Most of the decisions about a route are taken by airlines. But they must avoid no-fly zones. The area where the Malaysian airliner crashed had a no-fly zone in place up to 32,000ft (9,754m). The airliner was flying at 33,000ft (10,058m).
There are also national aviation bodies to consider. For example, the US’s Federal Aviation Administration in April issued a Notam (notice to airmen) that prohibited US airliners from flying over the Crimean region of Ukraine and nearby areas of the Black Sea and the Sea of Azov. After the crash of MH17 it widened that to cover the whole of Ukraine.
British Airways has been avoiding eastern Ukraine for some time, the BBC understands. But many airlines continued to fly over it. According to Flight radar24, which monitors live flight paths, the airlines that most frequently flew over Donetsk in eastern Ukraine in the last week were: Aeroflot 86 (flights), Singapore Airlines 75, Ukraine International Airlines 62, Lufthansa 56, and Malaysian 48. It was not necessarily a risky approach. The chance of a rocket reaching above 32,000 feet was considered remote, says Sylvia Spruck Wrigley, author of Why Planes Crash.