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The loud cracking noise and the restriction in the cyclic control reported by the pilot may have resulted from the fatigue failure of the engine-cooling fan. The investigation was unable to determine if the damage to the control rod would have been sufficient to adversely affect the flight controls.

Although it is likely that the specified torque was previously applied to the fan retaining bolts, the required clamping force may not have been achieved due to frictional torque effects and the painted surface of the fan.

The difference in the surface finish from that specified by the manufacturer would be expected to have a detrimental effect on the resistance of the fan to fatigue failure. Differences in assembly practices from those specified by the manufacturer may alter the magnitude of the alternating stresses created in the fan. A determination of which difference was the most significant, or whether failure resulted from the combination of the effects of several differences, would require further testing and analysis by the manufacturer.

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