Captain Zaharie Ahmad Shah, the pilot of the missing Malaysia Airlines flight MH370, was said to be "terribly upset" after his wife Faizah Khan decided to leave him, and was in "no state of mind to be flying," a close friend and fellow pilot revealed.

Shah's close friend, under condition of anonymity, told The New Zealand Herald that the 53-year old pilot could have taken the passenger jet for a "last joyride."

The friend said: "He's one of the finest pilots around and I'm no medical expert, but with all that was happening in his life Zaharie was probably in no state of mind to be flying."

Shah's friend had spoken to him on several occasions in the last year and the source said the pilot was obsessed by "three Fs" - Food, Family and Flying.

Shah and his wife, though separated, lived under the same roof. However, the pilot was reportedly seeing another woman.

Shah's separated wife, the mother of his three children, is likely to be questioned by Malaysian authorities about the possibility of her husband's suicide being behind the jetliner's disappearance.

Based on the routing, signalling and communications data transmitted from the aircraft, investigators are gravitating towards the suicide mission theory.

"The personal life of Zaharie Shah, however, is far more complex and is in the process of being unravelled," an investigation source told Malaysia Chronicle.

Shah has been under increasing scrutiny over the vanished airliner in the last week. It was earlier alleged that his political views could have prompted some kind of rogue behaviour.