Thrust Idler Bellcrank Fractures

 

 

             Whilst carrying out a turn-around flight servicing inspection [post-flight] the thrust idler bellcrank assembly, Part Number (P/N) 114C1408-1 was found completely fractured across the rigging pin hole. The aircraft had just completed a forty minute engine air test [ground run]. No unusual control or engine responses were noted. The barometric altitude hold was tested and found serviceable. [The Baro Hold actuator is connected to the Thrust Idler]. This is the first occurrence of this failure in the British Royal Air Force (RAF) Helicopter (HC) Mark 2 (Mk2) fleet. Aircraft Time Since New (TSN): 5453.55 hours, TSMLU: 1770.05 hours. Aircraft Serial Number (S/N): ZA-677. Boeing Build Number: M7012.

 

 

         
Fractured Thrust Bellcrank in British aircraft ZA-677.

 

 

         
Fractured Thrust Bellcrank in British aircraft ZA-677.

 

 

         
Fractured Thrust Bellcrank in British aircraft ZA-677.

 

 

             While this was the first for an RAF helicopter, there have been several on the U.S. Army helicopters. The problem is that the thrust idler bellcrank is not designed to have sideward forces applied. So, pilots and crew, be aware and do not touch the bellcrank. When rigging the flight controls, do not force the rigging pin into the hole, and do not let the weight of the flight controls rest on the bellcrank with the rigging pin installed. On a U.S Army CH-47D, should this bellcrank break with either or both engines running, the pilots can expect a duel engine high side emergency - What fun, eh?

 

 

          The CH-47 - 40 years old and still circling the world.

         

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