TM 1-1500-204-23-9 (d) Secure  the  load  to  the  rails.    Also, check that the rail stop assemblies are up. (e) Connect the trailer to a tow vehicle. (f) Release  the  trailer  brakes  and  tow the trailer to the work site. (g) Set   the   trailer   brakes   and   unhook the trailer. (h) To  unload  by  sliding  the  load  off, release  the  securing  device  and  slide  toward  the  rail ends. (i) Pull  the  rail  stop  assemblies  down when   the   load   is   near   the   stops   and   ready   to   the unloaded. (j) Slide the load off. (6) Inspection. Detailed inspection requirements can be found in the applicable maintenance  manual.    The  following  general  operator inspections should be made on the engine trailer. (a) Visual   inspection.      Look   over   the entire trailer for obvious discrepancies, such as loose or missing hardware or flat tires. (b) Tires.    Visually  check  for  apparent correct pressure.  If in doubt, check with a gauge. (c) Brakes.    Check  that  they  set  and release without sticking. (7) Maintenance.        Consult    the    applicable maintenance    manual    for    scheduled    and    preventive maintenance requirements. (8) Adapters available.  The adapters that can be  used  with  the  trailer  allow  it  to  be  used  with  many different   types   of   engines.      Consult   the   applicable maintenance manual for the engine repaired locally. f. Aviation   Ground   Power   Unit.        The    aviation ground power unit, shown in figure 9-13, is a gas turbine engine-driven,  wheel  mounted,  self-propelled  (up  to  3 mph on flat surface), enclosed unit.  The APGU can be towed (20 mph maximum), and is air transportable. (1) Purpose.  The aviation ground power unit provides    ac/dc    electrical,    hydraulic,    and    pneumatic power.  Power is available individually, or in any com- bination.        The    AGPU    provides    the    ground    power requirements for aircraft. (2) Principles of Operation.  Control and regulation of    the    AGPU    electrical    and    pneumatic    systems    is semiautomatic.  Electronic devices monitor and regulate electrical   voltage,   frequency,   and   current;   as   well   as pneumatic  outputs.    Control  of  the  hydraulic  system  is semi-automatic,  in  that  the  operator  must  set  hydraulic pressure, and select operating modes. (a) Engine/Gearcase.        The            gas turbine  engine  (GTE),  provides  pneumatic  power,  and shaft power to drive an ac/dc generator and a hydraulic pump.  The  generator  and  pump  are  mounted  on  pads on  the  gearcase.    Once  started,  the  engine  runs  up  to 100    percent    governed    speed    of    58,737    rpm,    and automatically    maintains    that    speed    until    shutdown. Engine    air    intake    is    through    an    air    cleaner/duct assembly  to  the  engine.    Engine  exhaust  is  through  an exhaust  ejector,  and  out  the  roof  through  an  exhaust flapper. (b) Starting  system.    The  accumulator of   the   mobile   servicing   unit   is   filled   with   a   nitrogen precharge.  The  hand  pump,  acting  on  hydraulic  fluid drawn   from   the   reservoir,   builds   up   pressure   in   the accumulator for the initial diesel engine start.  When the start  valve  is  depressed,  the  pressurized  fluid  flows  to the inlet of the starting motor to crank the engine.  The fluid   then   returns   to   the   reservoir.      The   accumulator charging pump pumps hydraulic fluid from the reservoir to charge the accumulator after the engine has started. (c) Propulsion System.  The propulsion system provides suspension, steering, brakes, and drive power    for    the    AGPU.        A    speed/direction    control assembly  mounted  on  a  tow  bar  allows  the  operator  to control the forward/reverse directions and speed (up to 3 mph on flat surface)in  self-propulsion  mode.    The  tow bar  is  also  used  to  steer  the  AGPU  when  using  self- propulsion   mode.      Drive   power   is   provided   by   a   dc traction motor driving a conventional rear axle assembly.  An electric brake assembly is provided.  The electric brake prevents the rear wheels turning with the clutch  engaged  until  electrical  power  is  applied  to  the traction   motor   and   electric   brake.      Application   of   dc drive   power   to   the   traction   motor   is   controlled   by   a motor controller and relays located on the upper tray in the    electrical    compartment.        The    motor    controller receives     signals     from     the     speed/direction     control assembly.    The  drive  train  consists  of  a  gear  box,  a chain drive, a manual clutch, and a rear axle assembly. A   dead   man   switch   on   the   speed/direction   control assembly must be held in while operating the propulsion system.      If   the   switch   is   released   (intentionally   or accidentally)during operation, 9-37