Washington — Mechanics agree that a properly maintained airplane can fly safely for decades.

But safety is one thing; cost is another. Older planes guzzle fuel and are expensive to inspect, mantain and update. One benefit of the proposed merger of Delta Air Lines Inc. and Northwest Airlines Corp. may be the opportunity to cut costs by combining their 975 planes into one fleet and retiring the oldest aircraft.

"Both have aging fleets that are not particularly fuel-efficient," said Ernest Arvai, president of the Arvai Group, an airline consulting firm based in Windham, N.H. "The high cost of fuel has created difficulties in operating older aircraft profitably, and neither has the cash to pay for the new aircraft they need as quickly as possible."

By retiring the oldest workhorses, such as McDonnell Douglas DC-9s, the merged airlines can eliminate their worst fuel guzzlers. Earlier this year, Eagan, Minn.-based Northwest said it would ground about a third of its 103 DC-9s, which average more than 30 years of service.

By parking some of those planes, Northwest already has lowered its average fleet age to less than 19 years, but it still has the oldest fleet among the six largest U.S. carriers, according to Airsafe.com, a Web site that collects aviation data.

Delta spokesman Kent Landers said it's too early in the merger process to say just how many planes would be retired, but at least some DC-9s would continue flying because they have roughly 100 seats, a good size for some routes. The immediate goal is not simply to retire planes, but to have "an aircraft to fit every market so that we can most efficiently match customer demand," he said.

The problem of aging aircraft is common throughout the industry. U.S. airlines were due to start making big investments in modern aircraft as the new century dawned, only to run up against a recession and the 9/11 terrorist attacks involving four airliners. The subsequent industry downturn put carriers into a financial tailspin, ultimately driving Delta, Northwest and others into bankruptcy.

As the carriers started emerging from bankruptcy, they were hoping last year to begin investing profits into aircraft orders to modernize their fleets. But with oil prices doubling since last spring, no U.S. carrier has been able to muster the financial strength to start buying significant numbers of new planes.

"The U.S. fleet is aging," said Michael Romanowski, vice president of civil aviation for the Aerospace Industries Association, a trade group. "There have been a number of new purchases, but as a whole, given their financial restraints, they have not been aggressively buying new aircraft."

The inability to modernize fleets will darken the industry's financial outlook for years. Having old planes "is not a safety issue if the proper maintenance is done," Romanowski said. "But what is an issue is the cost. It's going to mean higher maintenance costs as the planes get older, and they will not be fuel-efficient."

Another problem is in a global market, U.S. carriers may well find themselves operating at a competitive disadvantage as foreign carriers leapfrog them, offering travelers quieter, more modern equipment.

Most orders being placed with Chicago-based Boeing Co. and French-based Airbus S.A.S., the world's two largest plane manufacturers, are coming from foreign companies. So even if U.S. carriers were to suddenly become profitable again, they would not be able to cut in line ahead of orders already placed from China, Singapore, Qatar, Brazil, Russia, India and the other countries.

Dubai-based Emirates Airlines alone has 193 new aircraft on order, more than its entire existing fleet of 115 planes, which has an average age of just five years, according to a study released this month by Ascend Worldwide Ltd., an aviation consulting firm based in London.

The study found that because of order backlogs and delays, new aircraft orders placed today would not be delivered until 2012 to 2015.

U.S. carriers "will need huge numbers of new aircraft in the next decade," Ascend managing director Gehan Talwatte said in a statement accompanying the study. "The difficulty is that without already having the orders in place, those aircraft are not going to be delivered any time soon."

Talwatte said another difficulty for U.S. carriers is the global credit crunch. "The tightening of the credit markets we have seen this year means lenders are much more risk-averse," leaving carriers less able to buy planes, he said.

Romanowski said that if U.S. airlines do return to profitability and can get financing, aircraft manufacturers will rise to the occasion and build more planes more quickly.

"They already are making them at a rapid rate," he said. If U.S. passengers find themselves stuck on aging airplanes in coming years, he said, "the constraint won't be that we can't make them."

— Shelley Emling of the Cox Newspapers' London bureau contributed to this story.