utility hydraulic system. It can also limit the use of all available utility system pressure for the operation of the primary flight controls or prevent fluid loss during flight when damage to the utility system has occurred. This valve is sometimes referred to as a priority valve and normally has three modes or conditions of operation. CONDITION  ONE  (LANDING).—Flight control system pressure normal, switch in the landing position, solenoid deenergized, and the pilot ball on its lower seat, blocking the return port of the flight control system. See Figure 7-27, View A. In this condition, the pressure of the flight control system is allowed to act upon the lower working area of the poppet, moving it upward off its seat and compressing the poppet spring. This action will allow the fluid of the utility system to flow downstream from the location of the valves to the landing gear, flaps, speed brakes, etc. CONDITION TWO (FLIGHT).—Flight control system pressure normal, switch in the flight position, solenoid energized, and the pilot ball on its upper seat, preventing the pressure of the flight control system from working on the lower working area to the poppet. See Figure 7-27, View B. In this condition the return port of the flight system is open. The poppet spring will move the poppet onto its seat, preventing the fluid fron the utility system from flowing downstream from the location of the valve. This allows all available fluid to be directed to the components of the utility section, such as the ailerions, rudder, stabilizer, spoilers, of the flight control subsystem. CONDITION THREE (EMERGENCY).— Failure of the flight control hydraulic system. The flight control system pressureis 0 psi, and the utility system pressure is normal. During this condition, the poppet will remain on its seat, because the pressure of the flight control system is not available to work on the lower working area of the poppet to move it up to open the valve. See Figure 7-27 View C. Failure of the electrical system to the electro- hydraulic shutoff valve. The pressures of the flight control and utility systems are normal, and there is no electrical power to the solenoid In this conditon, the solenoid cannot be energized, the polit ball will remain on its lower seat, and the pressure of the flight control system will work on the lower working area. This holds the poppet of its seat and allows the pressure of the utility systems to flow downstream from the location of the valves. Manual Shutoff Valves Manual shutoff valves may be used as tire wall shutoff valves as well as subsystem shutoff valves. Some aircraft have a manual fire wall shutoff valve operated by cable linkage. Some aircraft use the needle-type shutoff valve in their landing gear and bomb bay systems. This needle-type valve consists of a handle, stem and valve, and body. Turning the handle in a clockwise direction places the valve on its seat within the body, stopping the flow of fluid. These shutoff valves are used during maintenance to shut off hydraulic fluid to the subsystems, thus allowing maintenance personnel to work safely in the wheelwell and bomb bay areas. Also, by closing the particular valve a desired amount, the speed of the operating unit can be controlled to aid in observing the sequence and full operation of the components being operated. HYDRAULIC FLUID COOLERS Hydraulic fluid coolers are used in some hydraulic systems for the purpose of lowering the temperature of the fluid within the system lines, thus preventing inadvertent overboard dumping of fluid from the reservoir due to thermal expansion. Fluid coolers are installed in systems in which the temperature of the fluid is likely to exceed the maximum allowable limit. According to the military specifications for aircraft hydraulic systems, 400°F is the maximum allowable temperature for any type of hydraulic system. In some systems, this temperature might be exceeded without some means of cooling the fluid. Several types of fluid coolers are used on naval aircraft. The most common is the radiator type, in which both the hydraulic fluid and engine fuel flow separately through the cooling unit. Another radiator type uses ram air in flight and an electric blower while on the ground to produce an air source as a cooling medium. Radiator Types Radiator-type fluid coders are also called heat exchangers and fluid coolers, as well as radiators. Their principles of operation are the same; however, the manner in which they obtain their objective may differ. On some aircraft, the radiator is a welded aluminum assembly with two semicylindrical and baffled hydraulic fluid chambers with multiple pencil diameter size tubes, which direct and contain fuel flow through the individual hydraulic chambers. The radiator is so constructed to prevent mixing of engine fuel with hydraulic fluid and one hydraulic system fluid with the other. As fuel flows through the radiator tubes, heat energy is transferred from the hydraulic fluid to the engine fuel prior to hydraulic fluid entry into the hydraulic reservoir. Figure 7-28 shows the cooling radiator used to cool two hydraulic systems; moreover, it has a fuel filter incorporated that filters the fuel supplied to the 7-28