Riser ducts Distribution manifold duct assembly Overhead duct Overhead supply ducts Figure 16-59.  Centralized manifolds from which air can be  distributed are common. Air Conditioning Systems Air Cycle Air Conditioning System Operation Cabin Pressurization Troubleshooting Pneumatic System Supply 16-35 The air cycle air conditioning system is supplied with air by  the aircraft pneumatic system. In turn, the pneumatic system  is supplied by bleed air tap-offs on each engine compressor  section or from the APU pneumatic supply. An external  pneumatic air supply source may also be connected while  the aircraft is stationary on the ground. In normal flight  operations, a pneumatic manifold is supplied by the engine  bleed air through the use of valves, regulators, and ducting.  The air conditioning packs are supplied by this manifold as  are other critical airframe systems, such as the anti-ice and  hydraulic pressurization system.  While pressurization systems on different aircraft operate  similarly with similar components, it cannot be assumed  that they are the same. Even those systems constructed by a  single manufacturer likely have differences when installed  on different aircraft. It is important to check the aircraft  manufacture’s service information when troubleshooting the  pressurization system. A fault, such as failure to pressurize  or failure to maintain pressurization, can have many  different causes. Adherence to the steps in a manufacturer’s  troubleshooting procedures is highly recommended to  sequentially evaluate possible causes. Pressurization system  test kits are available, or the aircraft can be pressurized by  its normal sources during troubleshooting. A test flight may  be required after maintenance. increases the time in service of these expensive components  and expedites expensive mandatory overhauls that are  performed at specified time intervals. Most high-performance,  medium size, and larger turbine-powered aircraft are fitted with  a receptacle in the air distribution system. To this a ground  source of conditioned air can be connected via a ducting hose.  The cabin can be heated or cooled through the aircraft’s air  distribution ducting using air from the ground source. This  limits the operating time on the engines and APU. Once  prefight checks and passenger boarding are completed, the  ducting hose can be disconnected for taxi and flight. A check  valve is used to prevent ground source air from flowing  upstream into the air conditioning system.  [Figure 16-61] Even with the frigid temperatures experienced at high  altitudes, bleed air is too hot to be used in the cabin without  being cooled. It is let into the air cycle system and routed  through a heat exchanger where ram air cools the bleed air.  This cooled bleed air is directed into an air cycle machine.  There, it is compressed before flowing through a secondary  heat exchange that cools the air again with ram air. The bleed  air then flows back into the air cycle machine where it drives  an expansion turbine and cools even further. Water is then  removed, and the air is mixed with bypassed bleed air for final  temperature adjustment. It is sent to the cabin through the  air distribution system. By examining the operation of each  component in the air cycle process, a better understanding  can be developed of how bleed air is conditioned for cabin  use. Refer to  Figure 16-63,  which diagrams the air cycle air  conditioning system of the Boeing 737.  Air cycle air conditioning prepares engine bleed air to  pressurize the aircraft cabin. The temperature and quantity  of the air must be controlled to maintain a comfortable cabin  environment at all altitudes and on the ground. The air cycle  system is often called the air conditioning package or pack.  It is usually located in the lower half of the fuselage or in  the tail section of turbine-powered aircraft.  [Figure 16-62] There are two types of air conditioning systems commonly  used on aircraft. Air cycle air conditioning is used on most  turbine-powered aircraft. It makes use of engine bleed air or  APU pneumatic air during the conditioning process. Vapor cycle air conditioning systems are often used on reciprocating  aircraft. This type system is similar to that found in homes  and automobiles. Note that some turbine-powered aircraft  also use vapor cycle air conditioning.