CHAPTER 3 UTILITY SYSTEMS 

Chapter Objective: Upon completion of this chapter, you will have a working knowledge of the operating principles and components of bleed-air utility systems.

The utility systems of an aircraft provide an additional measure of flight safety, pilot comfort and convenience, and contribute to the overall mission capability of the aircraft.

BLEED-AIR UTILITY SYSTEMS

Learning Objective: Recognize the operating principles and components for systems within the bleed-air utility system.

Many aircraft have utility systems that rely on a bleed-air system to function. The P-3C deicing system and the A-6E rain removal system are examples of such systems and are discussed in this chapter. This material will increase your proficiency in troubleshooting and maintaining these and similar systems.

DEICE SYSTEMS

An anti-icing system is designed to prevent ice from forming on the aircraft. A deicing system is designed to remove ice after it has formed. An aircraft deice system removes ice from propellers and the leading edges of wings and stabilizers. These systems may use electrical heaters, hot air, or a combination of both to remove the ice formation. As an AME, you are primarily concerned with hot air as a method to remove the formation of ice on wings and stabilizers. The P-3C wing deice system is used as an example in this chapter to describe a hot-air system.

Description and Components

The P-3C wing deice system uses hot compressed bleed air from the engines. The air is ducted from the 14th stage of each engine compressor, as shown in figure 3-1. The bleed air is maintained at a fixed percentage of engine airflow for all altitudes and flight speeds.

The hot bleed air is directed and regulated to the leading edge ejector manifold through shutoff valves, modulating valves, thermostats, skin temperature sensors, and overheat warning sensors.

SHUTOFF VALVES.- The wing deice system contains several shutoff valves. The fuselage bleed-air shutoff valves, installed in the cross-ship manifold on the right and left wings, isolate the wings from the fuselage duct section. In addition, they maybe used to isolate one wing duct from the other wing duct. Each valve is individually controlled by a guarded toggle switch mounted on the bleed-air section of the ice control protection panel.

A bleed-air shutoff valve is also installed in each engine nacelle. These shutoff valves are physically identical. They are of the butterfly-type, and they are actuated by an electric motor.

An indicator, located on top of the valve housing, shows the position of the valve-open or closed. This indicator enables you to visually check the operation of the valve while it is still installed in the deice system.

MODULATING VALVES.- The P-3C deicing system has three modulating valves installed in each wing. These valves are thermostatically controlled and pneumatically operated. They maintain the constant engine compressor bleedair temperature required for the wing leading edge. When deicing is not required, the valves operate as shutoff valves.

The modulating valves, shown in figure 3-2, have pilot solenoid valves that are electrically

3-2

Figure 3-2.-Anti-icing modulating valve.

controlled by three switches on the bleed-air section of the ice protection panel. When the solenoid is energized, it admits filtered, regulated, bleed-air pressure to one side of a diaphragm chamber in the valve. The other side of the diaphragm chamber is spring-loaded to the closed position. Movement of the diaphragm operates a main line butterfly valve.

When the valve opens, hot air is admitted to the leading edge distribution system. The hot air goes through the modulator valve to the ejector manifold, out the jet nozzles, and into the wing leading edge plenum area. The bleed air is then directed across a pneumatic thermostat. Increased temperature across the thermostat actuates the sensor and opens a bleed passage from the diaphragm chamber. This reduces the pressure on the diaphragm and allows a spring to close the main valve.