A collision between the co-axial main rotor blades caused the fatal crash of a Russian army Kamov Ka-50 attack helicopter in June 1998, says an official investigation into the accident.

The helicopter's pilot Maj Gen Boris Vorobyev, commander of the Army Aviation centre at Torzhok in the Tver region, was killed. He was one of the most experienced Russian military helicopter pilots.

The enquiry's official findings say "-the cause of the crash was the unintentional entry into an unexplored flight regime during complex combat manoeuvring exceeding current flight limitations". Eye witnesses claim the pilot was flying a simple terrain-hugging flight profile.

Many Russian helicopter experts claim that there is an unacceptable risk of collision between the upper and lower rotor blades during harsh combat manoeuvring, especially if abnormal rotor conditions have been caused by combat damage.

The crash of a Ka-50 prototype and those of earlier Kamov helicopters have been attributed to the same cause.

Source: Flight International