A system is disclosed for measuring displacement of a regulation system for a variable pitch axial fan. The system includes a hub having a plurality of adjustable pitch fan blades. A hydraulic cylinder is connected to the hub. A piston is disposed within the hydraulic cylinder. The piston is coupled to a piston shaft that is axially movable with respect to the hub. The piston shaft is coupled to an actuation assembly for adjusting the pitch of the plurality of adjustable pitch fan blades in response to an axial movement of the piston. A rotating oil union includes a housing and an interior element, the interior element coupled to the hydraulic cylinder so that rotation of the cylinder and the hub causes rotation of the interior element. The rotating oil union includes a static housing and a central probe fixed to the static housing. The central probe extends through respective openings in the interior element, the hydraulic cylinder, and the piston. The central probe is configured to sense an axial position of the piston and to generate a signal representative of said axial position. A method for using the system is also disclosed.