Passengers on missing flight MH370 died of oxygen starvation hours before the pilot deliberately plunged the plane into the Indian Ocean, a veteran air accidents investigator claims in a new book.

New Zealand-based commercial pilot Ewan Wilson blames pilot Ahmad Shah for the tragedy claiming he locked his co-pilot out of the cockpit then performed a controlled ditch.

All 239 people on board likely slipped into a coma and died minutes after the pilot deliberately depressurised the cabin, the conspiracy theory claims.

Oxygen masks would have dropped down but the air supply in the cabin would have lasted just 20 minutes, while the pilot had air for up to three hours.

The claims are made in the book 'Goodnight Malaysia 370', which is reportedly the first independent expert report since the plane went missing in March.

Mr Wilson believes Ahmad Shah was suffering from mental illness and tricked co-pilot Fariq Hamid into taking a break so he could carry out "the final act of his performance".

"Ahmad Shah was a man known for his methodical, thorough nature, for his love of the technical, and probably for his ego, too," Mr Wilson said.

"This would have been his final sad act to his family and to the world: 'find this one'."

The former commercial pilot and air safety investigator said he arrived at the conclusion after considering "every conceivable alternative scenario".

His controlled ditch theory also sheds light on why no wreckage has been found because the plane would have gone to the bottom of the ocean in one piece, he said.