The route that carries a dozen planes an hour high above eastern Ukraine is so popular it has a name: airway L980.
It is a primary pathway between the capitals of Europe and the mega-cities of Asia — Tokyo, Hong Kong, Mumbai, and Singapore.
Though the skies seem like a boundless, borderless domain, the paths flown by commercial aircraft are well beaten, constricted by a desire to conserve fuel and a system of waypoints that stand like figurative mileposts at 35,000 feet.
‘‘Typically, that flight plan doesn’t change much,’’ said Kees Rietsema, a professor at Embry-Riddle Aeronautical University in Phoenix. ‘‘If that airline flies every day from Amsterdam to Kuala Lumpur, they fly the same route every day.’’
The only variation — and Malaysia Airlines Flight 17 flew about 200 miles north of its normal path Thursday — would be if the pilot encounters bad weather, he said.
‘‘The airline is responsible for planning the route of an airplane, not the pilot,’’ Rietsema said. ‘‘The airline stays abreast of [notifications and warnings].’’
And the airline would assess the risk of flying over areas of conflict.
‘‘Airlines overfly conflicted areas all the time, whether it’s in the Middle East or wherever it might be,’’ said Rietsema, an Air Force veteran and former commercial pilot who also served as a staff member at the Joint Chiefs of Staff. ‘‘From a strictly legal perspective, you can overfly that closed airspace and there’s no one telling you can’t be there, but on the other hand, I would say that you also are assuming the risk on your own that something untoward could happen.’’
By late Thursday, that aviation highway over Ukraine was empty, abandoned for fear that a missile fired from the conflicted territory below might take down another aircraft. Dozens of flights between Europe and Asia will be rerouted from airway L980 on Friday and likely for weeks to come.
But until Malaysia Air Flight 17, flying its daily route from Amsterdam to Kuala Lumpur, was struck from the sky Thursday, it was business as usual on airway L980. US intelligence said the plane was brought down by an antiaircraft missile.
‘‘It’s an established route, and there’s no war declared,’’ said Robert Benzon, a former investigator for the US National Transportation Safety Board. ‘‘There are commercial planes flying in and out of Iraq all the time.’’
The FAA said Thursday that US airlines had agreed not to pass over the Russia-Ukraine border following the crash.
Before Thursday’s crash, about 300 commercial flights a day flew at cruising altitude above eastern Ukraine.