The likelihood that the Malaysian jetliner was hit by a missile could have profound consequences for the world's airlines.
Airlines might have to be more vigilant about avoiding trouble spots, making flights longer and causing them to burn more costly fuel. They may even be forced to reconsider many international routes.
Experts questioned the airline's decision to fly near the fighting, even as Malaysia's prime minister said that the plane's route from Amsterdam to Kuala Lumpur was declared safe by international aviation authorities.
"I find it pretty remarkable that a civil airline company - if this aircraft was on the flight plan - that they are flight-planning over an area like that," said Robert Francis, a former vice chairman of the National Transportation Safety Board.
In April, the U.S. Federal Aviation Administration cautioned airlines that Russia's claim to the airspace over Ukraine's Crimea could lead to conflicting air traffic control instructions.
A few weeks later, the FAA issued a tougher warning, telling pilots not to fly over the area, and the U.N.'s International Civil Aviation Organisation told governments to warn their airlines. Yesterday's crash, however, occurred outside the warning areas.
John Cox, a former airline pilot and accident investigator, said despite the cautions, the airspace was not closed. The Malaysia Airlines crew filed a flight plan and "Russia and the Ukraine both accepted the airplane into their airspace," he said.
Rerouting planes around war zones costs airlines money, as the planes burn more expensive jet fuel. Aviation expert Norman Shanks said many airlines continued to fly over Ukraine because it offered a shorter route that saved money on fuel.
Airlines quickly changed some routes. Flight-tracking services showed dense traffic to the west, light traffic to the east, but very few planes over Ukraine.
Emirates airlines said that one of its jets bound for Ukraine's capital of Kiev turned around and returned to Dubai. The airline suspended all flights to Kiev indefinitely..
Germany's Lufthansa rerouted all overflights to avoid eastern Ukraine, although flights to Kiev and Odessa were not affected.