The Boeing 747-400 Upper Rudder Control System with Triple Tandem Valve

To the casual airline passenger the appearance of the Boeing 747-400 jetliner is identical to that of the older model 747-300 with the exception of the winglets. However, modifications and improvements include modern digital avionics, a “glass” cockpit and more powerful rudder control surfaces to cope with the growth in engine thrust. The upper rudder is positioned by three parallel actuators and powered by two independent hydraulic systems. Flow control is provided by a triple tandem servovalve which maintains actuator force fight below allowable limits and synchronizes hydraulic flow to the three actuators. This paper describes the requirements, design, development, analysis and iron bird testing of some of the unique features of the upper rudder hydraulic control system.