The missing Malaysia Airlines flight may be proving so hard to find because it could have vanished in an aviation "black spot", an expert has told Sky News.
Former Boeing 777 instructor and United Airlines captain Ross Aimer said it was "disturbing" that there had been no distress call from flight MH370's cockpit and that the plane's emergency locator transmitters had not sent any signals.
He said: "These are very sophisticated equipment that should have been working under any condition - in the water, in the jungle, after a fire, after an explosion - and none of them have talked to the outside world yet.
"There are spots in the world, however, that are called blind spots, where you can not communicate for some reason.
"Unfortunately, that area near Vietnam, over the Gulf of Thailand, those are some of the black spots, so perhaps at that time there was no data transmission between the aircraft and the airline."
Flight MH370 vanished from radar screens early on Saturday about one hour into a flight from Kuala Lumpur to Beijing with 239 people on board.
When it last made contact, the Malaysia Airlines jet was at cruising altitude, 11km (35,000 feet) above sea level, largely considered the safest part of a flight.
Wreckage of Air France flight 447 is discovered after the crash While there is still no confirmation that the Boeing 777-200 crashed, aviation experts have put forward possible causes of its disappearance including a terrorist attack, extreme turbulence, human error or even suicide.
The failure of the pilots to send a distress signal has given rise to speculation there was a sudden catastrophe - possibly caused by a mechanical failure or an explosion.
Former Navy pilot Dr Simon Mitchell has told Sky News that despite flying becoming safer over the years, mistakes are still made.
"We've expended billions of dollars on developing very sophisticated aids to make the life of the pilot safer and more straightforward, but there are still opportunities whereby mistakes can be made," he said.
Investigations into Air France flight 447 that plunged into the Atlantic Ocean in 2009, killing 228 people, eventually concluded that both technical and human error were to blame for the disaster.
Authorities have extended the search area for the plane Closer to the area where MH370 vanished, Adam Air flight 574 with 102 people on board disappeared in January 2007, also at its cruise phase, during a domestic flight in Indonesia.
Authorities found the pilots lost control after becoming preoccupied with malfunctioning navigational equipment.
The lack of wreckage from MH370 also suggests a high-altitude disaster which spread debris across an area too wide to be easily detected.
Aviation experts say the size of the debris field will be one of the first indicators of what happened.
A smaller field would indicate the plane probably fell intact, breaking up upon impact with the water.
A large, widespread field would signal the plane probably broke apart at a high elevation, perhaps because of a bomb or a massive airframe failure.
Authorities are now searching a 100-nautical mile area But sudden, accidental structural failures are considered extremely unlikely in today's passenger aircraft.
This is especially so with the Boeing 777-200, which has one of the best safety records of any jet.
Authorities have not ruled out terrorism or hijacking, though, with suspicions over two of the passengers found to be travelling on stolen passports.
"There are two categories of people who use these (stolen passports) - criminals and terrorists," Rohan Gunaratna, a terrorism expert at Singapore's Nanyang Technological University, said.
In the 1988 Lockerbie bombing, there was substantial wreckage despite it being a mid-air explosion, and claims of responsibility came soon after the disaster.
The Lockerbie bombing killed 270 people, including 11 on the ground But no-one has come forward to claim the Malaysia incident as their attack.
Whatever caused the apparent crash, there would be some debris - but it could take a while to find.
It took two years to find the main wreckage of the Air France plane plunged into the Atlantic Ocean in 2009, killing 228 people.
An Indonesian navy ship detected metal on the ocean floor a week after the Adam Air flight disappeared in 2007.
It was a further two weeks before the US Navy picked up signals from the flight data and cockpit recorders, and seven months for the recorder to be recovered.
An Indonesian fisherman holds up part of the Adam Air aircraft The Malaysian jet could have made a U-turn shortly before it vanished, officials say, adding one more level of uncertainty to the effort to find it.
It is thought the plane could be hundreds of miles from where it was last detected, and the search has been widened in the hope of finding the plane.
Just about every major jet to disappear in the modern era has eventually been found. The rare exceptions did not involve passengers.
In September 1990, a Boeing 727 owned by the Peruvian Faucett Airlines ditched into the North Atlantic after running out of fuel on its way to Miami.
The accident was attributed to poor pilot planning and the wreck was never recovered.
More mysterious was the disappearance of another Boeing 727 being used to transport diesel fuel to diamond mines in Africa.
The owners had numerous financial problems and the plane took off without clearance and with its transponder turned off. It is believed to have crashed in the Atlantic Ocean.
One theory, never proven, is that it was stolen so the owner could collect insurance.
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An information message at Kuala Lumpur International Airport in Sepang 
A relative of a passenger on the flight waits for news in Beijing 
The two oil slicks seen off Vietnam 
A photo of the missing plane - seen taking off in Paris in 2011 
The jet is reported to have crashed in the ocean off southern Vietnam 
Family members have complained of a lack of information 
Distraught relatives and friends wait at Beijing International Airport 
