Eastern Ukraine, where a Malaysia Airlines jet with 298 people aboard was apparently destroyed by a surface-to-air missile yesterday, is obviously a dangerous spot. Pro-Russia separatists and Ukrainian forces have been fighting there for months. In the past few days, two Ukrainian military planes have been shot down in the region.

So what was a passenger plane doing anywhere near the area? As it turns out, aviation authorities had addressed the Ukraine situation and taken precautions. They just didn’t think non-combatant planes flying at 33,000 feet were at risk.

Dealing With Risks

The aviation industry is used to changing conditions that can suddenly render airspace too dangerous to move through. Under a well-established system, authorities like the Federal Aviation Administration (FAA), the European Aviation Safety Agency (EASA), and the International Civil Aviation Organization (ICAO) can issue orders recommending or requiring that aircraft avoid certain areas. These restrictions can be applied for reasons including armed conflict, movement of VIPs like the president, and natural disasters. The 2010 eruption of the Iceland’s Eyjafjallajökull volcano spewed ash—which is terrible for jet engines—and caused a six-day ban on most flights in European airspace.

However, in cases of military risk, flight restrictions often apply only under certain altitudes. Planes are most exposed to ground-level threats when they’re taking off and landing—that's when they're within range of small arms and shoulder-mounted anti-aircraft missiles (this is why the FAA recommends planes avoid Kenya). It requires much more sophisticated weaponry to take out a plane flying 33,000 feet above the ground. The risk posed by that kind of military hardware explains why almost no one flies over Syria—a full-blown war zone—or North Korea.

The Unexpected Threat

The situation in Ukraine was thought to be fairly safe, and local authorities cleared planes through their airspace as long as they stayed above 32,000 feet. The Malaysia jet, a Boeing 777-200ER en route from Amsterdam to Kuala Lumpur, was at 33,000 feet when it was hit.

It’s likely no one thought planes flying that high were at risk, says Mikael Robertsson, a co-founder of FlightRadar24, a site that tracks planes around the world. Malaysia Airlines had flown this route repeatedly over the past several weeks without incident, as had other carriers. When MH17 was shot down, it was right next to two other passenger planes operated by Air India and Singapore Airlines.

Plus, flying around Ukraine is a major pain: The country is right in the middle of a common direct route between Europe and southeast Asia. Longer routes mean more fuel and more chances for delays, so cost-conscious airlines avoid such maneuvers whenever they can.

Now that it’s clear flying at cruising altitude over eastern Ukraine isn’t safe after all, everyone’s avoiding the area. According to media reports, Alitalia, Lufthansa, Air France, British Airways, Aeroflot, Turkish Airlines, and Transaero have all announced they will divert flights away from eastern Ukraine for the foreseeable future. The Ukrainian civil aviation authority has closed all airspace in the area to flights, and the FAA says U.S. carriers have voluntarily agreed not to fly in the airspace near the Ukraine-Russia border until further notice.