TM   1-1500-204-23-2 CHAPTER  4 HYDRAULIC   MAINTENANCE   PRACTICES 4-1.  General.  This  chapter  provides  general  instruc- tions for maintenance of aircraft hydraulic systems, the materials required, and methods to be used. A systemat- ic  application  of  the  basic  information  in  this  chapter  will result in more efficient maintenance practices. In those cases  where  specific  instructions  for  a  particular  aircraft are  required,  refer  to  the  applicable  aircraft  maintenance manual.  Materials  and  processes  recommended  are identified by military or federal specification number or by manufacturer. 4-2.  Tubing  Systems  and  Repairs.  Tubing  used  in aircraft  hydraulic  systems  carries  pressure  to  distant points  in  the  aircraft  to  provide  control  of  various  critical components.  Failure  of  this  tubing  will  result  in  pressure loss  and  subsequent  loss  of  control,  resulting  in  possible destruction  to  aircraft  and  injury  or  death. a. Types.   Corrosion-resistant   steel   tubing   and   alu- minum  alloy  tubing  are  used  in  aircraft  for  fuel,  oil,  cool- ant, oxygen, instrument, hydraulic, and vent lines, as well as for electrical conduits and ventilating ducts. Copper tubing  generally  has  been  superseded  as  a  general  pur- pose tubing by aluminum alloy tubing because of its light- er weight, ease of forming, and resistance to corrosion and fatigue. Tubing material can be identified by visual inspection or by the aluminum alloy designation stamped on  the  surface. (1)  Corrosion-resistant  steel  tubing.  Corrosion- resistant  steel  tubing,  MIL-T-8504  and  MIL-T-6845,  is used  in  high  pressure  hydraulic  systems  (3000  psi)  such as  landing  gear,  wing  flaps,  and  brakes.  External  brake lines  should  always  be  made  of  steel  to  prevent  damage caused  by  flying  gravel  and  stones  and  ground  handling accidents. On hydraulic systems using 1/2-inch corro- sion-resistant  tubing  or  larger,  aluminum  alloy  nuts  and sleeves  may  be  used.  Steel  nuts  and  sleeves  should  be used on 3/8 inch corrosion resistant tubingor smaller. Corrosion-resistant  steel  tubing  does  not  have  to  be  an- nealed  for  flaring  or  forming.  The  flared  section  is  some- what  strengthened  by  cold  working  and  consequent strain  hardening.  The  high  tensile  strength  of  corrosion- resistant steel tubing permits use of thinner walls than those of aluminum alloy tubing, but the weight is about the  same  as  thick-walled  aluminum  alloy  tubing. (2) Aluminum alloy tubing.  Aluminum alloy tub- ing,  Federal  Specification  WW-T-700/1,  is  used  for  gen- eral purpose lines and conduits of low fluid pressure, such as instrument lines, and electrical and ventilating conduits. Aluminum alloy tubing Federal Specifications WWT-700/4  and  WW-T-700/6,  is  the  most  widely  used tubing for general purpose lines of low and medium pres- sures. It is easily flared, and is soft enough to be formed with  handtools.  It  should  be  handled  with  care  to  prevent scratches,  dents,  and  nicks.  Aluminum  alloy  tubing  is used  with  either  of  two  types  of  connections;  flared  joint for  mechanical  connections,  or  beaded  end  for  use  with clamps and flexible hose connections. In hydraulic sys- tems,  5052-0  aluminum  alloy  tubing,  Federal  Specifica- tion WW-T-700/4, is used for reduced pressure (1500 psi maximum)  and  return  lines.  Aluminum  alloy  tubing  6061 and 6062 MIL-T-7081, may be used for high pressure lines (3000 psi). Use of MIL-T-7081 tubing in aircraft is limited  by  MIL-H-5440.  Tubing  conforming  to  Federal Specifications WW-T-700/1 and WW-T-700/6 shall not be  used  in  hydraulic  systems. (3)  Copper  tubing.  High  pressure  oxygen  sys- tems   use   3/16-inch   diameter,   0.032-inch   wall   thickness copper  tubing,  Federal  Specification  WW-T-799,  Type  N. Fittings are silver soldered onto the tubing in accordance with  MIL-B-7883,  before  tubing  is  installed. (4)  Rigid  Tubing.  The  tubing  used  to  produce rigid tubing assemblies is sized by its outside diameter (OD) and wall thickness. Outside diameter sizes are in sixteenth-of-an-inch  increments,  the  number  of  the  tube indicating its size in sixteenths of an inch. For example, the number 6 tubing is 6/16 or 3/8 inch, number 8 tubing is 8/16 or 1/2 inch, and so forth. Wall thickness is speci- fied in thousandths of an inch. Most aircraft maintenance manuals  contain  a  table  which  lists  the  original  material and  acceptable  substitutes  and  gives  the  wall  thickness for  each. b. Tubing Identification. The Army uses two types of identification systems to code hydraulic plumbing lines of aircraft. The two systems used are the printed-symbol- ized  tape  system  (see  figure  4-1),  the  preferred  method, and the solidcolor band (see figure 4-2), the alternate method.  This  identification  will  be  applied  in  accordance with  TM  1-1500-204-23-1)  paragraph  6-3. c. Tubing Damage. Damage such as nicking and scratching of tubing can be minimized by using care. Most  damage  of  this  kind  occurs  from  careless  handling of tools during maintenance of aircraft. Any dent less than 20 percent of tubing diameter is not objectionable unless it is on the heel of a short bend radius. A nick no deeper than 15 percent of wall thickness of aluminum, aluminum alloy, copper, or steel, and is not in the heel of the  bend,  may  be  reworked  by Change 3      4-1