Qantas Airways and several other airlines altered their flight paths some time ago to avoid Ukrainian air space after fighting flared up in the region, raising questions about why others did not do the same.
The issue of whether to avoid flying over conflict zones has come into sharp focus after the downing of Malaysian Airlines Flight MH17 on Thursday, killing all 298 people on board.
International civil aviation regulators had imposed no restrictions on crossing an area where pro-Russian rebels are fighting Ukrainian forces, and the majority of carriers had continued to use a route popular with long-distance flights from Europe to southeast Asia.
But the fact that a handful of companies decided to circumnavigate the disputed territory underlined inconsistencies in airlines' approach to passenger safety.
Aviation experts said piecemeal and potentially conflicting advice from aviation regulators further confused the situation, and called for clearer guidance on which areas to avoid.
In addition to Qantas, Air Berlin, Asiana Airlines, Korean Air Lines and Taiwan's China Airlines decided to avoid Ukrainian airspace several months ago.
Cathay Pacific Airways changed its routes some time ago, but did not specify when, and a source familiar with the situation said British Airways had also been avoiding the area where the flight went down.
"Although the detour adds to flight time and cost, we have been making the detour for safety," said a spokeswoman for Asiana, which has been diverting its once-weekly cargo flight some 150 km (93 miles) below Ukrainian airspace since March 3.
The European Aviation Safety Agency did issue a safety bulletin, accompanied by recommendations from both the U.N.'s International Civil Aviation Organization (ICAO) and Brussels-based Eurocontrol, on April 3, advising that Crimean airspace should be avoided. Russia annexed Crimea from Ukraine in March.
But those directives did not apply to the airspace over Ukraine being traversed by Flight MH17 when it was brought down.
It was not immediately possible to verify which airlines had adopted which routes.
Flight paths and altitude vary according to factors such as weather, the amount of traffic on busy corridors and flight restrictions. Flying higher helps burn less fuel, but pilots do not always get to the altitude requested when airways are busy.
No undue risk
Malaysian Transport Minister Liow Tiong Lai said on Friday the national airline took no undue risk in flying over Ukraine, a route he stressed was approved by the ICAO and widely used by other airlines.
"We've flown this route for many years, it's safe and that's the reason why we are taking this route," Liow told a news conference where reporters repeatedly questioned why the airline chose to fly over a conflict zone.
The ICAO denied it had closed the route following the crash, saying it had no power to do so.
The International Air Transport Association (IATA) said airlines depended on governments and air traffic control authorities to advise which air space was available for flight, and that safety was carriers' "top priority".
German authorities warned the country's 144 aviation companies against lying over eastern Ukraine.
Geoff Dell, an accident investigation and safety specialist at CQ University in Australia, said airlines had their own intelligence operations which should be making decisions in such situations.
"It's blatantly obvious they shouldn't have been anywhere near it," Dell, who was working as a senior safety manager for Qantas during the first Gulf War, said of Flight MH17.
"Any sort of unrest breaks out, civil wars or such, you change your flight path so that you don't have to go anywhere near it. Of course it comes at a cost, because you have to fly further."
Suspected missile attack
Diverting planes is expensive for airlines, requiring more fuel and more time in the air and making some reluctant to do so without clear directives.
Flight MH17, en route from Amsterdam to Kuala Lumpur, was flying at around 33,000 feet over eastern Ukraine when it was brought down.
The United States said the plane was probably felled by a ground-launched missile strike, while Australian Prime Minister Tony Abbott said on Friday that Russian-backed rebels were responsible.
Immediately after the incident, several airlines announced that they were re-routing flights to avoid Ukrainian airspace, including Russian carrier Transaero.
As well as criticising some airlines, Dell and other experts said the onus was also on civil aviation regulators to provide clearer directives on avoiding conflict areas.
"The safety authorities themselves have much to answer for," said Chris Yates, of London-based aviation consulting firm Yates Consulting.
Ukrainian authorities had closed the flight path from the ground to around 32,000 feet, according to Eurocontrol, the agency responsible for coordinating European airspace. Flight MH17 was flying 1,000 feet above that.
After the crash, the International Air Transport Association (IATA) said that "based on information currently available it is believed that the airspace that the aircraft was traversing was not subject to restrictions".
Some conflict areas pose more of a threat than others.
In Ukraine, Soviet-era military hardware is common, and Kiev has accused pro-Moscow militants, aided by Russian military intelligence officers, of firing a long-range, Soviet-era SA-11 ground-to-air missile at the Malaysia Airlines plane.
On Monday, a Ukrainian Antonov AN-26 transport plane was downed in a rocket attack which Kiev said may have come from Russian territory.