The metering device of the present invention is characterized by controlling the simultaneous relative positions of two sliding valve elements so that the degree of opening between fuel inlet and outlet orifices always corresponds to the setpoint of the fuel flow. If the position of either of the two sliding elements in the metering device of the present invention can no longer be controlled for whatever reason, such as being jammed, the other element may always be controlled to move relative to the jammed element such that the fuel flow may be controlled by the other element to correspond to that of the fuel flow setpoint. Thus, the metering device according to the present invention will continue to operate effectively even if the position of either of its slide elements can no longer be properly controlled, either because the element has jammed or because there has been a malfunction in its control system.