The pilot of the MH370 flight that disappeared flew to 40,000ft so the aircraft would become depressurised and the passengers would die before he crashed it into the sea, a new report has claimed.

Captain Zaharie Ahmad Shah was the pilot-in-command when the plane carrying 238 other passengers and crew vanished in March 2014.

The Malaysian Airlines Boeing 777 is thought to have crashed in the southern Indian Ocean, and a safety report in July last year, revealed the plane was likely steered off course manually, before being flown for several hours.

In a new report for The Atlantic, aviation expert William Langewiesche explores the most likely scenario of what happened, and the troubled personal life of Shah.

On the night the aircraft went missing the controls were seized in the cockpit between 1.10am, when the plane was at around 35,000ft, until 1.21am when it disappeared from the radar.

Electrical engineer Mike Exner told The Atlantic, the plane climbed to 40,000ft in 'causing the rapid incapacitation and death of everyone in the cabin.'

"An intentional depressurisation would have been an obvious way—and probably the only way—to subdue a potentially unruly cabin in an airplane that was going to remain in flight for hours to come," writes Mr Langewiesche.

"In the cabin, the effect would have gone unnoticed but for the sudden appearance of the drop-down oxygen masks and perhaps the cabin crew's use of the few portable units of similar design.

"None of those cabin masks was intended for more than about 15 minutes of use during emergency descents to altitudes below 13,000 feet; they would have been of no value at all cruising at 40,000 feet.

"The cabin occupants would have become incapacitated within a couple of minutes, lost consciousness, and gently died without any choking or gasping for air."

MH370 pilot, Zaharie Shah (left), with his daughter Aishah. Photo / Facebook

The most prominent, and likely theory, at this stage is that Mr Shah deliberately downed the plane in a murder-suicide.

A lifelong friend, also a 777 pilot, who did not want to be named for fear of the consequences, came to the reluctant conclusion that Captain Zaharie Ahmad Shah crashed the plane.

The lifelong friend says Shah likely tricked his co-pilot Fariq Hamid, who was on his final training flight, into leaving the cockpit.

He told The Atlantic: "Zaharie was an examiner. All he had to say was 'Go check something in the cabin,' and the guy would have been gone."

Why Zaharie would supposedly choose to crash the plane is still a mystery but his friend has pointed to his chaotic personal life and fragile emotional state as a possible explanation.

He said: "Zaharie's marriage was bad. In the past he slept with some of the flight attendants. And so what? We all do. You're flying all over the world with these beautiful girls in the back. But his wife knew."

The man added he thought Zaharie's emotional state may have been a factor in the incident.

As well as a turbulent personal life, Shah was very active on social media, often leaving messages on the profiles on twin models, and making a number of political statements critical of the government.

In the 12 months before the Boeing 777 went missing, Zaharie stalked the Facebook pages of Qi Min Lan and her twin Lan Qi Hui, 26, leaving sexually suggestive comments.

Throughout 2013, the 53-year-old made 97 separate Facebook comments directed at Ms Lan. A vast majority of his messages were ignored.A photo from model QiMin Lan's facebook page. MH370 pilot, Zaharie Shah, commented on multiple posts of hers in the year leading up to the crash. Photo / Facebook

Shah implored the twins to come to Kuala Lumpur, where he lived with his wife.

"When in KL," he wrote in one message.

"How about KL?" he persisted, but got no reply.

In another, where Qi Min Lan posted a photo of herself in a bathrobe, he wrote: "Just shower?"

On top of his obsessive messages to the girls, Shah frequently criticised the Malaysian government, which happens to own the airline he worked for.

On his own public Facebook page, Zaharie labelled then-Prime Minister Najib Razak a "moron".

In April 2013, leading up to the Malaysian elections, Zaharie posted 119 times, all in reflection of his disgust with the Naijb government.

The following month, after the Najib party secured another five-year term, Zaharie wrote: "There is a rebel in each and every one of us. Let it out!"