TM 1-1500-204-23-9 power   is   removed   from   the   traction   motor   and   the electric  brake.    Removing  electrical  power  applies  the electric  brake.    Additionally,  the  speed/direction  control assembly  contains  a  mercury  switch  which  deactivates the   propulsion   system   when   the   tow   bar   is   raised. Conventional   drum   brakes   are   provided   by   the   rear wheels.  The brakes are set by a lever on the front of the AGPU, which is connected to the brake assemblies by a cable assembly. (d) Hydraulic   System.      The   hydraulic system  consists  of  a  hydraulic  pump,  interconnecting lines and hoses, and a hydraulic module.  The hydraulic pump  is  mounted  on  an  engine  gearcase  pad,  and  is driven   by   the   engine.      The   hydraulic   pump   output pressure   is   controlled   by   a   switch   on   the   hydraulic control    panel.    The    hydraulic    module    controls    the application  of  hydraulic  fluid  to  an  aircraft  for  power, fueling, or flushing requirements.  The hydraulic module contains a control panel, a reservoir, an accumulator, a cooler   (heat   exchanger),   a   manifold,   filters   (2),   and hydraulic lines, fittings, and valves. (e) Generator.        The    generator    is    a selfcooled, continuous duty, ac/dc, self-excited, brushless  unit.    It  includes  a  permanent  magnet  stator and  rotor,  an  exciter  stator  and  rotor,  and  a  main  dc rotating    field    and    ac    stator.        The    main    ac    stator incorporates  three  sets  of  three  phase  windings.    One set  of  main  stator  windings  provides  the  115/200  vac output.    The  ac  outputs  of  the  other  two  windings  are full-wave   rectified   to   provide   28   vdc   output.      The permanent    magnet    provides    a    three    phase    output whenever the generator is driven by the engine.  When the engine reaches 95 percent speed, a relay connects the  permanent  magnet  output  to  the  generator  control unit (GCU), located behind the control panel.  The GCU rectifies this ac voltage to provide dc control voltage for the  GC  U  and  dc  excitation  voltage  for  the  generator exciter field.  The exciter provides a three phase output which varies in magnitude with the field excitation.  The exciter voltage is half-wave rectified and applied to the generator main dc rotating field.  The magnitude of the three    phase    voltage    generated    in    the    main    stator windings  is  a  function  of  the  ampere  turns  of  the  field windings which is, in turn, a function of the exciter field excitation.    The  GCU  monitors  the  generator  ac  or  dc output  (depending  upon  which  output  is  selected  at  the AGPU control panel) and controls the exciter field as required to keep the selected output within limits. (f) Pneumatic System.  The pneumatic system provides pneumatic power (70 pounds/minute, at 40   psig).      The   pneumatic   system   consists   of   a   load control valve (LCV) mounted on the engine, ducts, and a  pneumatic  hose.    An  optional  deicer  kit  is  supplied with  some  units.    The  kit  consists  of  a  fifty-foot  hose assembly,     a     supply     valve,     and     a     deicer     nozzle assembly.    The  supply  valve  connects  to  a  normally capped outlet on the left interior bulkhead of the AGPU. (3) Safety   practices   and   procedures.      The following  special  precautions  should  be  observed  when using the aviation ground power unit. ·  Depressurize    the    hydraulic    hoses    before connecting or disconnecting them. ·  Be aware of the hazards associated with a fluid in a hose under high pressure. ·  Check    for    foreign    matter    before    connecting hydraulic hose. ·  Never  attempt  to  connect  or  disconnect  power cables   to/from   aircraft   with   power   applied,   or without  a  proper  ground  on  the  APGU  ground stud and aircraft. ·  A   ground   guide   is   required   when   operating AGPU in reverse. · ·  Remain   clear   of   downgrade   side   of   AGPU   if operating on a slope. (4) Operating     procedures. For detailed instructions, consult the applicable maintenance manual. (5) Inspection. The following general operator inspections apply to the aviation ground power unit.  Detailed  inspection  requirement  are  contained  in the applicable maintenance manual. (a) Liquid Levels.    Check  the  gas  tank, reservoir,   and   crankcase   for   proper   levels,   refill   as required. (b) Brakes. Check operation of handbrake lever and cable. (c) Tires.      Check   for   proper   inflation, cuts and damage. (d) Leaks.    Check  for  fuel,  engine  oil and hydraulic fluid leaks. (e) Ground  Stud.    Inspect  ground  stud for secure mounting. 9-39