There are 731 foreign repair shops certified by the F.A.A. around the globe. How qualified are the mechanics in these hundreds of places? It’s very hard to check. In the past, when heavy maintenance was performed on United’s planes at a huge hangar at San Francisco International Airport, a government inspector could easily drive a few minutes from an office in the Bay Area to make a surprise inspection. Today that maintenance work is done in Beijing. The inspectors responsible for checking on how Chinese workers service airplanes are based in Los Angeles, 6,500 miles away.

Lack of proximity is only part of the problem. To inspect any foreign repair station, the F.A.A. first must obtain permission from the foreign government where the facility is located. Then, after a visa is granted, the U.S. must inform that government when the F.A.A. inspector will be coming. So much for the element of surprise—the very core of any inspection process. That inspections have had the heart torn out of them should come as no surprise. It is the pattern that has beset the regulation of drugs, food, and everything else.

A facility in El Salvador By Rodrigo Flores/ImageBrief.

What effect does all this offshoring have on the airworthiness of the fleet? No one gathers data systematically on this question—which is worrying in itself—but you don’t have to look far in government documents and news reports to find incidents that bring your senses to an upright and locked position. In 2011, an Air France Airbus A340 that had undergone a major overhaul at a maintenance facility used by U.S. and European airlines in Xiamen, China, flew for five days with 30 screws missing from one of its wings. The plane traveled first to Paris and then to Boston, where mechanics discovered the problem. A year earlier, an Air France Boeing 747 that had undergone major maintenance at another Chinese facility was grounded after it was found that some of the plane’s exterior had been refinished with potentially flammable paint.

In 2013, yet another Air France aircraft, this one an Airbus A380 en route to Caracas from Paris, had to make an unscheduled landing in the Azores when all the toilets overflowed and two of the airplane’s high-frequency radios failed. The Air France pilots’ union said the incidents occurred on the airplane’s first commercial flight after heavy-maintenance work in China. The company that performed the work also does maintenance for American. (Air France has denied that the problems were associated with maintenance done in China.)

You don’t have to look far to find incidents that bring your senses to an upright and locked position.

In 2009, a US Airways Boeing 737 jet carrying passengers from Omaha to Phoenix had to make an emergency landing in Denver when a high-pitched whistling sound in the cabin signaled that the seal around the main cabin door had begun to fail. It was later discovered that mechanics at Aeroman’s El Salvador facility had installed a key component of the door backward. In another incident, Aeroman mechanics crossed wires that connect the cockpit gauges and the airplane’s engines, a potentially catastrophic error that, in the words of a 2012 Congressional Research Service report, “could cause a pilot to shut down the wrong engine if engine trouble was suspected.”

In 2007, a China Airlines Boeing 737 took off from Taiwan and landed in Okinawa only to catch fire and explode shortly after taxiing to a gate. Miraculously, all 165 people on board escaped without serious injury. Investigators later concluded that during maintenance work in Taiwan mechanics had failed to attach a washer to part of the right wing assembly, allowing a bolt to come loose and puncture a fuel tank. China Airlines does maintenance work for about 20 other carriers.

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Airline mechanics at U.S. airports who perform routine safety checks and maintenance tasks before an airplane takes off report that they are discovering slipshod work done by overseas repair shops. American Airlines mechanics contended in a lawsuit last January that they had been disciplined by management for reporting numerous safety violations they uncovered on airplanes that had recently been serviced in China. Mechanics in Dallas said they had discovered cracked engine pylons, defective doors, and expired oxygen canisters, damage that had simply been painted over, and missing equipment, among other violations. An American spokesperson denied the allegations, contending that the airline’s “maintenance programs, practices, procedures and overall compliance and safety are second to none.” Citing a lack of jurisdiction, a federal judge dismissed the lawsuit. The F.A.A., however, is investigating the allegations.