level of cleanliness must be maintained when working on hydraulic components. Only approved fill stand units are used to service naval  aircraft  hydraulic  systems.  By  following  a  few basic  rules,  you  can  service  hydraulic  systems  safely and keep contamination to a minimum. !    Never use fluid that has been left open for an undetermined  period  of  time.  Hydraulic  fluid that is exposed to air will absorb dust and dirt. !    Never   pour   fluid   from   one   container   into another. !    Use   only   approved   servicing   units   for   the specific aircraft. !    Maintain  hydraulic  fluid-handling  equipment in a high state of cleanliness. !    Always     make     sure     you     use     the     correct hydraulic fluid. Contamination   of   the   hydraulic   system   may   be caused by wear or failure of hydraulic components and seals.   This   type   of   contamination   is   usually   found through filter inspection and fluid analysis. Continued operation    of    a    contaminated    system    may    cause malfunctioning or early failure of hydraulic components. Q4-20. What  are  two  disadvantages  of  a  hydraulic system? Q4-21. On   a   basic   hydraulic   system,   what   is   the purpose of the selector valve? Q4-22. On   a   basic   hydraulic   system,   what   is   the purpose of the actuating unit? Q4-23. Define hydraulic contamination. PNEUMATIC SYSTEMS LEARNING    OBJECTIVE: Identify    the components of aircraft pneumatic systems and recognize their functions. There are two types of pneumatic systems currently used in naval aircraft. One type uses storage bottles for an air source, and the other has its own air compressor. Generally, the storage bottle system is used only for emergency operation. See figure 4-23. This system has an air bottle, a control valve in the cockpit for releasing the   contents   of   the   cylinders,   and   a   ground   charge (filler)  valve.  The  storage  bottle  must  be  filled  with compressed air or nitrogen prior to flight. Air storage cylinder  pneumatic  systems  are  in  use  for  emergency brakes, emergency landing gear extension, emergency flap extension, and for canopy release mechanisms. When the control valve is properly positioned, the compressed air in the storage bottle is routed through the shuttle valve to the actuating cylinder. NOTE: The  shuttle  valve  is  a  pressure-operated valve that separates the normal hydraulic system from the  emergency  pneumatic  system.  When  the  control handle   is   returned   to   the   normal   position,   the   air pressure  in  the  lines  is  vented  overboard  through  the vent port of the control valve. The other type of pneumatic system in use has its own    air    compressor.    It    also    has    other    equipment necessary to maintain an adequate supply of compressed air during flight. Most systems of this type must be serviced on the ground prior to flight. The air 4-21 ANf0423 Figure 4-23.—Emergency pneumatic system.