Ми-26Т RA-06121 20.12.2011

     The Investigation team of the Interstate Aviation Committee has completed the investigation of the accident with Mi-26T RA-06121 helicopter owned by "Nefteyugansk United Airline Transportation Company", LLC occurred in the Tyumen region (RF) on December 20, 2011.  
      The accident with Mi-26T RA-06121 helicopter was caused by the destruction of the central fuselage construction along the frame № 33 damaged downwards to the frame № 36 in straight flight.
      Damage of the construction had high-cycle fatigue and started from the frame № 33 in the space between № 2 left and right stringers.
      Critical fatigue failure size of the fuselage shell region in the accident flight was 560 mm.  
      Design critical size of the crack for normal operating loads is 1435 mm. Multifocal high-cycle fatigue failure and contraction of the size of this failure region indicate the increased stress of the central fuselage at the frame № 33 between № 2 left and right stringers.   
      Most probably the increased stress was caused by the impact of loads during the helicopter operation (their redistribution and increase) resulted in changing of stress-strain behavior of the construction in the upper area of the frame № 33. It was impossible to indicate the rate, type and time of loading.
      The increased stress of the dorsal section at the frame № 33 caused accelerated initiation and growth of cracks as well as the reduction of critical size of the fatigue failure area.
      The available instrumental monitoring facilities don't allow to identify the change of stress-strain behavior of the construction.
      Fatigue cracks were not identified during the helicopter maintenance.  
      Contributing factors could be:
      - defects of helicopter manufacture and maintenance work in the area of stringer № 2 at the frame № 33 that could cause the cracks initiation before;  
      - abnormal (violation of FOM and AMM regulations) external load that could accelerate initiation and cracks development during exploitation of this load.
      
The appropriate safety recommendations based on the investigation results were developed.