Figure  5-12.—Hose assembly INSTALLATION Flexible  hose  must identification  tags  (aircraft). not  be  twisted  during installation, since this reduces the life of the hose considerably and may cause the fittings to loosen as well. You can determine whether or not a hose is twisted by looking at the layline that runs along the length of the hose. If the layline does not spiral around  the  hose,  the  hose  is  not  twisted.  If  the layline  does spiral around the hose, the hose is twisted  (fig.  5-13,  view  B)  and  must  be  untwisted. Flexible  hose  should  be  protected  from  chafing by   using   a   chafe-resistant   covering   wherever necessary. The  minimum  bend  radius  for  flexible  hose varies  according  to  the  size  and  construction  of the  hose  and  the  pressure  under  which  the system   operates.   Current   applicable   technical publications  contain  tables  and  graphs  showing minimum  bend  radii  for  the  different  types  of installations.  Bends  that  are  too  sharp  will  reduce the  bursting  pressure  of  flexible  hose  considerably below  its  rated  value. Flexible  hose  should  be  installed  so  that  it  will be  subjected  to  a  minimum  of  flexing  during operation. Support clamps are not necessary with short  installations;  but  for  hose  of  considerable length (48 inches for example), clamps should be placed  not  more  than  24  inches  apart.  Closer 5-11 Figure  5-13.—Correct and incorrect installation of flexible hose. supports are desirable and in some cases may be required. A  flexible  hose  must  never  be  stretched  tightly between two fittings. About 5 to 8 percent of the total length must be allowed as slack to provide freedom   of   movement   under   pressure.   When under pressure, flexible hose contracts in length and  expands  in  diameter.  Examples  of  correct  and incorrect  installations  of  flexible  hose  are illustrated in figure 5-13. PFTE  hose  should  be  handled  carefully  during removal  and  installation.  Some  PFTE  hose  is  pre- formed  during  fabrication.  This  type  of  hose  tends to form itself to the installed position in the sys- tem. To ensure its satisfactory function and reduce the likelihood of failure, anyone who works with PFTE  hose  should  observe  the  following  rules: 1. 2. 3. 4. Do  not  exceed  recommended  bend  limits. Do  not  exceed  twisting  limits. Do  not  straighten  a  bent  hose  that  has taken a permanent set. Do not hang, lift, or support objects from PFTE  hose.