If debris spotted in southern Indian Ocean does in fact turn out to be flight MH370, it will be thousands of kilometres from where it turned around over the Malay Peninsula.

The theory being considered most likely in this scenario is that some mechanical failure led to a drop of cabin pressure that rendered the crew and passengers unconscious, leaving the plane on a ghost flight for several hours.

At some point, though, the fuel would have run out.

An aviation expert said most new-generation aeroplanes would continue to glide, even after all the plane's fuel reserves had been exhausted.

"All the aeroplanes glide," Professor Jason Middleton, from the School of Aviation at the University of NSW, said.

"They'll go 10 (kilometres) along for every one down."

In other words, if flight MH370 ran out of fuel somewhere over the Indian Ocean at a cruising altitude of around 10km, it could have glided for 100km without fuel in the tanks or a conscious pilot in the cockpit.

In such a "ghost flight" scenario, the final descent into the ocean would depend largely on the weather, Professor Middleton said.

Even with the best pilots, fully alert, the conditions in the Indian Ocean would guarantee a messy crash into the waves in which the plane would likely cartwheel and break apart, he said.

Fairfax Media reported earlier today that the theory favoured for some days now by professional pilots on chat sites and blogs is that the pilots had an event on board that took out the communications and led to a slow or rapid decompression - rendering the crew incapable of making an emergency landing.

Pilots have only a few minutes to bring a plane down to below 4000 metres before the passengers and crew will become disoriented, then unconscious and eventually die.

In 1999 a Lear jet carrying professional golfer Payne Stewart flew for several hours with its passengers and crew unresponsive, before it ran out of fuel and crashed in a field in South Dakota.