The NTSB held a hearing on the matter on Monday. Correspondent Bob Orr reports on CBS News' own investigation.
It's a place passengers never see. But what happens inside the cavernous hangar at Denver International Airport can directly affect air safety.
Mechanics for Frontier Airlines recently began the task of stripping down an old Boeing 737. They'll spend more than 6,000 hours over 19 days to make sure the jet is safe for passengers again.
Jon Bartram, who's been working on airplanes for 40 years, runs the operation. After four years of paying outside companies to do this work, Frontier just hired 100 mechanics to do the job in-house.
"We do a better job doing it ourselves," Bartram said. "Our people are dedicated to our aircraft. It's not using someone else's mechanic to do it. It's our mechanic."
But not all airlines operate as Frontier does. About half of the maintenance work done on U.S. airplanes is done, not by the airlines themselves, but by independent repair stations.
Crashes and accidents have raised doubts about some of the independents. Investigators found shoddy work by outside repair stations contributed to the ValuJet crash in 1996 and to the crash of an Atlantic Southeast commuter plane the year before. With the 1995 accident, a repair contractor failed to find a crack in the plane's propeller.
The Federal Aviation Administration was hit hard for lax oversight. In response, hundreds more inspectors were hired and training was tightened.
"The quality of work in the airlines' maintenance facilities and the quality of work at contract maintenance facilities have drastically improved," said FAA's Ava Mims.
But, the FAA doesn't have enough inspectors to check every panel, bolt and wire on the thousands of planes that continuously cycle through hundreds of repair shops.
"The FAA cannot be looking over every inspector's shoulder," said aviation safety analyst Michael Boyd.
That means it's up to the airlines, their mechanics and inspectors to ensure that all repair work is properly done, Boyd said.
Added Boyd: "The scope of the job is so huge that the airlines have to operate on the honor system. Some of them do it very well and some of them as we have seen in the past few years do [it] very poorly."
It's a system that demands that airlines know how each plane is being maintained.
Frontier's Bartram said, "We know it's safe. We know it is airworthy and in some cases when you're depending on other people to do that, it is a little bit....Sometimes you don't know."
But that uncertainty hasn't slowed an industry trend. Outside maintenance is up 30 percent in the 1990s and it continues to grow.
For more about NTSB's investigation, go to its Web site.
©1999, CBS Worldwide Inc., All Rights Reserved