MUNICH — On Thursday, soon after the first news of the disappearance of Flight MH17 hit, a curious image appeared on the flight website. This service enables people to follow live, rather like a flight controller, where a plane is in a specific region, its direction and its altitude. On Friday afternoon, the points on the screen started forming a large arc over the eastern part of Ukraine.

Some planes, like an Emirates flight from Dubai to Kiev, turned around and went back to the airport from which they’d taken off. By now no civil flights are flying through that area at all. The question many people are asking is why only now?

At the time the Malaysian flight is presumed to have been shot down, flight restrictions were already in place over Ukraine, but MH17 was in a part of air space open for civil aviation.

As early as April 3, the U.S. Federal Aviation Administration (FAA) prohibited U.S. airlines from flying over Crimean air space and adjacent regions in southeastern Ukraine. MH17, however, was hit by rockets far further north.

Last Monday, Ukrainian authorities themselves had ordered that flights over the region should avoid certain altitudes. They were no longer allowed to fly between 26,000 and 32,000 feet, or under a good 10-kilometer altitude. MH17 was flying at 33,000 feet, just 300 meters above the closed airspace.

The Ukrainians had assumed that planes flying at that height were no longer within range of small portable rockets, even if someone were to take aim at them. Apparently, they didn’t take into account that the separatists have access to large surface-to-air rockets mounted on heavy vehicles.

Who closes what airspace to whom in a fairly complicated business. As a rule, a country’s authorities are responsible for their own country’s airspace. But as we’ve seen, foreign aviation authorities can also order airlines from their area of supervision to avoid regions they consider dangerous. In those cases, however, it’s rare for there to be a real flying ban. Various types of restrictions are much more likely. They tend to be issued internationally as so-called “Notices to Airmen, or “Notams” for short.

The FAA also plays an international role in this: Although responsible for U.S. airlines only, other authorities tend to follow their recommendations — though not always, as FAA decisions can also be politically influenced.

The costs of detours

When the authority prohibited U.S. airlines from flying over Crimea, it was less about Washington fearing an attack such as that on MH17 and more about wanting to send a signal to Moscow.

Notams are released by the International Civil Aviation Organization (ICAO), a specialized UN agency. They are generally valid for a maximum of three months. If they are not rescinded, then they automatically become standard.

Ukrainian air space is right on the path of major connections between Europe and Southeast Asia. Flying around it usually means significant additional fuel and other costs for airlines because this makes the routes significantly longer. For most airlines, fuel has become the single biggest cost.

The decision of Malaysia Airlines to use the airspace over Ukraine on Thursday was fully in line with international practice. Nearly all large Asian airlines and many European ones, including Lufthansa, hadn’t until then changed their routes significantly. Only Australia’s Qantas had made the decision to fly around the region.

The airlines now face a dilemma. It is in their interests to fly the cheapest route, but they do generally respect restrictions set by Notams or other means. The large carriers have their own departments that try to assess the security situations in various regions around the world. But these departments are often swamped, so the companies must also depend on local authorities.

As the case of Flight MH17 has shown, these can also be wrong – with fatal consequences.