WASHINGTON — Rigorous airline maintenance audits recently ordered by federal regulators are likely to yield the most headaches for travelers flying Northwest, American and United airlines.

Those carriers have the oldest fleets, on average. The older the jet, the more likely it requires time-consuming, and potentially flight-grounding, government-ordered inspections, analysts and regulators agree.

"Older planes will usually have more (airworthiness directives) simply because they've been around longer," said Federal Aviation Administration spokesman Les Dorr.

Northwest Airlines Corp.'s fleet of roughly 350 planes has the oldest average age at nearly 18 years old, followed by AMR Corp.'s American Airlines at 15 years, and UAL Corp.'s United Airlines at 13, according to the companies' most recent annual reports.

"We don't have any concerns related to the age of the fleet," said Northwest spokeswoman Tammy Lee, adding that the carrier is retiring about 10 of its oldest aircraft this year.

Northwest is well positioned to pass the current review, having gone through a rigorous FAA maintenance oversight process after a mechanics strike in 2005, Lee said. Showing compliance with this FAA audit is "a voluminous process … (but) a procedural issue and record-keeping issue," she said.

Reports of shorted wires, evidence of worn-down power cables, and fuel system reviews conducted by the manufacturer, Boeing Co., led to the airworthiness directive on the MD-80 aircraft that were grounded last week by American, Alaska Airlines and other carriers, inconveniencing hundreds of thousands of travelers. That government order carried an effective date of Sept. 5, 2006, and airlines had 18 months to comply.

The first round of FAA audits over a two-week span last month — prompted by revelations that Southwest Airlines Co. flew dozens of planes that had missed inspections — checked 10 airworthiness directives that apply to each carrier's fleet. The second phase, which runs through June 30, will check 10 percent of the orders that apply to each airline's fleet.

American, which operates many different aircraft, said it was working to comply with about 180 different directives, according to its annual report filed in February. Northwest and United did not specify the amount. The FAA does not have an accurate count of how many apply to each carrier's fleet, Dorr said.

"We have no concerns about the age of our fleet in regards to maintenance," American spokesman Tim Wagner said in an e-mail Friday. "Our only concern about the age of our fleet, our MD-80s in particular, is that they are less fuel efficient than some of the more modern airplanes," which is why the carrier is taking delivery of some new Boeing 737-800s earlier than expected.

The nation's largest carrier canceled another 200 flights Saturday morning before returning all of its 300 grounded jets to service, bringing the total number of cancellations last week to nearly 3,300. The average age of American's MD-80s average 18 years old, Wagner said.

To be fair, flying on U.S. airlines has never been safer. The last U.S. crash of a jumbo jet was in November 2001, when an American Airlines flight plummeted into a New York City neighborhood, killing 265 people.

"We don't have old planes in the air," said Harlan Platt, a finance professor at Northeastern University in Boston who follows corporate turnarounds. The age of plane refers to the fuselage, while most of the parts are replaced every three to seven years, he added.

But as government scrutiny of safety procedures rises, flight delays and cancellations could get worse, particularly for carriers with older fleets, said Bob Harrell of New York-based travel and aviation consulting firm Harrell Associates. About 35 percent of the U.S. fleet is more than 25 years old, according to the International Air Transport Association.

United last month said it will ground and sell back to lessors 15 to 20 older aircraft that are less fuel-efficient than others in its 460-plane fleet. Megan McCarthy, a spokeswoman for the nation's second largest carrier, said Friday, "Our primary responsibility is the safety of our customers and our employees, and there is no obligation we take more seriously."