Investigators have hinted that pilot error will likely be deemed the cause of the disaster, with a focus on the engine fuel control switches.

A preliminary report into the deadly crash of an Air India jetliner in June is expected to be released by Friday, three sources with knowledge of the matter said, with one adding the probe had narrowed its focus to the movement of the plane's fuel control switches.

As the world waits for answers on the fatal Air India Boeing 787 crash, accident investigators have narrowed their focus on the engine fuel control switches.