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