Missing Malaysia Airlines flight MH370 could only have lost its transponder and other communications through human interference and its captain was best placed to do that.

Communications could have been lost through a fire but the plane would not then have continued flying another seven hours, former pilots have told the ABC’s Four Corners program.

As a Chinese navy survey ship prepares to start mapping the seabed off Western Australia where the crash site is believed to lie, the ABC has looked at how the search has failed so far.

Asuad Khan, brother-in-law of MH370 captain Zaharie Ahmad Shah, rejected suggestions the captain was unfit to fly because of personal problems.

Mr Khan also rejected speculation Captain Zaharie’s wife moved out just before the flight, saying it was normal for her to take the children and stay elsewhere while her husband was away.

It just points to one thing: human interference.

He did not know if his brother-in-law had a girlfriend – as some reports have indicated amid the struggle to find a motive or a cause for the plane’s disappearance.

“Why not? We can marry four. We are Muslim,” Mr Kahn said.

Officials believe the Boeing 777 veered far off course during a flight from Kuala Lumpur to Beijing on March 8 and crashed in the southern Indian Ocean.

The final communication occurred at 1.19am.

In the next 90 seconds, the aircraft’s transponder, which indicates identify, altitude, speed and destination, either failed or was turned off.

A separate system, transmitting technical data about the aircraft’s engines and other systems, also stopped working.

“It just points to one thing: human interference,” said Nik Huzlan, former Malaysia Airlines chief pilot.

Then the aircraft with 239 people aboard changed course, flying west across the Malaysian peninsular before heading south over the Indian Ocean, where it’s believed to have finally run out of fuel.

It was observed on Malaysia’s military radar. Even though contacted by Malaysia’s civil aviation authority about a missing airliner, the military opted not to send one of its aircraft to investigate.

“If you’re not going to shoot it down, what’s the point of sending it up,” Malaysian defence minister Datuk Hishamuddin Hussein told the ABC.

“Well, the Americans would,” he added.

The Chinese ship Zhu Kezhen should reach the search area off WA on Wednesday.