Few thought it was possible for a jumbo jet seemingly to vanish mid-flight until Malaysia Airlines Flight 370 from Kuala Lumpur, Malaysia, never made it to Beijing.
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The ensuing hysteria led to many false conclusions, but nothing has been proven.
The 239 people on board MH370, including the 12 crew members, are all believed to have died, though none of the plane's remnants nor any bodies were ever found.
Outlandish theories on the plane's fate have circulated in the deep recesses of the Internet -- but there also are more plausible theories that aviation experts have not been able to rule out, as well as one that seems the most likely.
There Was an Explosion or Fire on Board
The prospect of either a purposeful explosion, like a bomb on board the plane or a mechanical problem that resulted in the downing of the jet, were both suggested as possible explanations, first for MH370 and later for the AirAsia crash that took place eight months later in Indonesia.
Fire seems like a less likely cause in the case of MH370, however, because there appeared to be several purposeful actions taken before the plane vanished from all radar that may suggest something more sinister and planned was afoot, experts said.
The plane took a distinct turn towards the Indian Ocean, removing it from the intended flight path. That coupled with the fact that several layers of the plane’s radar tracking system appeared to be purposefully turned off suggest that someone with knowledge of how to fly a plane took over the controls with a goal in mind, according to the theory.
The Plane Flew in Another Plane’s Shadow
Another theory that has not been ruled out by experts is the prospect that MH370 could have “shadowed” another plane, flying either below or above another commercial airliner in order to avoid being tracked by radar.
While it might seem surprising that one plane can hide underneath another, former Air Force pilot and current ABC News aviation consultant John Nance said that it cannot be immediately ruled out.
“That idea is not outlandish, but the probabilities of that are extremely low,” Nance said.
One reason the theory may be unlikely is that pilots trained on military planes are the only ones who generally receive the required training for such a move. On top of that, they have specialized tools that help them do so.
ABC News aviation consultant Steve Ganyard, who previously served as a Marine Corps fighter pilot, noted that the pilots on board MH370 had not received the military training necessary to fly in formation.
“Even if they could,” he said, “military pilots have special lights. ... This idea that a 777 could tuck into 777‘s flight path is pretty hard to believe, especially at night.”
Hijackers Took Control of the Plane
“There’s just no question it was hijacked," Nance said. "The only question we don’t have an answer to is whether it was the pilot or a third party.”
Investigators searched into the backgrounds of the passengers on board but found nothing alarming, though that does not necessarily rule out the prospect.
“It is still possible that it was a third party but, if so, it was a dual team," Nance said. "A male would not have been able to get access to the cockpit so easily. An attractive female might.”
The strongest piece of evidence against the theory, however, is that there were no distress calls or suspicious activity transmitted via radio between the time of the last call into ground control and the time the transponder was turned off.
Hijackers Landed the Plane on Land and Hid It Somewhere Remote
The concept that a 777 could crash and not leave any debris after, presumably, shattering into thousands of pieces, led some to think that perhaps it didn’t crash at all, that perhaps the plane was diverted to a small, remote airstrip and was hidden to be used for some other mission.
Ganyard said that while we have not been able to find the plane, investigators “have a general idea of where that airplane was going” and have a rough search area approximately 1,000 miles west of Perth, in western Australia.
“I'm pretty comfortable with the satellite hits that we have now,” he told ABC News. “There’s pretty solid evidence that we have now of the airplane heading due south and the last satellite hit.”
Others pointed to Russia, saying that there could have been an operation to land it in a sympathetic country. The speculation was only fueled when pro-Russian separatist fighters shot down a different Malaysian Airlines plane months later amid fighting along the Ukrainian border, though Nance said it is extremely unlikely that Russia had anything to do with MH370’s disappearance.
“Putin isn’t an idiot,” Nance said. “This is not an operation he would ever approve because there is no gain.”
The Pilot Purposefully Crashed the Plane
Experts agree that the most likely scenario involves the pilot purposefully taking control of the plane and driving it into the ocean.
“The captain certainly had far more opportunity than anyone else that we know of,” Nance said.
The biggest indicators pointing to the pilot is that a significant amount of training is needed to know how to turn off the transponder as well as going through several levels of commands on the plane’s radar system, called ACARS, which only experienced pilots know how to do.
One fault against the pilot, however, is that nothing in his background suggested that he was depressed, had financial problems, or was dealing with suicidal thoughts, all of which have been warning signs for pilots in other scenarios, Nance said.
“All possibilities and probabilities settle on the captain's shoulders,” Nance said.