Over 65,000 flights have taken off from US airports in the past six years when they should have been grounded because of incomplete or improper maintenance, leading to hundred of deaths, according to a new investigation.

USA Today said its investigation found that substandard repairs, unqualified mechanics and lax oversight by airlines and the Federal Aviation Administration are not unusual.

Its investigation included an analysis of government fines against airlines for maintenance violations and penalty letters sent to them that were obtained through the Freedom of Information Act.

It found that maintenance was a “cause, factor or finding” in 19 accidents in the past six years. In one of them, 88 people died when an Alaska Airlines crashed into the Pacific Ocean off California because of the failure of screw threads on a device. The NTSB concluded that the threads had not been properly lubricated.

In eight other accidents a total of 43 more people were killed, among them a mechanic working for Julie’s Aircraft Services in Texas in January 2006.

Key findings from the study found that airlines contract about 70 percent of their maintenance work to repair shops in the U.S. and abroad, where mistakes are often made by untrained personnel.

Airlines also disregard FAA inspectors’ findings to keep planes flying, use unapproved parts and perform their own sloppy maintenance work, according to FAA documents.

The FAA levied $28.2 million in fines and proposed fines against 25 US airlines for maintenance violations that occurred during the past six years. In many cases, planes operated for months before the FAA found maintenance deficiencies. Airlines continued to fly planes after the FAA found deficiencies in them.

In its defence, the FAA said it “sets an exceptionally high bar” for the required level of safety for airlines and that the fines indicate that problems were detected and corrected.

US airlines “regard safety as their highest responsibility,” and “their maintenance programs reflect that commitment to safety,” Elizabeth Merida, a spokeswoman for the Air Transport Association, said.

The ATA said member airlines haven’t had a fatal accident “attributable to maintenance” since the Alaska Airlines crash on 31 January 2000.