18 March 2014 Last updated at 16:40 By Tom de Castella BBC News Magazine 
A Malaysia Airlines Boeing 777-200ER taking off from Narita Airport near Tokyo, Japan, last year As the search for Malaysia Airlines missing Boeing 777 moves into its 11th day, a multitude of theories about the plane's fate are circulating on forums and social media. Here, former pilots and aviation experts look at some of those theories.
Malaysia's government says the plane - with 239 people on board - was intentionally diverted and could have flown on either a northern or southern arc from its last known position.
The country's Department of Civil Aviation has stated that "pings" were picked up from the plane six hours after military radar last detected it over the Strait of Malacca at 02:15 on 8 March.
1. Landed in the Andaman Islands The plane was apparently at one stage heading in the direction of India's Andaman and Nicobar Islands, the most easterly part of Indian territory, which lies between Indonesia and the coast of Thailand and Burma. It has been reported that military radar there might not even have been operating, as the threat level is generally perceived to be low.
The editor of the islands' Andaman Chronicle newspaper dismisses the notion that the aircraft could be there. There are four airstrips but planes landing would be spotted, he told CNN. He also believed monitoring by the Indian military would prevent an airliner being able to land there unnoticed. But this is an isolated spot. There are more than 570 islands, only 36 of which are inhabited. If the plane had been stolen, this might be the best place to land it secretly, says Steve Buzdygan, a former BA 777 pilot. It would be difficult, but not impossible, to land on the beach, he says. At least 5,000ft (1500m) or so would make a long enough strip to land on.
It would be theoretically possible but extremely difficult. With such a heavy aeroplane, using the landing gear might lead to the wheels digging into the sand and sections of undercarriage being ripped off. "If I was landing on a beach I would keep the wheels up," says Buzdygan. But in this type of crash landing, the danger would also be damage to the wings, which are full of fuel, causing an explosion. Even if landed safely, it is unlikely the plane would be able to take off again.
2. Flew to Kazakhstan The Central Asian republic is at the far end of the northern search corridor, so the plane could hypothetically have landed there. Light aircraft pilot Sylvia Wrigley, author of Why Planes Crash, says landing in a desert might be possible and certainly more likely than landing on a beach somewhere. "To pull this off, you are looking at landing in an incredibly isolated area," says Wrigley. The failure so far to release a cargo manifest or passenger list has created wild rumours about either a valuable load on board or the presence of billionaires as a motive for hijacking.
But the plane would have been detected, the Kazakh Civil Aviation Committee said in a detailed statement sent to Reuters. And there's an even more obvious problem. The plane would have had to cross the airspace of countries like India, Pakistan and Afghanistan, which are all usually in a high state of military preparedness. But it's just possible that there are weak links in the radar systems of some of the countries en route to Central Asia, Wrigley speculates. "A lot of air traffic control gear is old. They might be used to getting false positives from flocks of birds and, therefore, it would be easy to discount it."
3. It flew south The final satellite "ping" suggests the plane was still operational for at least five or six hours after leaving Malaysian radar range. For Norman Shanks, former head of group security at airports group BAA, and professor of aviation security at Coventry University, the search should therefore start from the extremes of the corridors and work up, rather than the other way around. He thinks the southern corridor is more likely for a plane that has so far avoided detection by radar.
The southern arc leads to the huge open spaces of the Indian Ocean, and then to Australia's empty northern hinterland. Without knowing the motive, it is hard to speculate where the plane's final destination was intended to be. But the plane may just have carried on until it ran out of fuel and then glided and crashed into the sea somewhere north of Australia.
4. Taklamakan Desert, north-west China There has been speculation on forums that the plane could have been commandeered by China's Uighur Muslim separatists. Out of the plane's 239 passengers, 153 were Chinese citizens. One possible destination in this theory would be China's Taklamakan Desert. The region - described by Encyclopaedia Britannica as a "great desert of Central Asia and one of the largest sandy deserts in the world" - has no shortage of space far from prying eyes. The BBC's Jonah Fisher tweeted on 15 March: "Being briefed by Malaysia officials they believe most likely location for MH370 is on land somewhere near Chinese/Kyrgyz border."
