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
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.
Generator. The generator is a
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.
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.
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
· 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.
operator inspections apply to the aviation ground power
unit. Detailed inspection requirement are contained in
the applicable maintenance manual.
Liquid Levels. Check the gas tank,
reservoir, and crankcase for proper levels, refill as
handbrake lever and cable.
Tires. Check for proper inflation,
cuts and damage.
Leaks. Check for fuel, engine oil
and hydraulic fluid leaks.
Ground Stud. Inspect ground stud
for secure mounting.