Such systems operate a great deal like hydraulic systems,  except they employ air instead of a liquid for transmitting  power. Pneumatic systems are sometimes used for: For high-pressure systems, air is usually stored in metal  bottles at pressures ranging from 1,000 to 3,000 psi,  depending on the particular system.  [Figure 12-70]  This  type of air bottle has two valves, one of which is a charging  valve. A ground-operated compressor can be connected to  this valve to add air to the bottle. The other valve is a control  valve. It acts as a shutoff valve, keeping air trapped inside  the bottle until the system is operated. Although the high- pressure storage cylinder is light in weight, it has a definite  disadvantage. Since the system cannot be recharged during  flight, operation is limited by the small supply of bottled  air. Such an arrangement cannot be used for the continuous  operation of a system. Instead, the supply of bottled air is  reserved for emergency operation of such systems as the  landing gear or brakes. The usefulness of this type of system  is increased, however, if other air-pressurizing units are added  to the aircraft.  [Figure 12-71] Both pneumatic and hydraulic systems are similar units and  use confined fluids. The word confined means trapped or  completely enclosed. The word fluid implies such liquids  as water, oil, or anything that flows. Since both liquids and  gases flow, they are considered as fluids; however, there is  a great deal of difference in the characteristics of the two.  Liquids are practically incompressible; a quart of water still  occupies about a quart of space regardless of how hard it is  compressed. But gases are highly compressible; a quart of  air can be compressed into a thimbleful of space. In spite of  this difference, gases and liquids are both fluids and can be  confined and made to transmit power. The type of unit used to  provide pressurized air for pneumatic systems is determined  by the system’s air pressure requirements. Pneumatic systems are often compared to hydraulic systems,  but such comparisons can only hold true in general terms.  Pneumatic systems do not utilize reservoirs, hand pumps,  accumulators, regulators, or engine-driven or electrically  12-48 Figure 12-70.  High-pressure pneumatic system. Abstactor Oil and water trap Charging valve Dehydrator Pressure relief valve Nonreturn valve Porous metal filter Sensing line Pressure switch To nose wheel steering Normal brakes Normal gear control Pressure gauge Pressure gauge Pressure reducing valve Pressure relief valve Main bottle Isolating valve Alternate/emergency bottle Alternate/emergency brakes Alternate/emergency gear control Air filter Warning light Compressor Pressure regulator LH Nacelle RH Nacelle Pneumatic System Components High-Pressure Systems • Brakes •  Opening and closing doors •  Driving hydraulic pumps, alternators, starters, water  injection pumps, etc. •  Operating emergency devices