The pilot of missing Malaysia Airlines flight MH370 made a series of deliberate turns and speed changes to avoid radar detection before plunging into the Indian Ocean, new research suggests.
Aerospace engineer Richard Godfrey, who has spent years investigating the flight's 2014 disappearance, said his research suggested that pilot Zaharie Ahmad Shah took a "carefully planned" flight path to avoid "giving a clear idea where he was heading".
The Boeing 777 with 239 people on board, dropped off radar screens after taking off from Kuala Lumpur International Airport, bound for Beijing.
The plane took an unexplained U-turn from its planned flight path and headed back across the Malay Peninsula and the Malacca Strait before vanishing.
Mr Godfrey said the plane's final movements could be mapped out using data from Weak Signal Propagation (WSPR), a global network of radio signals that can trace the movement of planes as they cross signals and set off invisible "electronic trip-wires".
"WSPR is like a bunch of trip-wires or laser beams, but they work in every direction over the horizon to the other side of the globe," Mr Godfrey said in his report.
His research found MH370 crossed eight of these "trip-wires" as it flew over the Indian Ocean, which is consistent with previous studies of the plane's flight path.
But he said the plane's change in movements and speed appeared to suggest it was trying to avoid leaving clues about where it was heading.
"The flight path appears carefully planned," he added.
"The level of detail in the planning implies a mindset that would want to see this complex plan properly executed through to the end."
Friends of the pilot said he was “lonely” and “sad” while aviation specialist William Langewiesche wrote in The Atlantic that “there is a strong suspicion among investigators in the aviation and intelligence communities that he was clinically depressed."
One theory put forward by electrical engineer Mike Exner, from Colorado, is that the pilot probably made a climb which "accelerated the effects of depressurising, causing the rapid incapacitation and death of everyone in the cabin."
With oxygen still available in the cockpit, Mr Ahmad Shah could have kept flying for hours.
The Australian Transport Safety Bureau's (ATSB) $200 million search for MH370 scoured more than 120,000sq km of Indian Ocean floor using high-resolution sonar between 2014 to 2017 but could not locate the plane.
A second search sponsored by the Malaysian government was also fruitless.
In its final report, the ATSB identified an area of less than 25,000sq km "which has the highest likelihood of containing MH370".
While the aircraft has not been located, 33 pieces of debris – either confirmed or highly likely to be from MH370 – have been discovered in Mauritius, Madagascar, Tanzania and South Africa.