Congress last year mandated the Federal Aviation Administration step up oversight of the growing trend by U.S. carriers of outsourcing aircraft maintenance to foreign repair stations.

The 2012 aviation authorization (PL 112-95) requires the FAA to ensure that foreign repair stations are inspected to guarantee standards consistent with U.S. requirements. The law also requires foreign repair stations allowed to conduct FAA-sanctioned maintenance to administer drug and alcohol tests consistent with U.S. requirements. The issue is gaining new attention with the proposed merger of U.S. Airways and American Airlines — one of the last major domestic airlines to conduct most of its maintenance in-house.

Before 2001, most U.S. airlines performed the bulk of their aircraft maintenance themselves. But by 2011, 44 percent of maintenance dollars were spent on outside contractors, according to the Transportation Department. The agency’s inspector general said that 71 percent of the airframe heavy maintenance for nine major U.S. carriers in 2007 was outsourced, with 27 percent of the work going to foreign repair stations, where labor costs are lower.

North America and Europe still hold the biggest share of airline maintenance work, but Chinese government subsidies, along with investments by aircraft manufacturers Boeing Co. and Airbus, have turned China into a major player.

With easy proximity to the U.S., El Salvador — with three FAA-certified repair stations — handles major maintenance work for many American carriers. There has been concern about the quality of work at foreign stations since 1995, when the crash of a DC-9 flown by the defunct ValuJet was blamed on errors by a Turkish maintenance facility. But a Congressional Research Service report in December found little evidence to support a decline in safety in recent years as dependence on outsourcing has grown.

“Although some experts believe that safety is being compromised and the regulation and oversight of foreign repair stations needs to be improved, analyses of recent trends do not provide obvious evidence that maintenance outsourcing has adversely affected airline safety,” the CRS concluded.