest valuation.

Some analysts say that is too generous.

"We continue to hold a bearish view on commercial aerospace as we believe the high valuations do not reflect the risk of a faster-than-expected downturn due to the weak position of the airlines nor the fact that industry orders have peaked," George Shapiro, an analyst at Citigroup, said in a research note.

Growing competition among world airlines and elevated fuel prices could even place at risk the record 2,057 net orders that Boeing and Airbus won last year, some analysts warn.

Worldwide airline losses are seen narrowing to $4.2 billion this year from $6 billion in 2005, according to industry body the International Air Transport Association, which predicts they could return to profit in 2007.

"Going by history, which perhaps is the best gauge, there have been some levels of cancellations based on a variety of factors including financial health of individual airlines," said Gregg Agens, U.S. Aerospace & Defense leader at PricewaterhouseCoopers.

INDUSTRY'S WEAKLINGS

Boeing and Airbus both acknowledged that orders this year will probably lag 2005's breakneck pace. And Airbus is forecasting that the world airliner market could slump by more than half this year, to about 800 planes.

But whether sales dive or see a soft landing will depend largely on whether U.S. and European airlines call a halt to their recent order drought.

European carriers are closer to doing so than their U.S. rivals. Boeing Chief Executive James McNerney told analysts earlier in February that British Airways Plc and Deutsche Lufthansa could both look at potential orders this year.

He also said there were "incipient signs" of interest in new planes from U.S. traditional carriers.

A move by U.S. carriers to replace just the oldest 20 percent of their fleets would mean orders for 1,100 aircraft, double the deliveries Boeing is projecting for India in the next 20 years and half of that projected for China, brokerage D.A. Davidson said in a report.

But while U.S. discount airlines like Southwest Airlines Co. (LUV) and JetBlue Airways Corp. (JBLU) are investing heavily in new planes, traditional airlines like AMR Corp.'s

American Airlines and UAL Corp.'s United Airlines (UAUA) have adopted a "go slow" approach.

American's outgoing finance head, James Beer, recently said the No. 1 U.S. airline, whose fleet has one of the oldest average ages among U.S. carriers, has no plans to buy new planes anytime soon as it concentrates on paying down $20 billion in debt. UAL agrees.

"Our focus right now is on meeting or beating the business plan," the No. 2 U.S. carrier's Chief Financial Officer Jake Brace said in a recent conference call.

Bankrupt Northwest Airlines has the oldest fleet, with an average age of 18 years, according to D.A. Davidson, followed by American, with an average of 12.5 years.

"To a certain extent, high fuel prices sort of spur fleet renewal," said D.A. Davidson's JB Groh, with airlines like Northwest likely to feel particular pressure to replace older gas-guzzling McDonnell Douglas DC-9's and MD-80s.

Still, Groh said: "I think there will be a sharp drop-off" in orders this year from last year.