But again, this theory rests on an extraordinary run through the radar systems of several countries.
5. It was flown towards Langkawi island because of a fire or other malfunction The loss of transponders and communications could be explained by a fire, aviation blogger Chris Goodfellow has suggested. The left turn that the plane made, deviating from the route to Beijing, could have been a bid to reach safety, he argues. "This pilot did all the right things. He was confronted by some major event onboard that made him make that immediate turn back to the closest safe airport." He aimed to avoid crashing into a city or high ridges, Goodfellow argues. "Actually he was taking a direct route to Palau Langkawi, a 13,000ft (4,000m) strip with an approach over water at night with no obstacles. He did not turn back to Kuala Lumpur because he knew he had 8,000ft ridges to cross. He knew the terrain was friendlier towards Langkawi and also a shorter distance." In this theory it would be assumed that the airliner did not make it to Langkawi and crashed into the sea.
But Goodfellow's theory has been disputed. If the course was changed during a major emergency, one might expect it to be done using manual control. But the left turn was the result of someone in the cockpit typing "seven or eight keystrokes into a computer on a knee-high pedestal between the captain and the first officer, according to officials", the New York Times reported. The paper says this "has reinforced the belief of investigators - first voiced by Malaysian officials - that the plane was deliberately diverted and that foul play was involved."
6. The plane is in Pakistan Media tycoon Rupert Murdoch has tweeted: "World seems transfixed by 777 disappearance. Maybe no crash but stolen, effectively hidden, perhaps in northern Pakistan, like Bin Laden." But Pakistan has strenuously denied that this would be possible. The country's assistant to the prime minister on aviation, Shujaat Azeem, has been reported as saying: "Pakistan's civil aviation radars never spotted this jet, so how it could be hidden somewhere in Pakistan?" Like the Kazakhstan theory, this all seems far-fetched, not least because the junction between Indian and Pakistani air space is one of the most watched sectors in the world by military radar. And despite the remoteness and lawlessness of northern Pakistan, the region is watched closely by satellites and drones. It seems scarcely believable to think an airliner could get there unspotted.
7. The plane hid in the shadow of another airliner Aviation blogger Keith Ledgerwood believes the missing plane hid in the radar shadow of Singapore Airlines flight 68. The Singaporean airliner was in the same vicinity as the Malaysian plane, he argues. "It became apparent as I inspected SIA68's flight path history that MH370 had manoeuvred itself directly behind SIA68 at approximately 18:00UTC and over the next 15 minutes had been following SIA68." He believes that the Singaporean airliner would have disguised the missing plane from radar controllers on the ground. "It is my belief that MH370 likely flew in the shadow of SIA68 through India and Afghanistan airspace. As MH370 was flying 'dark' without a transponder, SIA68 would have had no knowledge that MH370 was anywhere around, and as it entered Indian airspace, it would have shown up as one single blip on the radar with only the transponder information of SIA68 lighting up ATC and military radar screens." The Singapore Airlines plane flew on to Spain. The Malaysian jet could have branched off. "There are several locations along the flight path of SIA68 where it could have easily broken contact and flown and landed in Xinjiang, Kyrgyzstan, or Turkmenistan," Ledgerwood argues.
Prof Hugh Griffiths, radar expert at University College London, says it sounds feasible. But there is a difference between military and civilian radar. Civilian radar works by means of a transponder carried by the aircraft - a system known as secondary radar. The military use primary radar and this "ought to be higher resolution". So how close would the two planes need to be? He estimates about 1000m (3300ft). It is possible military radar would be able to pick up that there were two objects, he says. "It might be able to tell the difference, to know that there are two targets." If this happens, though, there's then the question of how this is interpreted on the ground. Is it a strange echo that would be discounted? When the Japanese attacked Pearl Harbor in 1941, although the US radar operator detected the incoming aircraft, they were dismissed as US bombers arriving from the mainland.
