You can hear terror in the voice of the captain on US Airways Express flight 5481.

"We have an emergency in the cockpit for Air Midwest 5481," she said on a cockpit voice recorder tape.

The plane, operated by Air Midwest, crashed in Charlotte in January 2003, killing all 21 people on board.

"It came own nose first into a fireball," said Tracy Right, who saw the crash.

Flight 5481 was headed to Greenville-Spartanburg, South Carolina, then to Raleigh-Durham International Airport. The NTSB blamed the crash, in part, on an outsourced maintenance company in Huntington, West Virginia. Two nights before the crash, a mechanic who had never worked on that kind of plane made a major mistake. He incorrectly rigged critical cables that control the plane.

Just last week, the president of Air Midwest, Greg Stephens, apologized to the people who lost loved ones in the crash.

"We are truly sorry and regret and apologize to everyone affected by this tragedy," he said.

Ernie Kiss has spent his career as an airline mechanic. Now he's in charged of safety at the nation's largest mechanics union. When Steve Daniels met him at RDU International Airport, he told Eyewitness News what his union members are seeing on the job.

"Approximately 84 to 90 percent of our technicians have seen mistakes made at third-party vendors," Kiss said. "We're having to re-work maintenance that is farmed out, and it's costing the carrier more to farm out than if we did it in house to start with."

In fact, our Eyewitness News investigation is uncovering disturbing safety issues at a TIMCO in Greensboro. United Airlines, Delta Airlines, Fed Ex and America West all outsourced maintenance work to TIMCO.

A source showed us pictures of a panel above the wing that flew off a United Airlines Boeing 757 on a flight leaving TIMCO.

The source says workers at TIMCO forgot to screw in panels during a maintenance overhaul. Another United plane – a wide-body Boeing 767 – also had serious safety problems after it left TIMCO, according to our source and documents obtained by Eyewitness News.

A panel under the wing nearly ripped off in flight. Cockpit warning lights indicated repeated problems with the slats and flaps on the wings. The cockpit control sticks were not working properly, and the emergency evacuation lighting system wasn't working properly.

"It sounds like it's a total quality management problem, there at TIMCO with their quality assurance and their lack of training," Kiss said.

We also obtained documents revealing problems on another United Boeing 767, which our source says was at TIMCO in December 2003. Pilots reported a series of cockpit warnings connected to the slats and flaps on the wings. Our source says another 767 had problems with the emergency evacuation lighting after going to TIMCO last October.

Back in 2001, the FAA fined United Airlines for work that TIMCO did on a Boeing 737. The report said TIMCO "failed to properly re-install fuel system components, rendering the aircraft unairworthy."

TIMCO would not answer our questions about the documents we obtained. They did release a statement saying TIMCO is an industry leader in safety:

"In fact, our safety standards surpass those required by both the FAA and commercial airlines. TIMCO performs millions of maintenance tasks annually. Just like airline maintenance operations, we are not immune to an infrequent service issue. If there are service issues, we are quick to investigate to determine the root cause and to implement corrections actions."

During our investigation, we discovered the NTSB has blamed maintenance outsourcing at other companies in several other crashes, including the ValuJet crash in Miami in 1996 and the crash of an Emery cargo plane in Sacramento, Calif., in February 2000.

"It doesn't cross anybody's mind that these things are being maintained by unlicensed technicians at a third-party vendor," Ernie Kiss said. "I don't think the American flying public knows that."

Greg Stephens, the man whose company operated the US Airways Express flight that crashed in Charlotte, says his airline has learned some tough lessons.

"We have taken substantial measures to prevent similar accidents and incidents in the future, so that your losses will not have been suffered in vain," he said. Stephens also said his company is following all of the NTSB safety recommendations that emerged from the crash investigation.

United Airlines says in a statement:

"We have worked cooperatively with TIMCO for maintenance work for years. At no time has the safety of United's passengers, employees or aircraft ever been compromised. Safety -- the cornerstone of United's business -- is the company's number one priority. Outsourced and internal maintenance operations are in strict compliance with United Airlines' FAA-approved maintenance program and all applicable Federal Aviation Regulations. FAA safety inspectors and United's own Quality Assurance division also provide additional oversight to our maintenance programs."