Abstract: A hardover protection system for an aircraft. A differential motion sensor is mounted to a sector for receiving cockpit pitch commands input via a cable system. The motion sensor includes a pair of cranks held together by a predetermined spring preload. One of the cranks is coupled to a rod that forms a portion of a power linkage for hydraulically driving an elevator surface while the other is coupled to a rod that is linked to a shutoff valve. A linkage for mechanically driving an elevator control tab is also mounted to the sector. The design of the shutoff valve and its location within the aircraft's hydraulic control system interact to disable powered operation of the elevator surface upon a predetermined degree of deflection of the shutoff valve piston. Such deflection is responsive to the detection of a predetermined mismatch between the input command motion to the sector and the output motion of the power linkage.