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The cause of the fatal plane crash in South Korea last month is still to be determined, but it thrust into the spotlight a subject little understood by the general public: how airplanes are maintained.

Jeju Air, the airline that operated the flight in which 179 people died, said that checks before the flight had raised no concerns and that the jet, a Boeing 737-800 that was about 15 years old, had no history of accidents.

While there is no evidence that poor upkeep played a role in the crash, experts say that the plane’s repair history, which includes work performed by maintenance firms in other countries, will be an important part of the crash investigation, as is typically the case.

Here’s what you should know about how airplanes are maintained.

The Basics

Airline executives, pilots, mechanics, regulators and others play vital, and overlapping, roles in taking care of commercial jets.

At the front line is a practice that will be familiar to many travelers even if they don’t know what it is called: line maintenance, which involves regular inspections and repairs between flights. Before a jet takes off, pilots perform visual and other inspections for damage or other problems. Airline mechanics also conduct periodic checks. Often, a mechanic can troubleshoot a problem quickly, fixing it or safely patching it up until the issue can be addressed later. Serious problems can result in a long delay or cancellation.

In addition, every part of a plane, down to the nuts and bolts, is subjected to routine inspections, repair or replacement. Those checks are typically scheduled based on how many hours the plane has been used, the number of flights, time or some combination of those factors. These inspections are overseen by aviation authorities around the world, with many following the lead of regulators at the Federal Aviation Administration or its European Union counterpart.

When it comes to more intensive maintenance that might take a plane out of service for days or weeks, the world’s largest airlines perform much of that work on their own. Some, including Lufthansa and Delta Air Lines, even sell those services to others. But every airline outsources at least some work to the companies that make parts and planes, like Boeing and Airbus, or to third parties. Small carriers, especially, depend on the practice.

“If you look at the long tail of aircraft operators, the majority are very small, so it never made sense for them to invest the capital,” said Jonathan Berger, a founding partner of Alton Aviation, a consulting firm. “Outsourcing has been going on since the dawn of time.”

The Rise of Outsourcing

While it has always played a role in aviation, outsourcing accelerated in recent decades. In the United States, the share of spending on airline maintenance that was outsourced more than doubled from 1990 to 2011, rising from about 20 percent to more than 44 percent, according to a 2012 Congressional Research Service report. Airlines do not typically disclose such spending and few outside the industry track or estimate it.

Worldwide, airlines still conduct almost all of their own line maintenance. But they often send out more comprehensive work to companies that specialize in maintenance, repair and overhaul of planes. That work is usually divided into three categories, covering the airframe — or body, wings and tail of a plane — the engine and various other components.

Labor accounts for most of the cost of airframe maintenance, so airlines in wealthier countries often send that work to countries where wages are lower. Some U.S. airlines send planes to El Salvador, for example, and Western European carriers send them to Eastern Europe.

With engines, materials make up more of the cost of repairs, so outsourcing to lower-income countries doesn’t save airlines as much. Still, the much of that work is also outsourced, often to engine manufacturers, because fixing these large machines is expensive and complex, according to Mr. Berger. Most jet engines for large commercial planes are made by several U.S. and European companies.

The global network of maintenance providers is a growing market valued at more than $100 billion. Proponents say that it allows airlines, particularly budget carriers, to plan costs and focus on what they do best: plan, sell and operate flights efficiently. Airlines also benefit because they can turn to specialists who may focus on certain kinds of maintenance or aircraft.

Is Outsourcing Safe?

Many aviation industry experts say that outsourcing is safe and necessary and note that the F.A.A. still oversees maintenance of U.S. planes wherever it occurs. But some consumer groups and labor unions representing American mechanics have raised concerns about it.

“There is a quality issue and for the industry to deny that is just false,” said William J. McGee, who has spent decades as a passenger advocate before lawmakers and regulators and is a senior fellow at the American Economic Liberties Project, a progressive group.

In the 2000s and early 2010s, the Transportation Department’s inspector general looked into the F.A.A.’s oversight of foreign and domestic repair stations and found it lacking. Airline mechanics have reported finding obvious mistakes in planes returning from foreign repair shops. And their unions, including the International Brotherhood of Teamsters and the Transport Workers Union, have raised concerns that workers abroad are not subject to the same rigorous standards as those in the United States.

The F.A.A. recently addressed some of those concerns. This month, a new agency rule will start requiring foreign repair sites to carry out drug and alcohol tests of employees who perform certain safety-sensitive maintenance, affecting as many as 977 locations in 65 countries.

“This rule will ensure these employees are held to the equivalent high level of safety standards, regardless of where they are physically located,” Mike Whitaker, the outgoing F.A.A. administrator, said in a statement last month.

Defenders of outsourcing say that oversight by airlines and the F.A.A. today is rigorous, noting that despite a few shocking crashes and incidents, flying on commercial planes is far safer than other common modes of travel, including cars, buses or trains.

Maintenance Needs Are Rising

Despite a substantial slowdown early in the pandemic, air travel came roaring back much more quickly than the industry had expected. Airlines are now eager to expand and upgrade their fleets.

But Boeing and Airbus, the two big jet manufacturers, have struggled to churn out planes. Boeing has had to slow production in recent years, first after two fatal crashes in 2018 and 2019 of its most popular plane, the 737 Max. Last year, it was forced to do that again after a panel blew off a Max jet in flight, and in the fall it largely stopped production of the Max during a seven-week strike.

Airlines have also had to pull Airbus jets from service to carry out unexpected inspections of Pratt & Whitney engines after the engine manufacturer discovered a quality defect. Airbus and Boeing have also struggled with lingering pandemic-era problems, including shortages of parts and skilled workers.

The slow delivery of new planes has forced airlines to use older planes longer, which is safe but often requires more maintenance. Newer engines, which are packed with new technology that makes them more efficient, have also required more fixes and upkeep than expected.

But the number of people pursuing careers in airplane maintenance has lagged behind demand.

“Not only are the airlines hiring, but general aviation is hiring, corporate aviation is hiring,” Chuck Horning, a professor of aviation maintenance at Embry‑Riddle Aeronautical University, said referring to planes used by many businesses to transport people and goods. “And then you’ve got industries that didn’t even really exist when I graduated in 1986, like the space industry and unmanned aerial systems.”