20070708017629A

(-23)The location of the accident was figueroa mountain rd, 5 miles north of state route 154 near the community of Los Olivos. This accident investigation involves a CH-54A, registration number N44094, serial number 67-18421 operated by heavy lift helicopters, inc., FAA operating certificate number H3VL008H. This twin-engine turbine powered helicopter's maximum gross weight is 38,000 pounds, and it was being operated under an "on call" contract for the state of California (calfire) in support of wild land fire suppression activities on the zaca fire incident located in santa barbara county. During the on-scene investigation, physical evidence consisting of a nut, bolt, and plate was found approximately six feet from the wreckage. This pin assembly secures one of three servo units to the helicopter transmission on one end and the helicopter swash plate on the other end. Attached to the pin via a bolt, is a pin locking holder with a receiving hole on the opposite end. This pin is placed through an outside attach lug which is integral to the transmission case, then through lower servo attach lug and into an inside attach lug (again integral to the transmission case), which secures the collective servo (control device) to the swash plate. This gives the pilot directional control to the main rotor. The pin locking holder attached to the outside end of the pin is a safety retention device which slides over a stud on the helicopter's transmission case. There is a nut that is fastened onto the stud then safety-wired to the bolt which attaches the pin locking holder to the pin. This entire assembly was located several yards away from the main wreckage. Further investigation of the transmission case pin retention stud which retains and secures the pin that attaches the collective servo to the transission case has excessive werar from fretting at the lower 3/16 inch of the stud from bearing contact from the pin retention strap. The threads in the area of the retaining nut contact (center and upper portion of the stud) had deformed threads that were deformed upward (or away from the transmission case). The thread deformation appears to have been the result of tension/compressive overload due to pn and strap separation. The 14 inch x 28 retaining nut had separated from the stud and was attached to the pin retention bolt by safety wire. The threads on the nut appeared to be deformed from wear and overload. The inside attach lug, integral to the transmission case, has a closed end bushing installed which provides a bearing surface for the pin and also prevents main transmission lubrication oil from leaking out of the case. This bushing is manufactured with a flat end surface perpendicular to the round bushing od. The installed bushing appeared deformed from contact of an interior transmission part or component. It is not known what force is requried, during helicopter operation, to "push" the pin and strap assembly out of the bushings bores. The force however to rollover and deform the threads on the stud would be more than normal vibration forces and would have required an external/internal force to cause the retaining nut to be pulled off the threads. The stud thread signatures substantiate the application of an internal force that pushed the pin out of its installed position. The applied force was strong enough to push the pin out of the bushing bores and to cause the retaining plate to depart the stud and push the threaded nut off the stud. The nut, pin, retention strap and stud are flight safety critical components. However, if the inside bushing, manufactured by heavy lift helicopters, inc is not manufactured to correct tolerances and as wear takes place inside the main transmission and tolerances change, internal mechanical forces could "push" this pin assembly and nut off of the stud. This will cause the separation of the servo unit, and consequently cause main rotor blade pitch control loss. The loss of control would likely manifest itself in a rollover event if the helicopter were in the process of taking off, or a loss of flight control if the event were to occur while airborne. Witness and photographic evidence gathered thus FAR documents that the pilot of the accident helicopter lost control during his attempted takeoff. As a result, the helicopter rolled over. Observations from participating on-scene FAA and calfire personnel indicated that separation of the servo assembly was likely related to the pilot's loss of helicopter control, and its subsequent rollover.