While travelers may be surprised that a commercial airline was flying over Eastern Ukraine at a time of a military conflict, commercial airlines often fly over hostile territory—unbeknown to those onboard. In fact, experts say no-fly zones for commercial aircraft can change daily or even hourly.

On Thursday, Malaysia Airlines Flight 17 crashed in the Donetsk region of Ukraine, killing all 298 people onboard, and on Friday U.S. officials said the plane was most likely downed by a surface-to-air missile. The Federal Aviation Administration added another Ukrainian region, Eastern Crimea, to its prohibited areas in its “Notice to Airmen.” “Events have indicated the potential for continued hazardous activities,” it said. On Friday, flight tracker app Flightradar24 said in a statement, “Several airlines that have been tracked over Ukraine as late as yesterday, claim they have been avoiding Ukraine for months.”

(See MarketWatch’s ongoing coverage of the Ukraine plane crash.)

The latest no-fly zone for commercial U.S. aircraft expands a prohibition of U.S. flight operations issued by the FAA over the Crimean region of Ukraine and adjacent areas of the Black Sea and the Sea of Azov. A previous Notice to Airmen prohibited flights in an area near where Flight MH17 crashed, but this only applied to flights between 26,000 and 32,000 feet. Flight MH17 originally filed a flight plan at 35,000 feet, Malaysia Airlines MY:MAS said in a statement, but was requested to fly at 33,000 feet by Ukraine air traffic control.

U.S. calls for cease-fire in Ukraine

“Civil aircraft fly over hostile parts of the world every day,” says Patrick Smith, an airline pilot and author of “Cockpit Confidential.” Dozens of commercial aircraft pass through Iraqi airspace every day, he adds; Smith himself flew directly over Baghdad recently on a commercial passenger flight headed to the U.S. “The midnight view of the city was beautiful,” he says. Individual countries determine what areas to restrict and regulators like the United Nations and Europe’s air traffic control body Eurocontrol can impose further restrictions.

As such, there’s a big difference between FAA instructions and actual closure of airspace. Although the FAA prohibited U.S.-registered jets from flying to the area where Flight MH17 crashed, for instance, airspace was still open for European and Asian airlines. On Thursday, Eurocontrol announced the closure of airspace over Eastern Ukraine. “Since the crash, the Ukrainian authorities have informed Eurocontrol of the closure of routes from the ground to unlimited (altitude) in Eastern Ukraine,” the organization said in a statement.

To be found among the FAA’s active notices about troubled regions: advice to “avoid” operating in the Damascus region of Syria, citing surface-to-air missile firings, as well as military activity in the vicinity of airfields. The FAA warns airlines flying in and around the Pyongyang region: “North Korea has a history of launching short-range and medium-range ballistic missiles with no warning.” It also says there’s “significant risk” to flight operations in Yemen “due to terrorist activities and civil unrest” there.

While all this might sound dangerous, most warnings from regulators don’t amount to an outright ban on commercial flights. Flying over hostile territory is routine, and easy-to-follow protocols are in place, restricting flights to particular routes or altitudes, Smith says. “We don’t just fly into dangerous airspace and say, ‘I hope nobody shoots at us,’” he says. “The locations and boundaries of restricted airspace will often change, but they are marked on maps and we receive detailed information as part of our flight planning paperwork. Thousands of commercial flights deal with this every day.”

But flying over these areas isn't without risks. In 1983, Korean Air Lines Flight 007—a Boeing 747 flying from New York to Seoul in South Korea that strayed off course—was shot down by a Soviet fighter jet, killing 269 people. In 1988, the U.S.S. Vincennes, a guided missile cruiser, shot down an Iranian Airbus A300 en route from Tehran to Dubai, killing 290 people. In 2001, Ukraine military accidentally shot down a Siberian Airlines plane traveling from Tel Aviv to Novosibirsk in Russia, killing 78 people.

Also see: Fabien Costeau: It may take years to find Flight 370.