Helicopter control mechanisms were failing at a higher than normal rate on high-altitude flights in mountainous regions. All of the failures occurred at or near attachment points on pressurized tubes, causing a pressure drop and partial loss of function. Visual and SEM examinations revealed cracks along the inner surface of the tubes, some of which had propagated through the thickness of the wall. Cracks emanating from weld toes were also visible. Based on their observations, investigators concluded that the tubes were subjected to excessive flexural load, causing cracks due to fatigue. They also provide recommendations for avoiding such failures...