NEW YORK CITY, New York - Mechanics' unions are strongly criticizing American and Southwest Airlines after an eight-month CBS News investigation raised questions about the oversight of aircraft maintenance.

In interviews with 26 airline mechanics at Southwest and American Airlines, all expressed concerns about undo pressure to cut corners or not write up issues to get planes back in service faster.

The claims are consistent with FAA whistleblower complaints and at least 32 anonymous industry-wide reports since 2015.

A 2015 FAA whistleblower investigation substantiated allegations that American Airlines managers pressured six Chicago mechanics to "not record discrepancies, take short cuts… or improperly sign off on work which was not actually completed," warning: "it also appears the concerns may be much more prevalent across American's organization." 

Twenty-one FAA follow-up investigations were launched, relying, in part, on the airline's own internal reviews. Two resulted in letters of correction, reports CBS News correspondent Kris Van Cleave.  

Gary Santos, an American Airlines mechanic in New York, said he's risking his job by speaking to us on camera.

"Do you feel like the FAA is doing enough in its role as the oversight here to be policing these maintenance issues?" Van Cleave asked Santos.

"They're in an unenviable position because they have to both police the airline and promote the airline. The fact that the mechanics can be harassed and it's not considered a regulatory violation – that's a problem," Santos responded.

Mechanics can file complaints with their airline, their union or the FAA, but the FAA office investigating whistleblower complaints can only recommend other offices take follow-up action. CBS News has found indications that FAA investigators may be interpreting regulations differently. 
 
Complaints to the FAA alleging mechanics were pressured were substantiated -- essentially confirmed -- at American. But when different investigators found similar behavior at Southwest, it was not substantiated despite those investigators noting, "the motivation behind management questioning [mechanics]... when they discover anything outside the scope of a maintenance task…appears as a tool used to influence the relaxing of standards, to look the other way."

"Shouldn't the FAA all be on the same page about what the regulations are?" Van Cleave asked former NTSB member John Goglia.

"That's the hope,"  said. "There was efforts 25 years ago to try to get the FAA personnel consistent… there's still inconsistency, even today. The FAA was putting a lot of faith in the safety management systems… of the airlines."

U.S. aviation is currently experiencing an unparalleled period of safety, but in the last few years, the FAA changed how it regulates airlines. In 2015 the FAA moved from focusing on enforcement to compliance and relying heavily on the airline's own safety programs to meet FAA standards. Since 2014 the number of enforcement actions against airlines has dropped roughly 70 percent.

"Do you think there's a willingness by the FAA to come down hard on an airline?" Van Cleave asked.

"The inspectors in the FAA are out there doing their jobs when they can do it, but then you've got the oversight or the control of the top of the FAA, saying we're not going to persecute, we're not going to fine," one mechanic said.

American and Southwest are currently in contract negotiations with mechanics. Both said they do not tolerate a hostile work environment and the FAA oversight system is working.

"It's important that everybody understand at American if it's not safe, it's not flying," American Airlines senior vice president David Seymour said.

"I have the highest confidence in the work that… our mechanics do. And also that any issue that's brought up, any issue, is dealt with appropriately," Southwest Airlines senior director of safety management Dave Hunt said.

Several American mechanics agreed to speak on camera but asked we protect their identity for fear of retaliation. "They do retribution against mechanics that find problems," one mechanic told CBS News.

"Sooner or later, somebody's not going to see something because yesterday they were intimidated by what a manager or a supervisor might have said to them," another said.

The FAA declined our interview request, but in a statement to CBS News said: 

"The nation's aviation system is safer than ever. Commercial aviation remains the safest form of travel because multiple and redundant levels of safety are built into the system. 

Safety enforcement is never static. The FAA constantly works to improve consistency, safety data collection, and risk analysis. The U.S. has the largest, most diverse, and most complex airspace system in the world. Oversight is a dynamic process that requires the FAA and the airline industry to constantly strive for safety improvements. We welcome any opportunity to enhance what already is the safest aerospace system in the world. 

The FAA's enforcement is designed to identify and mitigate potential risks before they affect safety. We investigate all allegations of safety standards violations, regardless of the source. We continue to be involved in investigations related to both American and Southwest Airlines. If those safety allegations are substantiated, we will take swift and appropriate action. We cannot discuss specific details until those investigations are complete."

The DOT inspector general is currently investigating the FAA's oversight of maintenance programs at Allegiant, Southwest, and American.

The Transport Workers Union (Local 591) and Aircraft Mechanics Fraternal Association released a joint statement in response to CBS News' report. See Southwest Airline's statement to CBS News.