The search for Malaysia Airlines Flight 370 has rekindled a debate over the iconic "black box" flight recorder and whether it's time for aircraft to start live-streaming in-flight data in real time.

Civil aviation industry sources agree the technology exists for commercial airliners to immediately relay via satellite vital technical information otherwise compiled by a flight data recorder in the course of a flight.

But it's another question whether airlines, forever struggling to keep costs down in a highly competitive business, want to front up the money involved -- or even if it's truly worth the expense.

"There are no technical barriers ... and the cost barriers can be addressed," said Peter Goelz, a former managing director of the National Transportation Safety Board (NTSB), the US government agency that investigates major aviation accidents.

"But the reality is that air carriers don't want to do anything unless they're ordered to do it," he told AFP.

Commercial airliners typically carry two black boxes, which in fact are bright orange in color. One monitors cockpit conversations, the other records a vast array of technical data from airspeed to engine performance.

Whenever a crash occurs, investigators scramble to recover both devices -- and if the tragedy happens on terra firma, they typically find them in short order.

At sea, however, it's another story.

- Digital link yields clues -

By then investigators already had a vague clue what might have happened thanks to the doomed flight's Aircraft Communications Addressing and Reporting System (ACARS), a digital datalink for brief text messages.

It puts out limited information about location and airspeed -- but nothing compared to the several thousand parameters that a black box can monitor during the course of a flight.

Malaysia Airlines has said all its aircraft are ACARS-equipped, but it has so far declined to release whatever data it got from Flight 370.

Twelve years ago, US avionics manufacturer L-3 estimated it would cost $300 million a year for a global airline to transmit flight data in real time, Bloomberg Businessweek magazine reported.

But Goelz said "there's no reason to stream all the data all the time."

Systems could be programmed, for instance, to emit only a limited amount of data in normal circumstances -- and then put out lots of data when an in-flight abnormality is detected.

The main hurdle is getting airlines to invest in such systems. Goelz said the onus is on governments to make them mandatory, as in the case of mid-air collision avoidance systems and smoke detectors in cargo holds.

"The technology does exist, but the question is: why spend the money?" added John Cox, a former airline captain who is chief executive officer of aviation consultancy Safety Operating Systems in Washington.

Not only might there be "massive amounts of data" to be managed, but there would also be a potential for misinterpretation and misuse, on top of data protection concerns, he told AFP.

"If you look back in history, it's not as if we have a large number of these (lost) airplanes that we don't find... (and) it is not as though we are not finding the cause of aircraft accidents using the technology that we have."

No idea where to look: the hunt for MH370
Phu Quoc, Vietnam (AFP) March 11, 2014 - Helicopters and planes criss-cross the sky as scores of boats search below -- but officials say the multi-national hunt for missing flight MH370 is like looking for a needle in a haystack.

Malaysia on Monday doubled the search radius to 100 nautical miles (equivalent to 185 kilometres) around the point where Malaysia Airlines MH370 disappeared from radar over the South China Sea early Saturday.

"The biggest problem is just knowing where to look -- especially at night," Vo Van Tuan, a top Vietnamese military officer who is leading Vietnam's search effort, told AFP.

The vastness of the search zone reflects authorities' bafflement over the plane's disappearance. On the fourth day of searching, the operation had grown to involve 42 ships and 35 aircraft from Southeast Asian countries, Australia, China, New Zealand and the United States. Japan said Tuesday it was sending a plane to join the search efforts.

Vietnam has mobilised its first major search and rescue operation, deploying aircraft, boats and its commercial fishing fleet to help Malaysia search for the jet -- even as relatives of the 239 people aboard said their hopes for a miracle were ebbing away.

The hunt to discover the plane's fate will likely be "a long mission that requires patience," Vietnamese Major General Do Minh Tuan told AFP as he flew on a military helicopter near the country's southern Tho Chu island.

"If the plane crashed and sank, some debris will surface, and if we find that we will be able to pinpoint the location of the plane," he said.

But multiple reports of "suspicious floating objects" have revealed nothing but flotsam, tired Vietnamese rescue officials, putting in 20-hour days, concede.

"In terms of our assessments and predictions - we have little hope of a positive outcome," Pham Quy Tieu, deputy minister of transport, said Tuesday.

In southern Phu Quoc, normally a sleepy tourist town, hundreds of foreign journalists -- who usually face strict visa restrictions -- have arrived after the government set up a search and rescue base at the airport.

Officials have taken over rooms in the air traffic control tower at the new Phu Quoc international airport, where the atmosphere is calm and organised, but sparsely furnished rooms hint at Vietnam's limited resources.

The communist country "has minimal capabilities for search and rescue at sea," said Vietnam expert Carl Thayer, adding it was geared more towards dealing with natural disasters such as typhoons.

"The longer the search continues (Vietnam) will have problems sustaining its commitment," he said.

The total search sphere now includes land on the Malaysian peninsula itself, the waters off its west coast, and an area to the north of the Indonesian island of Sumatra. Vietnam has said it will search on land if needed.

That covers an area far removed from the scheduled route of MAS flight MH370, which officials say may have inexplicably turned back towards Kuala Lumpur.