CHARLOTTE, N.C. (AP) — A commuter airline's maintenance contracts will receive attention from federal investigators as they look into a plane crash here that killed 21 people, The Charlotte Observer reported Sunday.

US Airways Express Flight 5481 crashed seconds after takeoff Jan. 8 from Charlotte/Douglas International Airport. The National Transportation Safety Board is looking at the plane's weight with shifting baggage and malfunction of a tail flap as possible contributing causes of the crash.

But the crash has raised questions about what experts call the uncommon contract maintenance arrangement that Air Midwest, which flew the plane for US Airways, had for the Beech 1900 turboprop.

The NTSB will "most certainly" look at the maintenance arrangement in its crash investigation, the Observer reported, citing a source familiar with the probe.

The board has said the flight control cables on the plane were improperly adjusted two days before the crash during work performed at a hangar in Huntington, W.Va.

Air Midwest contracted with Raytheon Aerospace LLC to maintain its planes. In turn, Raytheon contracted with Structural, Modification and Repair Technicians Inc. of Edgewater, Fla., to provide staff. The shop had been open for six months and employed about a dozen people at the time of the crash.

The management structure is unusual because the Federal Aviation Administration allowed the hangar to operate under Air Midwest's operating certificate, but other companies provided all but one of the workers.

Federal regulations say Air Midwest was responsible for maintenance. It's unclear how closely Air Midwest monitored the maintenance work performed by the other companies.

Air Midwest, Raytheon and the Florida company have repeatedly declined to comment on issues related to the investigation.

Typically, the FAA relies on the airlines to oversee employees at their maintenance shops to make sure procedures are followed. But the standards don't specifically address situations in which the airline's presence may be extremely limited.

"There's nothing in the rule book that deals with this growing situation of contract workers," said Bart Crotty, an aviation maintenance and safety consultant in Springfield, Va.

Crotty said it's unusual for an airline to operate a maintenance base under its own certificate with the vast majority of workers provided by a contract agency. There are no statistics that show how rare it is.

A key question is whether the sequence of events that preceded the crash would have happened at a more traditional airline maintenance hangar, where the airline's permanent mechanics have their work overseen by a safety inspector with fewer additional responsibilities.

The mechanic who adjusted the flight-control cables on the turboprop was doing the job on that type of airplane for the first time, the Observer reported, citing sources. He expected an inspection by his supervisor, who both were hired through the Florida company, according to the newspaper. Whether the inspection ever occurred is part of the investigation.

The FAA should require that airlines retain the role of inspector when they contract out for maintenance, said Jim Burnett, NTSB chairman from 1982 to 1988.

"The FAA should say it's the responsibility of the airline, and not allow them to delegate that to anyone," Burnett said.

Third-party maintenance companies now do about half of all maintenance work for U.S. airlines. In 2001, major carriers spent $2.9 billion for outsourced aircraft maintenance — $1.3 billion more than they spent five years earlier, according to the Department of Transportation inspector general. mThe inspector general's office is now examining the FAA's oversight of third party repair stations.

Contracting out can enable airlines to sidestep costly union contracts. Some airline experts say it has become tougher for airlines to ensure maintenance work is done to their standards when the mechanics are contractually so far removed.

"The more removed you get from the maintenance, the more training it takes," says Sarah MacLeod, executive director of the Aeronautical Repair Station Association, a group that represents repair shops.