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Why was Malaysia Airlines MH17 flying over Ukraine? Time, money

The likelihood that Malaysia Airlines Flight MH17 was downed by a missile has raised questions over whether the plane should have been flying over war-torn eastern Ukraine—and could have profound consequences for air travel.

The doomed Boeing 777 was following an air traffic routing between Europe and Asia that is issued by dozens of jets a day and was in a section of air space deemed safe by international aviation authorities.

Within hours of Thursday's disaster, all air space around eastern Ukraine was shut down. But some airlines had already been avoiding the area amid concerns over the deteriorating security. Two low-flying Ukrainian aircraft—a fighter jet and a military transport plane—were shot down by pro-separatist rebels earlier this week.

Aviation regulators—including the FAA—had issued a series of notices to pilots in recent weeks prohibiting air space very close to the crash site.

"This was a very commonly used route and passenger jets fly at high altitudes over many of the world's hotspots all the time," said Norman Shanks, professor of aviation security at Britain's Coventry University. "They chose the most direct and economic flight route possible, which keeps their fuel costs down and is something we expect as customers. They were no different from any other international airline."

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MH17 was flying at 33,000 feet when disaster struck—well above the trajectory of missiles commonly used by militias in ground conflict, and high enough that its routing was approved by the airline's flight planners, air traffic controllers and ultimately the pilots.

However, with pro-Russia separatists apparently in possession of surface-to-air missile, airlines might have to be more vigilant about avoiding trouble spots.

"This incident is unprecedented really," Greg Waldron, Asia managing editor Flight Global told the BBC. "A lot of the weapons used by separatists and other guerrilla groups simply don't have the range to get [to 33,000 feet], they don't have the accuracy to hit something like an airliner."

Why was MH17 over a conflict zone?

Flight plans are drawn up by airlines and pilots and submitted to air traffic controllers for approval. Ultimately, pilots fly the agreed routing—unless they receive or request adjustments from air traffic controllers while en route.

MH17's exact flight plan—a routing defined by a series of waypoints and air corridors—has not been confirmed but appears to have included airway L980, a busy section of air space that acted as an airborne freeway between northern Europe and southern Asia.

British Airways, Air France, Lufthansa and KLM are among the airlines to have used exactly the same routing over eastern Ukraine in recent days, according to website FlightRadar24.

"Fifteen out of 16 airlines in the Association of Asia Pacific Airlines fly this route over Ukraine," Malaysia's transport minister Liow Tiong Lai said Friday. "European airlines also use the same route, and traverse the same air space."

Airlines can choose to avoid these areas. Australia's Qantas stopped flying over Ukraine several months ago and shifted its London-Dubai route 400 miles to the south. Korean Air said it had rerouted cargo and passenger flights in early March amid the worsening situation over the Crimean peninsula.

Read MoreLive blog: US says Russian separatists hit MH17

On April 23, the FAA issued a Notice to Airman (NOTAM) prohibiting U.S. carriers from flying over the Crimean region and portions adjacent to the Black Sea and the Sea of Azov. It didn't cover the air space where MH17 crashed. "This action was taken due to the unilateral and illegal action by Russia to assert control over Crimean air space, including international air space administered by Ukraine, without agreement by the International Civil Aviation Organization (ICAO)," the FAA said in a statement. Similar notices were issued by other countries.

Another NOTAM was issued on Tuesday prohibiting flights in an expanded area close to where MH17 crashed—but crucially it only applied to operations between 26,000 feet and 32,000 feet.

The pilots of MH17 filed a flight plan asking to fly at 35,000 feet throughout Ukrainian air space, the airline said in a statement Friday. However, upon entering Ukrainian air space, MH17 was instructed by Ukrainian air traffic controllers to fly at 33,000ft.

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"MH17's flight plan was approved by Eurocontrol, who are solely responsible for determining civil aircraft flight paths over European air space," the Malaysia Airlines statement said. "Eurocontrol is the air navigation service provider for Europe and is governed under ICAO rules."

How could air travel change?

Airlines have rushed to assure passengers they are rerouting flights between Europe and Asia to avoid Ukraine entirely. In part, they have no choice because Ukraine quickly closed all air space over its eastern zone after news of MH17 broke.

Longer routings mean longer flights for passengers and higher fuel bills for airlines. Carriers may even be forced to reconsider many international routes. Airline stocks such as Lufthansa dipped in early trading in Europe Friday, Reuters reported, as major travel companies rerouted flights to avoid Ukraine.

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Thomas Routh, an aviation attorney in Chicago, said it was up to airlines and pilots to decide whether a flight will be safe for crew and passengers. "There are airlines flying through Afghanistan's air space every day," Routh said.

Some of the other places that the FAA also currently warns pilots to avoid parts of include Iran, Yemen, the Sinai peninsula and North Korea. Gulf-based carrier Emirates stopped flying over parts of Syria as a civil war there expanded.

Last month, a gunman in Pakistan fired on a jetliner that was landing in Peshawar, part of the country's volatile northwest region, killing a passenger and wounding two other people. Emirates suspended flights to Peshawar, and other carriers canceled some flights while they reviewed airport security.

Greg Raiff, an aviation consultant in New Hampshire, said that if airlines must avoid flying over all the world's hot spots, flight times would be extended, requiring extra fuel and pilots. That might make some routes uneconomical, forcing airlines to abandon them.

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No commercial airliner is fitted with missile defense technology because of the prohibitive cost relative to the risk. A 2008 Department for Homeland Security report outlined a feasibility study in which missile jamming technology was briefly fitted to three American Airlines Boeing 767s and to MD-10 cargo planes operated by FedEx.

—By NBC News' Alastair Jamieson. The Associated Press contributed to this report.