Via: The Atlantic:
Over the past decade, nearly all large U.S. airlines have shifted heavy maintenance work on their airplanes to repair shops thousands of miles away, in developing countries, where the mechanics who take the planes apart (completely) and put them back together (or almost) may not even be able to read or speak English. US Airways and Southwest fly planes to a maintenance facility in El Salvador. Delta sends planes to Mexico. United uses a shop in China. American still does much of its most intensive maintenance in-house in the U.S., but that is likely to change in the aftermath of the company's merger with US Airways.
The airlines are shipping this maintenance work offshore for the reason you'd expect: to cut labor costs. Mechanics in El Salvador, Mexico, China, and elsewhere earn a fraction of what mechanics in the U.S. do. In part because of this offshoring, the number of maintenance jobs at U.S. carriers has plummeted, from 72,000 in the year 2000 to fewer than 50,000 today.