CHAPTER 4 AIR-CONDITIONING SYSTEMS 

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

AMEs maintain the air-conditioning and pressurization systems of naval aircraft. These systems provide heating and cooling of the cabin and, at altitude, the pressurization required for breathing. As an AME, you will be assisting aircrews and troubleshooting discrepancies. A good knowledge of the systems is necessary to perform effectively. This chapter uses the S-3 environmental control system as the basis for discussion. To simplify matters, we have divided the system into two subsystems: bleed air and air-conditioning.

BLEED-AIR SYSTEM

Learning Objective: Identify the operating principles and components of a bleed-air system.

The bleed-air system is the air source for the environmental control system (ECS) and for deicing functions. There are three sources of bleed air available. The primary source is the compressor sections of the two aircraft engines. Secondary sources are from the auxiliary power unit (APU) and from an external air supply such as support equipment (SE).

SYSTEM OPERATION

As previously stated, the source of air for the bleed-air system may be from the aircraft engines, the APU, or SE. Operation of the system using each of these sources is presented in the following paragraphs. Frequent referral to the bleed-air system schematic (fig. 4-1) will aid you in understanding the material.

Engine Bleed Air

The engine bleed air is extracted from the 10thand 14th-compression stages of each engine. The low-stage bleed-air check valve supplies the 10thstage air, which is the primary source for operation of the ECS. When 10th-stage air is insufficient to meet ECS demands, 14th-stage air is supplied through the high-stage, bleed-air regulator valve.

One bleed-air shutoff valve is installed in each engine pylon downstream of the 10th- and 14thstage engine-compressor bleed ports. The bleedair shutoff valves are controlled by switches on the eyebrow panel in the flight station. Lights on the instrument panel indicate the position of the bleed-air shutoff valves. The lights illuminate when the valves are closed regardless of the position of the switches. When open, the bleed-air shutoff valves allow engine compressor bleed air to flow into the bleed-air manifolds.

The bleed-air manifold distributes bleed air from both engines into the air-conditioning and pressurization systems. Two crossover duct isolation check valves prevent the possibility of an overbleed of both engines should a rupture occur in the left or right bleed-air manifold.

The check valves, located in the left and right manifolds, allow bleed air to flow in one direction

Figure 4-1.-Bleed-air system schematic.

only. If the left or right engine bleed air is secured, or if a rupture occurs in the left or right bleed-air manifold, the appropriate check valve closes. This allows the air-conditioning and pressurization subsystems to operate from the opposite bleedair source. An open engine bleed-air bypass and shutoff valve allows bleed air to bypass the check valves and flow from the left-to-right or right-to-left manifolds. The engine bleed-air bypass and shutoff valve is open during engine starting. It is also open when operating the deicing system with one engine secured.

Bleed-air pressure is sensed by the bleed-air pressure transmitter located in the bleed-air supply duct downstream of the crossover duct isolation check valves. The pressure is displayed on the bleed-air pressure indicator on the environmental panel.

Bleed air from the left and right manifolds flows through the crossover duct isolation check valves to the bleed-air flow control and shutoff valve. The bleed-air flow control and shutoff valve is electrically controlled and pneumatically actuated to modulate the bleed-air flow to the airconditioning and pressurization systems in response to predetermined flow schedules.

Two alternate air supply sources, APU air and ground start air, connect to the left and right bleed-air manifold. The APU air duct supplies bleed air through two check valves to the left manifold. The ground start duct supplies highpressure air through a check valve to the right manifold. These alternate air supply sources are used primarily for starting engines and for ground operation of the air-conditioning system.

APU Bleed Air

Bleed air flows from the APU compressor through two one-way check valves in the APU duct to the left one-way check valve in the crossbreed manifold. Bleed air is also supplied to the bleed-air shutoff valve, the left side of the engine bleed-air bypass and shutoff valve, empennage deice valve, the left wing deice valve, and the ram air anti-icing valve. With the bleed-air switch in the ON position, the left bleed-air shutoff valve is opened. In the open position, bleed air is supplied to the left engine starter control valve.

To provide bleed air to the right side of the crossbreed manifold, the engine bleed-air bypass and shutoff valve is opened to allow bleed air to the right bleed-air shutoff valve and to the right wing deice valve. With the bleed-air engine No. 2 switch set to ON, the right bleed-air shutoff valve is opened. In the open position, bleed air is supplied to the right engine starter-control valve.