Editor's note: This is the first article in a series examining the growing trend of outsourced aircraft maintenance.

CHARLOTTE, N.C. -- The last maintenance-related fatal commercial aviation accident involving a major U.S. carrier occurred more than four years ago, when a Beech 1900D crashed just after takeoff from Charlotte, killing all 21 people on board.

Considering that more than 40 million flights have departed and arrived at their destinations since, domestic airlines have continued to build on the enviable record of safety that already existed prior to the incident in January 2003.

However, at the same time, a number of carriers have dramatically increased their dependence on outside contractors to perform their maintenance rather than relying on in-house employees, and that's raising not insignificant safety issues.

The most notable transformations have come at

United

(UAUA)

and

Northwest

(NWA)

, according to data from the government's Bureau of Transportation Statistics.

Last year, United outsourced 45% of its maintenance, spending $787 million. By contrast, in 2000, United outsourced only 16% of the work on its planes. Northwest, meanwhile, outsourced 70% of its maintenance in 2006, paying $624 million, up from 24% six years earlier.

Only two carriers, American, a unit of

AMR

(AMR)

, and

Frontier

, resisted the trend. Both contracted out less than one-quarter of their work.

The rapid shift in the industry has tested the ability of the Federal Aviation Administration, which is charged with ensuring aviation safety, to keep up.

Spreading Blame

The Charlotte crash that killed 19 passengers and two crew members reflected a breakdown in the oversight of outsourced maintenance, not only by the FAA but also by an airline and a third-party contractor.

The Beech 1900D involved in the incident was being operated for the old US Airways by commuter carrier Air Midwest and maintained by contractor Raytheon Aerospace -- now L-3 Communications Vertex Aerospace -- at a Huntington, W.Va., hangar. Raytheon hired most of its mechanics from another firm, Structural Modification and Repair Technicians Inc., or SMART, of Edgewater, Fla.

Only two days before the crash, a mechanic hired from SMART improperly adjusted the flight control cables on the plane, the National Transportation Safety Board said after it reviewed the crash.

Although licensed, the mechanic had never before adjusted the cables on a Beech 1900. His instructor also inspected his work. And the FAA inspector charged with overseeing the hangar, who was based in Wichita, Kan., had visited just once since it began operating.

The NTSB found multiple causes for the accident. One was a formula, since changed, that used inaccurate passenger weights to calculate an airplane's weight and balance before takeoff. Primarily, though, the NTSB focused on faulty maintenance procedures and inadequate oversight.

Blame was assigned to Air Midwest for lacking proper administration of its hangar, the Raytheon Aerospace inspector for failing to detect the incorrect rigging of cables and the FAA for a lack of oversight of Air Midwest's maintenance program.

"There were some big holes there," says consultant John Goglia, who served on the NTSB while it investigated the Charlotte crash. "The airline's repair station was staffed by people who didn't work for the airline. The FAA inspector sitting in Wichita watching that certificate was unable to see whether those people were trained or not trained. He might have raised the flag if he had."

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To be sure, the North Carolina accident was one event, but it does give rise to questions about the extent to which outside workers are used, how well they're qualified and to what degree they're monitored internally and externally.

The shift is continuing unabated, and the maintenance, repair and overhaul businesses, known in the industry as MRO, is booming. Worldwide, third-party maintenance is a $41 billion business, and it's growing at nearly 5% annually, according to recent estimates.

A survey by

Overhaul and Maintenance

, an industry trade magazine, predicted that the MRO market for commercially operated jets will reach $52 billion by 2012. Engine work accounts for 42% of the business, the magazine says, while airframe heavy maintenance makes up 21%.

Smaller players aren't the only ones getting the contracts, either. American,

Delta

(DAL) - Get Report

and United all do work for other airlines.

For instance, even though it has led the move by U.S. carriers to outsourcing, United did about $200 million worth of airframe maintenance for outside carriers in 2006. The carrier is now considering a spinoff of its maintenance division.

Concerns about the FAA's power to assure that planes are safe, particularly when maintenance is performed abroad, were addressed at a March hearing of the House Aviation Subcommittee.

"U.S. airlines are relying more heavily on foreign contractors to perform everything from routine maintenance to major overhauls," chairman Rep. Jerry Costello (D., Ill.) said at the time. "We must ensure that the FAA has a robust system to oversee maintenance work conducted outside of the U.S."

In a recent report, the Transportation Department inspector general says "the continuous growth in outsourcing underscores the need for the FAA to remain vigilant," adding that the agency lacks adequate systems to determine which repair stations airlines use for critical maintenance or to allocate inspectors to those facilities.

The FAA says it hasn't dropped the ball. "We still do oversight, but the complexion has changed," says Jim Ballough, FAA director of flight standards, in an interview. Increasingly, "inspectors travel from location to location to make sure work is being performed," he says. "No matter where the maintenance is being performed, oversight is provided."

For instance, Ballough says, FAA inspectors travel quarterly to San Salvador, El Salvador, to visit Aeroman, the maintenance division of Taca, the El Salvadoran airline that performs maintenance for carriers including

JetBlue

(JBLU) - Get Report

,

US Airways

(LCC)

and America West.

Improving Oversight

In response to the Charlotte accident, the FAA has been developing regulations that would better identify airline maintenance training rules, says agency spokesman Les Dorr. Other changes in FAA procedures and policies include a requirement that inspectors place special emphasis on outsourced maintenance.

The agency also established new guidelines for Air Midwest training documentation. It was Air Midwest, Ballough notes, that operated the Huntington hangar and had the responsibility to provide training for its mechanics.

"The regulatory infrastructure for that type of operation stays in place," he says. "

On the issue of training, the airline could have done better."

Goglia says the FAA has taken steps to address the issues that led to the Charlotte crash, but more work is needed. A key is that the FAA must ensure that inspectors make regular visits to maintenance bases, wherever they are, he says.

Additionally, rules are needed to firmly establish that airlines keep training records not only for their own employees, but also for repair station employees who work on their planes.

"Manpower places have grown like weeds," Goglia says. "The FAA needs to be sure it has access to the records of the personnel that they provide to maintenance organizations."