The BellBoeing CV-22 crash in Afghanistan on 8 April was not caused by a mechanical failure, according to a source familiar with preliminary findings of the US military investigation.
The fatal crash, which killed four and injured others, occurred after the pilot lost situational awareness while landing in a wadi around 1am under brown-out conditions, the source says.
The incident killed the pilot, a flight engineer, an army Ranger and an unidentified civilian.
Air Force Special Operations Command (AFSOC), which owns the CV-22 fleet, was not immediately available to comment.
US military officials have previously stated the cause of the CV-22 crash in Afghanistan was still under investigation. Military spokesmen, however, have ruled out enemy fire as a potential cause.
The 8 August crash is the first fatal accident involving a V-22 Osprey tiltrotor since December 2000, and is the fifth fatal crash in the programme's chequered history.
In 2000, two fatal crashes within eight months caused by a combination of design flaws and mechanical failures forced military leaders to put the programme on hold for two years while contractors re-designed systems and the airframe to improve safety.
After declaring the MV-22 fleet operational in 2007, the US Marine Corps has deployed its version of the Osprey in Iraq and Afghanistan without suffering a fatal crash.
USMC officials have praised the MV-22's performance, although the service has acknowledged concerns about unexpectedly high costs to operate and maintain the unique tiltrotor fleet.
AFSOC, meanwhile, had deployed six CV-22s delivered so far to Africa, Iraq and Afghanistan before sustaining the crash.
The brown-out scenario during landing is recognized as a major safety concern for all rotorcraft operating in areas with loose sand. A recent study by the Office of the Secretary of Defense has concluded that 80% of the US millitary's 320 rotorcraft crashes during the last decade has been caused by degraded visual awareness.