primarily an airframe engineering and manufac- turing problem. 2. There must be a means of limiting the maximum pressure differential to which the cabin walls will be subjected. This is provided by the cabin safety valve. 3. The aircraft must have an adequate supply of compressed air. This is provided through the compressor section of the jet engine. A separate compressor or supercharger is used on aircraft having reciprocating engines. On all jet aircraft, the air is taken directly from the compressor section of the jet engine. This is generally referred to as bleed air. 4. There must be a means of cooling the bleed air before it enters the cabin. This is provided by an aircraft refrigeration unit. 5. There must be a means of controlling the cabin pressure. This is provided by the cabin pressure regulator, which regulates the outflow of air from the cabin. In addition to the major components, various valves, controls, and other allied units are necessary to complete an aircraft pressurization and air-conditioning system. The design, construc- tion, and use of these components may vary somewhat with different manufacturers; however, the systems on all jet aircraft operate on the same principles. The system used as an example in this text is in the F-18 aircraft. The environmental control systems of most aircraft include cabin air conditioning and pressurization, equipment cooling, defogging, windshield washing and rain removal, and equip- ment pressurization subsystems. Coverage in this section is limited to air cycle cabin and equipment pressurization and air conditioning. BLEED-AIR SYSTEM Bleed air is supplied by the last compressor section of each engine (fig. 3-1, a foldout at the end of this chapter). This bleed air flows from the engines through two engine bleed-air pressure regulation and shutoff valves. The valves are spring-loaded closed when the system is not in use. When air conditioning is selected, the valves open and regulate bleed air to a predetermined pressure. The bleed air then passes through two engine bleed-air check valves, which prevent reverse flow from one engine to the other. At this point the bleed air from both engines enters a common duct and flows through the engine bleed-air secondary pressure-regulating and shutoff valve. This valve is spring-loaded open and regulates the pressure of the combined flow of bleed air from both engines. The regulated bleed air then flows into the primary heat exchanger of the ACS. There are two overpressure switches (primary and secondary) incorporated in the system to prevent overpressure damage to system com- ponents in case of a pressure regulator mal- function. An air isolation valve is located in the system to provide a means of providing bleed air to the ACS when required and during cross starting of engines. These bleed-air components are discussed in the following paragraphs. Engine Bleed-Air Pressure Regulation and Shutoff Valve These two valves (fig. 3-1) act as system shutoff valves when air conditioning is not required. They are spring-loaded closed. When air conditioning is selected, an electric solenoid is energized, which unseats a poppet from the vent line. As air flows from the engine, a line downstream of the butterfly valve routes a small amount of the bleed air to the butterfly diaphragm. This air is called control air since its action on the diaphragm is the controlling force for the valve. As pressure builds on the diaphragm, it overcomes spring pressure holding the butterfly closed and the valve opens. As the bleed air passes through the valve, another line upstream of the butterfly routes bleed air to the regulator portion of the valve. As pressure builds and overcomes spring pressure, a poppet is reseated, allowing some of the control air pressure from the open side of the butterfly diaphragm to bleed off. Spring pressure can now start closing the butterfly, thus lowering the bleed-air pressure downstream of the butterfly. In this manner bleed-air pressure is controlled to 75 ± 15 psi. Engine Bleed-Air Check Valve These dual-flapper check valves are located downstream of the pressure regulator and shutoff valves (fig. 3-1). They prevent cross-flow of bleed air from one engine to the opposite engine in the event of single engine operation. Engine Bleed-Air Secondary Pressure-Regulating And Shutoff Valve This valve is located in the common ducting upstream of the bleed-air check valves (fig. 3-1). 3-3