Douglas Barrie/LONDON
A CRITICAL PART of the Westland Lynx main-rotor- blade assembly is being replaced on a precautionary basis, with the UK manufacturer admitting to being "confused" over the underlying cause of a fatal Lynx accident in September 1994.
The loss of a British Army Air Corps Lynx in Germany was attributed to a catastrophic tie-bar failure. The tie-bar is situated between the rotor head and blade, and absorbs centrifugal and torsional blade stress.
Although the tie-bar was originally intended to be stressed for the airframe life of the helicopter, Westland is now replacing the part every 1,500 flight hours. Inspections are also being carried out every 200h.
Westland says that it has so far been unable to find a " definitive cause" for the failure. The tie-bar was originally stressed to a 90,000kg breaking tension, although, during its life, this could reduce to 55,000kg. Westland says that the maximum tension it would expect on any tie-bar during flight is 41,000kg.
Westland is being particularly cautious in not writing the incident down as a one-off. It has also found " unexpected wear characteristics" in a "minority" of the Lynx tie-bars it has inspected.
It is also anxious to understand fully the cause of the failure because it plans to upgrade the Lynx airframe to allow it to stay in service until 2035 (Flight International, 21-27 June).
The company appears to have ruled out a manufacturing problem as the root cause of the failure, or as a cause of greater-than-projected wear on other tie-bars.
"If anything, the pattern is more akin to the type of use," says the company.
Source: Flight International
