The Federal Aviation Administration (FAA) needs to be more aggressive about policing critical airline maintenance work performed by noncertified shops, a report by the Transportation Department's Inspector General concludes.

Cost-squeezed airlines are increasingly farming out more and more aircraft maintenance to independent contractors -- many of them in foreign countries -- that are not FAA-certified.

The practice has already been blamed for one fatal accident -- the January 2003 crash of a US Airways flight in Charlotte, N.C. Investigators said the non-certified mechanics who worked on the plane the day before the crash incorrectly adjusted a flight-control device that contributed to the accident, which killed all 21 people on board.

The National Transportation Safety Board (NTSB) found that the operator of the plane floying under US Airways colors, Air Midwest, did not provide the proper level of oversight.

It is the airlines' responsibility to oversee the work being done by the contractors but the report says that the level of scrutiny is not high enough to ensure that all work is up to FAA standards.

The report warns that non-certified facilities are "performing more significant work than anyone realized."

The Inspector General's office studied six U.S. airlines, which are not named in the report. It found that none were providing an adequate level of training. One airline provided 11 hours of classroom and video training while another simply provided a one-hour video. One airline simply mailed a workbook to each shop and required mechanics to sign a form saying they had read it.

JetBlue, Southwest, America West, Northwest and United are among the carriers who outsource major maintenance of their aircraft to contractors in other countries, The Wall Street Journal reported earlier.

To provide a broader overview, the Inspector General reviewed 19 airlines and found that one of then outsourced just 1 percent of its total maintenance while another outsourced 39 percent. At one unnamed airline, noncertified contractors performed 74 percent of critical repairs, those that require an inspection before the airplane goes back into service.

The Inspector General said the FAA needs to determine whether it should be allowing noncertified mechanics to perform so many critical maintenance functions. In response, the FAA said it believe its practices had had "no adverse impact on safety."