TM 55-1520-240-23-8
10-1
FUEL SYSTEM
(Continued)
10-1
THEORY OF OPERATION FUEL DELIVERY SYSTEM
The fuel pumps for all tanks operate on 115 vac from
number 1 or 2 ac bus and are controlled by 28 vdc.
When the ac circuit for any pump is closed, 115 vac is
connected to a relay in the fuel pump control relay box.
Closing the pump dc circuit breaker connects dc voltage
to the pump control switch on the overhead fuel control
panel.
NO BREAK - WORK HARDER
On the main tank fuel pumps, placing the overhead panel
switch to ON operates the relay directly, connecting ac
power to the pump to operate it.
NO BREAK - WORK HARDER
On each auxiliary tank fuel pump, placing the panel
switch to ON connects 28 volts to a thermistor control
unit in the cabin.
NO BREAK - WORK HARDER
Two thermistors are mounted on each auxiliary fuel
pump. As long as both thermistors are covered by fuel,
28 volts passes through the control relay connecting ac
power to operate the pump. When the fuel level drops
enough to uncover both thermistors, their resistance
changes, opening the control relay and shutting the
pump off. Two thermistors are used to ensure that the
pump does not shut down in response to fuel sloshing.
NO BREAK - WORK HARDER
During normal operation with all fuel pumps operating,
each of the two main pumps in the left and right systems
delivers fuel through its check valve to a common engine
fuel line. Fuel passes through the feed line through a
system check valve, a pressure switch, a main engine
fuel valve and a fuel flow transmitter to the engine fuel
system.
NO BREAK - WORK HARDER
The check valve has a thermal relief feature that allows
fuel to flow back into the tank before the temperature
within the line from check valve to engine reaches a point
where the line could rupture because of fuel expansion.
NO BREAK - WORK HARDER
The pressure switch lights a capsule on the master
caution panel when fuel pressure drops below 10 psi.
NO BREAK - WORK HARDER
The main engine fuel valve is motor-operated by 28 vdc.
It is controlled through the cockpit fire warning handle
for that engine. A lever on the valve allows it to be also
shut off manually.
NO BREAK - WORK HARDER
The flow transmitter provides a signal to a gage on the
center instrument panel.
10-14
