TM  55-1510-221-10 4. 2-101. intervals   to   assure complete coverage. Removal  of  ice  accumulations  using  undi- luted  defrosting  fluid  is  expensive  and  slow. If  tires  are  frozen  to  ground,  use  undiluted defrosting   fluid   to   melt   ice   around   tire. Move  aircraft  as  soon  as  tires  are  free. APPLICATION OF EXTERNAL POWER. Before  connecting  the  power  cables  from the  external  power  source  to  the  aircraft, insure  that  the  GPU  is  not  touching  the aircraft  at  any  point.  Due  to  the  voltage drop   in   the   cables,   the   two   ground   sys- tems   will   be   of   different   potentials. Should   they   come   in   contact   while   the GPU  is  operating,  arcing  could  occur. Turn  off  all  external  power  while  connect- ing  the  power  cable  to,  or  removing  it from   the   external   power   supply   recepta- cle.   Be   certain   that   the   polarity   of   the external  power  source  is  the  same  as  that of  the  aircraft  before  it  is  connected.  3ini- mum  GPU  requirement  is  400  amperes continuous  and  1800  amperes  for  one tenth  of  a  second. An  external  power  source  is  often  needed  to  sup- ply  the  electric  current  required  to  properly  ground service  the  aircraft  electrical  equipment  and  to  facil- itate  starting  the  aircraft’s  engines.  An  external  DC power  receptacle  is  installed  on  the  underside  of  the right  wing  leading  edge  just  outboard  of  the  engine nacelle.  An  external  AC  Power  receptacle  is  installed on  the  underside  of  the  left  wing  leading  edge  just outboard  of  the  engine  nacelle. 2-102.  SERVICING  OXYGEN  SYSTEM. The   oxygen   system   furnishes   breathing   oxygen to   the   pilot,   copilot   and   first   aid   position   Oxygen cylinder  location  is  shown  in  figure  2-19. a. Oxygen  System  Safety  Precautions. Keep   fire   and   heat   away   from   oxygen equipment.  Do  not  smoke  while  working with  or  near  oxygen  equipment,  and  take care  not  to  generate  sparks  with  carelessly handled  tools  when  working  on  the  oxy- gen  system. (1.)  Keep   oxygen   regulators,   cylinders, gages,  valves,  fittings,  masks,  and  all  other  compo- nents  of  the  oxygen  system  free  of  oil,  grease,  gaso- line,   and   all   other   readily   combustible   substances. The  utmost  care  shall  be  exercised  in  servicing,  han- dling,  and  inspecting  the  oxygen  system. (2.) Do  not  allow  foreign  matter  to  enter oxygen  lines. (3.)  Never   allow   electrical   equipment   to come  in  contact  with  the  oxygen  cylinder. (4.)  Never   use   oxygen   from   a   cylinder without  first  reducing  its  pressure  through  a  regula- tor. b.   Replenishing   Oxygen   System. 1. Remove  oxygen  access  door  on  outside of aircraft (fig. 2- 19). 2. Remove  protective  cap  on  oxygen  sys- tem  filler  valve. 3.  Attach  oxygen  hose  from  oxygen  ser- vicing  unit  to  filler  valve. If  the  oxygen  system  pressure  is  below 200   PSI,   do   not   attempt   to   service   sys- tem.     Make     an     entry     on     DA     Form 2408-13. 4. 5. 6. Insure   that   supply   cylinder   shutoff valves  on  the  aircraft  are  open. Slowly  adjust  the  valve  position  so  that pressure   increases   at   a   rate   not   to exceed  200  PSIG  per  minute. Close  pressure  regulating  valve  on  oxy- gen  servicing  unit  when  pressure  gage on   oxygen   system   indicates   the   pres- sure  obtained  using  the  Oxygen  System Servicing  Pressure  Chart  (fig.  2-31). 2-84