SYDNEY (BLOOMBERG) - The man leading the search for Malaysia Airlines flight MH370 is showing the strain after almost two years of fruitless toil.

Mr Martin Dolan, head of the Australian Transport Safety Bureau (ATSB), said he struggles to sleep at times, gnawed by thoughts that wreckage from the Boeing 777 may have slipped through the sonar net scanning 120,000 sq km of the southern Indian Ocean.

MH370 is weeks away from becoming aviation's biggest unsolved mystery since American aviation pioneer Amelia Earhart disappeared in 1937.

Of the three million components in the jet, only one has turned up - a barnacle-encrusted wing flap - on the French island of Reunion.

There have been no traces of the 239 people on board, their luggage or even the life jackets that were supposed to float.

"There's always this question: Have we missed something?" Mr Dolan, 58, said at his office in Canberra.

"That's the sort of thing that will occasionally keep me awake at night."

Some of the world's most experienced search-and-rescue experts increasingly accept that the A$180 million (S$180 million) search may fail.

Without fresh clues, the hunt should end about June, when four ships are due to finish combing the seas off western Australia, Mr Dolan said.

Within a rectangle the size of North Korea, vessels have scoured most of the patch believed to be the likely impact point - and come up empty.

Nor are investigators any closer to ascertaining what happened inside the plane after it took off from Kuala Lumpur on March 8, 2014, for Beijing.

A FAILURE POSSIBLE

"We were ready for most things, but MH370 has been unpredictable all the way through," Mr Dolan said. "It's a possibility we will not succeed."

In the aftermath, the International Civil Aviation Organisation (ICAO) is pushing for commercial aircraft to report their positions every 15 minutes. And the underwater locator beacons inside their black boxes will have to last for 90 days instead of the current 30, under European Union proposals.

But neither rule takes full effect before 2018.

Asia's aviation industry, in the meantime, is expanding at a breakneck pace with 100 million new passengers taking to the skies every year, according to Boeing. In the next two decades, the world's fleet of 22,000 aircraft is set to double.

Mr Larry Stone, chief scientist at Reston, Virginia-based consultant Metron, has tracked missing aircraft and ships for half a century. Mr Stone mapped out the resting place of Air France Flight 447, which was found two years after plummeting into the Atlantic Ocean with 228 people aboard in 2009.

The location, size and characteristics of the underwater search for MH370 make it the toughest he's ever seen.

"I wouldn't be surprised if they didn't find it," Mr Stone said.The waters in the search area are up to 6km deep and peppered with trenches and submerged peaks. Last month, a towed sonar vehicle collided with a volcano rising 2,200m from the seabed. The device was severed and sank to the bottom.

Vessels still have to scan about 35,000 sq km - an area bigger than Belgium. The newest reinforcement is a Chinese ship with high-definition sonar. The Dong Hai Jiu 101 will focus on areas of the ocean floor that are difficult to scan with conventional sonar when it arrives this month, Mr Dolan said.

HOT SPOTS

Ships are rechecking about 100 locations, some of them inside an oval-shaped patch towards the southern end of the search area, Mr Dolan said. This hot spot is most likely to contain the wreckage, according to Australia's Defence Science and Technology Group, a government agency normally devoted to national security.

But even the most comprehensive search won't satisfy the victims' families if it fails.

"We are going to try as hard as we can to lobby for the search to continue beyond June," said Ms Grace Subathirai Nathan, 28, whose mother was a passenger. "I need an answer."

The disaster unfolded when air-traffic controllers lost contact with MH370 less than an hour after takeoff as it approached Vietnam. Military radar showed the plane took a left turn, looped back across Malaysia and headed north-west up the Strait of Malacca.

SATELLITE PINGS

Radar contact then was lost, but an orbiting satellite picked up pings from the plane. Analysis of those hourly check- ins indicates MH370 cruised south over the Indian Ocean for about six hours.

"They've done a remarkable job to get anything useful out of it at all," said Mr Vaughan Clarkson, a specialist in radar and tracking at the University of Queensland, Australia who helped calculate the flight path. "You're trying to track a fast-moving aircraft with updates only about once an hour."

The extent of human intervention in the silent disaster isn't known. The last recorded words from MH370's pilots, at 1.19am on March 8, were: "Good night Malaysian Three Seven Zero."

In a world where a US$100 (S$140) smartphone can be tracked for free, the US$250 million jet vanished. Malaysia Prime Minister Najib Razak has said the plane was deliberately steered off course, and the homes of the pilot and co-pilot were searched. The US Federal Bureau of Investigation also analysed the pilot's personal flight simulator to no avail.

"All the evidence says classic autopilot flight," Mr Dolan said, emphasising that other authorities are trying to reconstruct events inside the cabin.

MATHEMATICAL PROBABILITY

The disaster was the beginning of the end for Malaysia Airlines as a listed entity. Four months after MH370 disappeared, Flight MH17 was shot down over Ukraine. A Malaysian government investment company bought out the airline the same year as passenger traffic slumped and losses widened.

A failed search "would leave a bad impression on the industry as a whole, but particularly on Malaysia Airlines", Mr Christoph Mueller, the chief executive officer, told Bloomberg Television on Monday (Feb 15).

Back in Canberra, the ATSB is investigating about 130 incidents in total - ranging from a freight train derailment in Queensland to a seaplane crash off northeast Australia.Yet Mr Dolan says he's consumed by MH370. He speaks mostly in a soft, low tone, pausing often as he chooses his words.

At a table in his office, he refers to a map of the search zone in front of him as he outlines the analysis of the plane's final moments: The right engine runs out of fuel first, and within 15 minutes so does the left. There's just enough fuel remaining for the satellite data unit to reboot and beam a final message.

Then the plane probably banks left and spirals into the ocean.Based on that sequence, searchers are prioritising an area within 20 nautical miles of the aircraft's last transmission.

LAST CHANCE

Investigating the other possibility - that someone was steering the plane and glided it without power until it hit the water - would mean tripling the search area. That's a very unlikely scenario, and it risks overwhelming investigators, Mr Dolan said.

"Governments are just not willing to put the resources into that sort of potential extension," he said.

Ships scanning the seafloor already collected about 20 petabytes of imaging data. That's enough to house the entire digital collection of the US Library of Congress - several times over.

Even after acknowledging the difficulties of searching a massive, remote and deep area of the ocean, Mr Dolan said he was confident the plane will be found.

"Every morning I wake up and check what's going on, and I hope that today's going to be the day," he said.