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polarity are within acceptable limits. The acceptable voltage limits are
approximately 24 to 32 volts. The GPU should be regulated to 28-vdc
and limited to 1,500 amps.
The EXT PWR switch is located on the cockpit electrical control panel .
The green AVAIL caption on the EXT PWR switch will illuminate if the
plugged-in GPU is within acceptable parameters. Depressing the EXT
PWR switch when the green AVAIL light is illuminated will close the
GPU anti-flash contactor, connecting the GPU output to the left
generator bus. The ON caption will illuminate and the green AVAIL
caption will extinguish. The GPU may be deselected with the same
switch.
The bus-tie contactor automatically closes when a GPU is connected to
the airplane electrical system. The entire dc system is powered,
assuming the AV MSTR and BATT switches are on and no buses have
been deselected via the electrical control panel.
Neither the airplane generators nor the APU will come on line with the
GPU selected ON; and if they are on when the GPU is selected, they
will drop off line. Airplane generators will automatically come on line
after engine start, but not if the GPU is selected ON. The aircraft main
batteries do not have to be on to close the GPU anti-flash contactor;
however, only the bus-tie and non-essential bus contactors will close if
neither battery is selected On. In this case GPU power would only be
available to the left and right generator buses, the left and right
non-essential buses, and the hot buses.
GPU output voltage is indicated on the EICAS/MFD SUMRY page and
on the ELEC system schematic under “L/R ESS VOLTS.” The EMER
BUS VOLTS will also show GPU voltage. No indication of amps drawn
from the GPU is provided.
The CAS will provide an EXTERNAL POWER message when an
external power cable is connected and EXT PWR will also be
annunciated on the EICAS/MFD ELEC system schematic. These
indications will appear whenever a GPU cable is connected to the
airplane, and a voltage of greater than 5 volts is sensed by the power
monitor. It is not an indication that the GPU meets acceptable
parameters nor does it indicate that the GPU is powering the airplane
electrical system.
Pilot’s Manual
4-8 PM-126A
Operation of the EXT PWR switch on the electrical control panel is also
described under ELECTRICAL SWITCHES in this section.
AUXILIARY POWER UNIT (APU)
The optional APU generator can provide the same service as an
airplane engine generator and can be operated in conjunction with one
or both airplane generators. The APU is only certified for ground use.
After starting the APU using the APU control panel on the center
pedestal, the green APU RUN annunciator on the APU control panel
will illuminate and at the same time, the green AVAIL caption on the
APU GEN switch, located on the electrical control panel, will also
illuminate. After depressing the APU GEN switch on the electrical
control panel, the ON caption will illuminate and the AVAIL caption
will extinguish to indicate that the APU contactor has closed,
connecting the APU generator output to the right generator bus. The
APU GEN switch is also used to reset the APU generator when a failure
has been detected and cleared. Operation may be restored by depressing
the APU GEN switch for one second. If the AVAIL caption illuminates,
depressing the switch again will bring the generator on line.
The bus-tie contactor automatically closes with the APU generator on
line. A GPU and APU generator cannot provide airplane electrical
power simultaneously. The APU generator will automatically drop off
line if the GPU is selected on line.
The following CAS illuminations are specific to the APU generator:
ELECTRICAL CONTROL PANEL
The dc power system electrical control panel is designed to provide
ease of operation and dark cockpit integration. The automatic load
shedding design for operation with single or dual generator failure
relieves the pilot of manual deselection of electrical buses to prevent an
overload. The control panel reflects and displays batteries, generators,
or buses that have been isolated (automatically shed) in the event of a
fault. The pilot has a manual override option of selecting, recycling or
deselecting some of the buses on the dc system. During engine start, the
generator autostart feature reduces engine start switch selections and
pilot workload.
CAS Color Description
APU AMPS HIGH Amber APU generator amperage exceeds upper
limit.
APU AVAILABLE White APU is operating and available for bleed air
and electrical power.
Pilot’s Manual
PM-126A 4-9
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