Nine U.S. airlines outsourced more than 70 percent of their major aircraft maintenance last year, and federal aviation officials' oversight of repair facilities is lagging, according to a government report.

One-fourth of the outsourced maintenance is being handled by contractors overseas.

The Transportation Department's inspector general said the outsourcing, which has more than doubled in four years, was of concern because the Federal Aviation Administration has failed to closely track how much maintenance is farmed out and where it is performed.

Although the FAA has taken steps to improve, "the agency still faces challenges in determining where the most critical maintenance occurs and ensuring sufficient oversight," investigators said in the report issued this week.

In their effort to lower costs, the report said, airlines continue to shift their heavy airframe maintenance from their own in-house mechanics and engineers to hundreds of repair companies in the United States, Canada, Mexico and countries in Central America and Asia.

Nine major airlines examined by the inspector general outsourced 71 percent of their heavy air frame maintenance — repairs and servicing to an aircraft's body, wings and tail — in 2007, up from 34 percent in 2003. More than a quarter of that maintenance — 27 percent — was performed at foreign repair facilities.

The airlines examined in the report were AirTran Airways, Alaska Airlines, America West Airlines, Continental Airlines, Delta Air Lines, JetBlue Airways, Northwest Airlines, Southwest Airlines, and United Airlines. American Airlines, the nation's largest domestic carrier, was not included, the inspector general said, because it handles most maintenance in-house.

The FAA relies heavily on the airlines — and the repair facilities themselves — to make sure outsourced repairs meet the air safety standards and requirements of the individual airlines.

FAA requires each repair station to undergo a government inspection at least once a year, FAA spokesman Les Dorr said. The report says those inspections often are not being conducted by agency inspectors most familiar with standards and requirements of the airline whose planes are being repaired.

As much as five years lapsed between visits to some major maintenance facilities by inspectors assigned to individual airlines. Inspectors not assigned to a specific airline may not be familiar with the special maintenance requirements of that airline's planes, which are often customized.

The report cited a foreign facility, which repairs engines for an unidentified airline, that had not been inspected by an FAA inspector assigned to that airline in five years, a period in which the facility had repaired 39 of the air carrier's engines.

The report recommends FAA require airlines to provide more complete information on the extent and location of outsourced repairs, ensure air carriers and repair stations are better able to spot and correct problems, and improve the documentation of inspection results.

The FAA agreed it needs to do more. "We actually concur with all the inspector general's recommendations," Dorr said. "We have procedures in place that already address some of the recommendations, and we have some projects in progress that address others."

One safety expert, however, said the report underscores that FAA still has a long way to go toward resolving the outsourcing issue, which has been source of controversy for the agency for several years.

"What this report tells me is there is still a big problem with oversight — the FAA is not verifying that the oversight being provided by the air carriers is doing the job it's supposed to," said John Goglia, a former member of the National Transportation Safety Board.