TM 1-1520-238-10
Change 9
2-41
2.36.8 Nitrogen Inerting Unit (NIU).
The NIU reduces
fire hazards associated with fuel cell ullages (airspace) by
filling the ullage with oxygen-depleted air. The NIU is self-
contained and automatically operated whenever pressur-
ized air and 115 vac power is available. A go/no-go press-
to-test monitor is located in the aft avionics bay. The NIU
uses pressurized air from the pressurized air manifold and
purges about 70% of the oxygen present. This air is then
sent to the aft fuel cell and onward to the forward fuel cell.
2.36.9 Fuel System Electrical Power Sources.
Elec-
trical power for the fuel system is controlled by the follow-
ing circuit breakers (fig 2-39):
a. FUEL FILL Circuit Breaker.
The refuel panel re-
ceives 24 vdc power directly from the battery through the
FUEL FILL circuit breaker on the pilot overhead circuit
breaker panel.
b. FUEL BST, FUEL TRANS, and FUEL XFEED Cir-
cuit Breakers.
Circuits controlled from the pilot and
CPG FUEL control panels receive 28 vdc from the emer-
gency dc bus through the FUEL BST, FUEL TRANS, and
FUEL XFEED circuit breakers on the pilot overhead circuit
breaker panel. The EXT EMP light on the pilot caution/
warning panel receives 28 vdc from the No. 2 essential dc
bus through the FUEL TRANS circuit breaker on the pilot
overhead circuit breaker panel.
c. FUEL BST and APU HOLD Circuit Break-
ers.
Fuel supply system components, automatically
controlled by the APU control switch, receive 28 vdc from
the emergency dc bus through the FUEL BST and APU
HOLD circuit breakers on the pilot overhead circuit break-
er panel and the APU circuit breaker in the aft avionics
bay.
d. ENG INST Circuit Breakers.
All fuel system
gauges and caution/warning lights (except those on the
refueling panel and the pilot EXT EMP caution light), re-
ceive 28 vdc from the emergency dc bus through the ENG
INST circuit breakers on the CPG No. 1 circuit breaker
panel and pilot overhead circuit breaker panel.
e. ENG START Circuit Breaker.
Fuel supply sys-
tem components, automatically controlled by the automat-
ic-engine start circuits, receive 28 vdc from the emergen-
cy dc bus through the ENG START circuit breaker on the
pilot overhead circuit breaker panel.
f. JETT Circuit Breakers.
External jettison circuits
(pilot selectable stores jettison switches) receive 28 vdc
from the No. 2 essential dc bus through the JETT circuit
breaker on the pilot overhead forward circuit breaker pan-
el. In the event of a dual generator failure, this circuit is in-
operative as the No.2 essential dc bus is not powered.
The second JETT circuit breaker (center circuit breaker
panel) receives 28 vdc through the emergency dc bus and
allows stores jettison through the collective stores jettison
switches.
