TM 55-1520-240-23-8
10-1
FUEL SYSTEM
(Continued)
10-1
FUEL QUANTITY INDICATING SYSTEM
The basic sensing elements of the indicating system
are the fuel quantity probes in each tank. Each probe
is a capacitor in an indicator bridge circuit. Since
the dielectric constant of fuel is twice that of air, the
capacitance of a probe is twice as great when the tank
is full as it is when the tank is empty. At intermediate
levels, the capacitance is proportional to fuel level.
NO BREAK - WORK HARDER
Information from all probes is directed through a switch
box to one of the two selector switches. From the switch,
information from the selected tank or the total of all tanks
is directed to the accompanying fuel quantity indicator.
Each indicator contains two servo amplifiers, two servo
motors, elements of bridge circuits, and calibration
potentiometers.
NO BREAK - WORK HARDER
The change in probe capacitance as the fuel level varies
creates an imbalance in the bridge circuits resulting in a
gage correction output signal. This signal is amplified
to drive a servo motor. The motor works through
reduction gearing to move a variable resistor wiper arm
to re-establish bridge balance. A pointer on the wiper
shaft moves across the calibrated scale of the indicator
to indicate the amount of fuel remaining in the selected
tank.
NO BREAK - WORK HARDER
The wiper arm is also mechanically linked to a counter
mechanism that always displays the total fuel quantity in
pounds, regardless of selector knob position.
NO BREAK - WORK HARDER
The choice of cockpit or refueling station panel indicator
is made at the overhead fuel control panel through relays
in the switch box. If the REFUEL STATION switch on
the overhead panel and the PWR switch at the refueling
station are OFF, probe signals are connected to the
cockpit selector switch. When the switches are ON, 28
vdc is connected to the switch box to operate the relays.
This connects the probe signals to the refueling station
switch. At the same time, dc power is supplied to an
inventer that provides 115 volt 400 Hz single phase ac
power to the refueling station indicator.
10-17
