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the essential bus. In the unlikely event of a total ac/dc failure inflight, the battery provides the last
level of essential bus backup capability, providing about 5 to 10 minutes of power for the FCCs, after
which aircraft control is lost.
Regardless of BATT switch position, the battery charger supplies charging power to the battery
anytime the right 115 vac bus is energized.
2.4.3.4 FCC Electrical Redundancy. FCC electrical redundancy is provided by several sources of dc
power (figure 2-10). The primary dc source for each FCC channel is its respective PMG output. If a
PMG output should fail, that FCC channel is powered by the essential bus, which also has several
sources of redundancy (the TRs, the PMGs, and the battery). The BATT switch must be ON for either
a PMG or the battery to power the essential bus. Additionally, ``keep alive'' circuits connected directly
to the maintenance bus provide each FCC channel with an uninterrupted backup power source during
normal bus power transients.
2.4.3.5 BATT Switch. The BATT switch is located on the electrical power panel on the right console.
ON Allows the battery or either PMG to power the essential bus when TR power is not
available.
OFF Prevents the battery or either PMG from powering the essential bus when TR
power is not available.
2.4.3.6 Automatic Battery Cutoff. The automatic battery cutoff circuit is provided to conserve
battery power. On the ground with the BATT switch ON, the circuit disconnects the battery from the
essential bus and returns the BATT switch to OFF 2 minutes after ac power is removed from the
aircraft. When battery cutoff is activated, the battery can be reconnected to the essential bus by
reselecting the BATT switch ON. The automatic battery cutoff circuit is disabled when the APU comes
online.
A1-F18EA-NFM-000
I-2-29 ORIGINAL
2.4.3.7 Battery Gauge. A battery gauge is installed on the electrical power panel on the forward right
console in the front cockpit only. Depending on generator status, the battery gauge provides an
indication of either essential bus voltage (both GENs offline) or maintenance bus voltage (either GEN
online).
With both GENs offline and the BATT switch ON (e.g., prior to first engine start), the battery gauge
is connected to the essential bus and indicates battery voltage. Nominal voltage for a ²good² battery
should be 23 to 24 vdc. Minimum battery voltage is that which provides a successful engine start (e.g.,
APU remains online and the EFD remains powered to provide indications of RPM and TEMP). EFD
blanking and/or uncommanded APU shutdown should be anticipated with battery voltage at or below
18 vdc.
With at least one GEN online, the battery gauge is connected to the maintenance bus and indicates
28 vdc output of the battery charger. If the battery gauge fails to jump to approximately 28 vdc with
one GEN online, a battery charger malfunction has occurred which requires maintenance action prior
to flight.
Figure 2-10. FCC Electrical Redundancy
A1-F18EA-NFM-000
I-2-30 ORIGINAL
If a dual GEN failure occurs, the battery gauge is reconnected to the essential bus and must be
referenced to determine the essential bus power source. If the battery gauge remains at approximately
28 vdc, an EBB PMG is powering the essential bus, and FCC operating time is not limited. However,
if the battery gauge indicates 24 vdc or below, the battery is powering the essential bus, and FCC
operating time is limited to about 5 to 10 minutes. The FCCs should continue to operate down to a
battery gauge voltage of approximately 18 vdc.
2.4.3.8 BATT SW Caution and Caution Light. The BATT SW caution and caution light are only set
to alert the aircrew of an improperly placed BATT switch. The battery gauge must be referenced to
determine whether an EBB PMG or the battery is powering the essential bus following a dual GEN
failure. These cautions are set in only two circumstances.
1. The BATT switch is ON on the ground in the absence of ac power (e.g., first engine start). The
battery is depleting and the switch should be placed to OFF unless APU start is about to be made.
2. The BATT switch is OFF inflight and should be placed to ON to provide essential bus backup
capability from the PMGs and the battery.
2.4.4 External Electrical Power. External electrical power may be connected to the aircraft bus
system through an external power receptacle located on the left forward fuselage. If external power is
not of the proper quality, the external power monitor prevents application of power to the aircraft.
Actuation of 1 to 4 ground power switches is required to energize certain aircraft systems following
application of external power.
The aircraft buses are energized by external power in the same manner as if a generator were
　
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