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.

Source: Flight International