A fitting in a mechanical control system having a jack for the displacement of an actuator. When a stroke of the jack is impeded by a problem in the kinematic chain, the jack continues to push with increased intensity and may damage one or more elements of the mechanism for transmitting the movement, or even more. A frangible part, having a weak point that can withstand up to a certain force threshold is provided in the fitting. Beyond this threshold, the part breaks and allows the jack to continue its stroke so as to not damage the overall system, in both directions of displacement of the jack.