A controllable pitch propeller device is provided with a pitch locking system that prevents a controllable pitch propeller blade from being inadvertently forced to an extreme end of travel in the event that hydraulic pressure is lost in either of two conduits or as a result of a pump failure. The hydraulic pressure in each of two conduits is used to overcome a check valve in the opposite conduit to allow return hydraulic fluid flow through that opposite conduit. In the event of a loss of hydraulic pressure, the pressure needed to overcome the check valve is absent and the check valve operates normally to block return flow to a source of hydraulic pressure, such as a pump. This blockage of return flow, in turn, prevents a hydraulically responsive element, such as a piston, from moving subsequent to the loss of pressure.