the  hydraulic  liquid,  the  hydraulic  system  must  be flushed. NOTE:  The presence of foreign particles in the hydraulic   system   indicates   a   possible   component malfunction that you should correct before flushing the system. A  hydraulic  system  in  which  the  liquid  is  con- taminated  should  be  flushed  according  to  current applicable  technical  instructions.  Flushing  procedures are  normally  recommended  by  the  manufacturer.  The procedure varies with different hydraulic systems. One method is as follows: Drain out as much of the contaminated liquid as possible. Drain valves are provided in some systems for this purpose; while on other systems, lines and fittings must be disconnected at the low points of the system to remove any trapped fluid in the lines and components. Close all the connections and till the system with the applicable  flushing  medium.  Any  of  the  hydraulic liquids  approved  for  use  in  power-transmission  systems may be used for flushing purposes. CAUTION Diesel fuel oil must not be used for flushing hydraulic systems in active service, because of its   poor   lubricating   qualities   and   its contaminating effect on the subsequent till of hydraulic  liquid. While being flushed with an approved hydraulic liquid,  power-transmission  systems  can  be  operated  at full  load  to  raise  the  temperature  of  the  liquid. Immediately  following  the  warming  operation,  the system  should  be  drained  by  opening  all  drain  outlets and disconnecting the hydraulic lines to remove as much of the flushing medium as possible. All filter elements, screens, and chambers should be cleaned with new fluid before filling the system with the required service liquid. CAUTION The system should not be operated while or after draining the liquid. Power-transmission systems and their intercon- nected hydraulic controls whose inner surfaces have been inactivated and treated with a corrosion prevention or  preservation  compound  must  be  flushed  to  remove the  compound.  The  latest  current  instructions  for flushing  and  other  operations  required  to  reactivate  a particular  system  must  be  strictly  followed  to  prevent damage. Some hydraulic systems are flushed by forcing new liquid  into  the  system  under  pressure,  forcing  out  the contaminated  or  decomposed  liquid. Hydraulic  liquid  which  has  been  contaminated  by continuous use in hydraulic equipment or has been expanded as a flushing medium must not be used again but should be discarded according to the prevailing instructions. CAUTION Never permit high-pressure air to be in direct contact with petroleum base liquids in a closed  system,  because  of  the  danger  of ignition. If gas pressure is needed in a closed system, nitrogen or some other inert gas should be  used. REFERENCES Aviation  Hydraulics  Manual,   NAVAIR  01-1A-17, Commander,   Naval   Air   Systems   Command, Washington, D.C., 1989. Aviation  Support  Equipment  Technician,  Naval Education  and  Training  Program  Management Support Activity, Pensacola, Fla., 1990. Base  Vehicle  Equipment  Mechanic,  Extension   Course Institute,  Air  University,  Gunter  Air  Force  Station, Montgomery,  Ala.,  1986. Fluid Power,  Naval  Education  and  Training  Program Management  Support  Activity,  Pensacola,  Fla., 1990. 10-34