As the mystery over the missing Malaysia Airlines (MAS) plane deepens, new theories have been floated by pilots and aviation enthusiasts to explain its disappearance. We look at some of these theories, and what experts say about them.

1. The plane could have caught fire mid-air

A fire probably broke out onboard MH370 and the pilot was trying to save the plane by making a sharp left turn to land on the Malaysian island of Langkawi, said an experienced Canadian pilot.

The flight crew, however, might have been overcome by smoke and the aircraft continued flying on autopilot until it ran out of fuel, said Mr Chris Goodfellow. Another possible scenario: the fire could have destroyed the control surfaces and it crashed.

The loss of transponders and communications made sense in a fire, he wrote in an article, adding it was likely electrical. The pilot's first response would be to shut down and restart the circuits.

Another possible cause of fire was overheating of one of the landing gear tyres which blew on takeoff and started burning slowly.

"Fire in an aircraft demands one thing: Get the machine on the ground as soon as possible," he said, adding that Langkawi is closer than Kuala Lumpur.

What experts say:

Some say this explanation makes sense. But others quoted reports which said the left turn was achieved using a computer system on the plane. That would involve typing seven or eight keystrokes into the computer. If the course was changed during a major emergency, it was more likely done using manual control.

Some also pointed out that the plane is believed to have made a series of turns after the first one. Such vigorous navigating, they say, would have been impossible if the crew were unconscious.

Moreover, the electronic "ping" detected by the Inmarsat satellite at 8.11am on March 8 - the day it went missing - narrowed its location at that moment to one of two arcs - one in Central Asia and the other in the southern Indian Ocean. Both areas are not in the direction of Langkawi.

2. The plane could have "stalked" another aircraft to avoid radar detection

Some believe the missing Boeing 777-200ER could have hidden in the shadow of another plane. With its transponder and lights switched off, MH370 could trail another aircraft undetected, said pilots and aviation enthusiasts.

To a ground radar controller, the planes would appear as one or two "blips" depending on how close they were.

Aviation blogger Keith Ledgerwood believes that MH370 could have trailed the Barcelona-bound Singapore Airlines (SIA) Flight 68, which left Changi Airport at about 1.05am, about 25 minutes after MH370 took off from Kuala Lumpur for Beijing. Both planes were in the same vicinity, he said.

"There are several locations along the flight path of SQ68 where it could have easily broken contact and flown and landed in Xinjiang, Kyrgyzstan, or Turkmenistan," he added.

When contacted, a SIA spokesman would only say: "All queries related to MH370 have to be directed to the investigating authorities."

What experts say:

While it sounds feasible on paper, it would be difficult to closely shadow a plane at night without radar help.

Some also pointed out that military radar, which has higher resolution, would still be able to identify that there were two objects. The two planes would need to be no further than about 1,000 m to appear as one on a military radar, radar expert Hugh Griffiths told BBC News.

3. The plane could have used "terrain masking" technique to avoid detection

MH370 could have dropped to an altitude of 5,000 ft, or possibly lower, to avoid commercial radar coverage after it turned back from its planned route, Malaysia's New Straits Times reported quoting officials.

It is also possible that MH370 had hugged the terrain in some areas that are mountainous to avoid radar detection. The technique, called terrain masking, is used by military pilots to fly to their targets stealthily.

What experts say:

Aviation expert Jason Middleton of New South Wales University told British paper Guardian that avoiding radar was a well-known technique. "Radar goes in a straight line. If you are in the shadow of a mountain or even the curve of the Earth - if you are under the radar beam - you can't be seen," he said.

But flying a large aircraft this way is dangerous because it puts tremendous stress on the airframe. Flying at such low altitude would also require a much higher fuel burn and result in a lower speed.

4. The plane was hijacked by crew or passenger

Investigators have zoomed in on pilot Captain Zaharie Ahmad Shah, 53, and co-pilot Fariq Abdul Hamid, 27, or passengers with aviation expertise after Malaysia said the plane went off course because of "deliberate action by someone on the plane".

The police were looking into whether the pilot had practised landings on his home flight simulator at airports located in the two vast tracts of territory where the search is focused.

Malaysian reports claimed police found five airports on Capt Zaharie's simulator. The five runways are in Diego Garcia, Male (Maldives), two in India and one in Sri Lanka. Diego Garcia is an atoll in the Indian Ocean where the United States Military has a naval base as well as an air base and runway able to accommodate large aircraft.

What experts say:

Police investigations have not found anything suspicious, even though the Malaysian authorities revealed yesterday(wed) that data logs had been recently deleted from the pilot's home flight simulator.

If the plane had been hijacked by a third party, no statement or demand had been made.

5. The plane could have crashed or exploded mid-air

Some believe that the plane might have crashed. Others said it might have exploded mid-air, which would explain why no debris has been found so far. There were also reports of sightings by people in countries from Indonesia to the Maldives. But these reports turned out to be false leads.

What experts say:

Austria-based Comprehensive Nuclear-Test-Ban Treaty Organisation (CTBTO), which has extremely sensitive sensors throughout the world, says it did not detect any explosion or crash - both on land or at sea.

CTBTO stations have detected some plane accidents in the past, including the crash of an aircraft at Narita airport in Japan in March 2009.

6. Flight had "structural issue"

Stanford computer science student Andrew Aude put forward a theory that the plane had a structural issue.

He cited a Federal Aviation Authority directive which pointed to the fuselage cracking at a spot where the satellite antennae is located. That could lead to rapid decompression and damage to the structure of the aircraft.

Mr Aude said that could explain why no alert was raised by those onboard because they could have been rendered unconscious by a slow decompression of the plane.

What experts say:

Boeing has since clarified that the missing Boeing 777-200ER was not subject to a new U.S. safety directive that ordered additional inspections for cracking and corrosion on certain 777 planes.