At a Senate estimates hearing in Canberra on Tuesday, Peter Foley from the Australian Transport Safety Bureau outlined a possible theory – which he stressed was one of many – for the plane’s disappearance four years ago.

The official explanation from the ATSB and Malaysian authorities is that the captain, Zaharie Ahmad Shah, was unconscious when MH370 crashed into the Indian Ocean, resulting in a steep dive.

But recent reports, including a one-hour special aired by Australia’s 60 Minutes, claimed Zaharie was conscious at the end of the flight, and deliberately piloted his plane in a “controlled ditching” into the sea.

The program’s experts said Zaharie depressurised the plane to incapacitate passengers and other crew through hypoxia (oxygen deficiency), and used an emergency air supply to stay conscious. He then repressurised the plane for the rest of the journey.

On Tuesday, Foley said this was “plausible”, but Zaharie himself would have been knocked out by the sudden pressure change.

“Most of the people out there are speculating about a long period of depressurisation after the transponder went off,” he said. “[They say] this may have been as long as an hour. 

“What they fail to understand is that while you don an oxygen mask and prevent the worst of the hypoxia situation, you are flying an aircraft at 40,000 feet. You are taking an aircraft from sea level to Mt Kosciuszko in 20 minutes, then you are talking it, over the course of a couple of minutes, to the height of Mt Everest plus 1,000 feet. You’ll get decompression sickness too.”

He said a similar situation happened to a cargo aircraft in 1994, documented by the US National Transportation Safety Board.

“During the climbout the flight crew was unable to pressurise the aircraft, and the captain elected to proceed with the flight. The crew donned their oxygen masks and shortly thereafter the captain became incapacitated from decompression sickness. The first officer took command and they landed the plane.”

He said this occurred “within several minutes”.

“The pilot in this particular aircraft was 51 and overweight. The pilot in command of MH370 was 53 and overweight. I’m not saying that happened and I hate to speculate, but that is one plausible scenario.”

However, senator Rex Patrick continued to press Foley on the possibility that the plane was in a controlled descent.

“Today we have an analysis of the flap that tells us it is probably not deployed,” Foley said. “We have an analysis of the final two transmissions that say the aeroplane was in a high rate of descent. We have 30 pieces of debris, some from inside the fuselage, that says there was significant energy at impact ... We have quite a lot of evidence to support no control at the end.”

He added that the ATSB had spoken to experts who supported the controlled descent theory, including former Boeing 777 instructor Simon Hardy.

“Some of Simon’s initial area where he was postulating it was controlled to the end, but not a ditching – we actually searched. We went a long way to the east in that search – 42 miles.

“We certainly listened very carefully to what Simon had to say. We certainly read the articles in The Australian where [another pilot] Byron Bailey said ‘Clearly the pilot has done this ... He must have been in control at the end.’ But it wasn’t substantiated.

“We haven’t ever ruled out someone intervening at the end. It’s unlikely.” 

Foley has been the ATSB’s director of the search for MH370 for the last four years. He also told the committee he believed Zaharie, or someone else, was in control of the plane for at least the first one and a half hours of its flight.

“Early in the flight an aircraft doesn’t turn itself,” he said. “There must have been someone in control of the aircraft up until 18:25 (UTC).”

More about: #MH370