8. There was a struggle One of the hardest things to account for so far with an innocent explanation is the way the plane was flown erratically. It went far above its "ceiling", flying at 45,000ft (13,716m) before later flying very low. Big fluctuations in altitude suggest there might have been a struggle, says Buzdygan. Post-9/11, cockpit doors have been strengthened against the possibility of hijack but there are still scenarios where access could be gained. Pilots talk to each other "over a beer" about how they'd deal with hijackers, he says. Buzdygan would have had no qualms about flying aggressively to try to resist a hijack. "I'd try to disorientate and confuse the hijackers by throwing them around," he says.
9. The passengers were deliberately killed by decompression Another theory circulating is that the plane was taken up to 45,000ft to kill the passengers quickly, former RAF navigator Sean Maffett says. The supposed motive for this might have been primarily to stop the passengers using mobile phones, once the plane descended to a much lower altitude. At 45,000ft, the Boeing 777 is way above its normal operating height. And it is possible to depressurise the cabin, notes Maffett. Oxygen masks would automatically deploy. They would run out after 12-15 minutes. The passengers - as with carbon monoxide poisoning - would slip into unconsciousness and die, he argues. But whoever was in control of the plane would also perish in this scenario, unless they had access to some other form of oxygen supply.
10. The plane will take off again to be used in a terrorist attack One of the more outlandish theories is that the plane has been stolen by terrorists to commit a 9/11 style atrocity. It has been landed safely, hidden or camouflaged, will be refuelled and fitted with a new transponder before taking off to attack a city. It would be very hard to land a plane, hide it and then take off again, Maffett suggested. But it can't be ruled out. "We are now at stage where very, very difficult things have to be considered as all sensible options seem to have dropped off," he says. It is not clear even whether a plane could be refitted with a new transponder and given a totally new identity in this way, he says. Others would say that while it is just about feasible the plane could be landed in secret, it is unlikely it would be in a fit state to take off again.
The even more far-fetched Many of the above theories might seem far-fetched but there are even more outlandish-sounding ones out there.
If the plane had flown up the northern corridor, experts maintain it would probably have triggered primary radar. Key countries whose airspace it might have crossed are Kazakhstan, Uzbekistan, Kyrgyzstan, Turkmenistan, Pakistan, Bangladesh, India, China, Myanmar, Laos, Vietnam, or Thailand. After 9/11, an unidentified airliner entering sovereign airspace is likely to lead to fighters being scrambled, says Maffett. "If the plane is in the northern arc it could easily have been shot down." It's a theory circulating on some forums. The notion is that no-one would want to admit shooting down an airliner full of passengers, Maffett says, and thus might currently be concealing the event.
But there are a host of holes in the theory. Firstly, the plane would still have had to avoid numerous radar systems before finally triggering one. And the nation responsible would be trying to keep secret the fate of the world's currently most-searched for object. Covering up the incident for so long would arguably make the shooting down look far worse.
Then there are other conspiracy theories. Some forum postings have pointed to the US military base in the middle of the Indian Ocean, on the tropical atoll of Diego Garcia. The island is owned by the UK but leased to the US. One of the more extreme theories circulating online claims that the Kremlin believes that the US "captured" the plane and flew it to its base. With a conspiracy theory of this magnitude it is difficult even to know where to start with the rebuttals.
A completely different thread of conspiracy theory assumes a sympathetic regime. The scepticism about flying undetected through radar changes somewhat if the hijackers are in cahoots with a country's government. There are several authoritarian regimes within the aircraft's range, but the conspiracy theory doesn't even require a government's co-operation - the hijackers could just be in cahoots with radar operators. Again, this seems to be a conspiracy of incredible complexity to be kept secret for this length of time. And what would the motive be for those colluding?
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