a hydraulic control system which is of an open center parallel circuit arrangement for aerial boom devices with two return lines , said system having control valves for upper and lower boom arms , wherein the lower boom control valve overrides the upper boom control valve . the system includes a valve connected to a pump actuating engine and responsive to hydraulic pressure to increase the speed of the fluid pump when the flow or pressure in one return line is reduced due to diversion of fluid within the system into the second return line . the system also includes a normally open relief valve carried by the boom supporting turntable adjacent its pivotal connection to the lower boom and operable to control a winch actuator and to deactivate said winch actuator when the boom is in an unstable position .