TM 1-1520-238-T-5
8–30
8–7.
SYSTEM DESCRIPTION (cont)
8–7
(e) The FSC (fig. 8–25) operates on 28 VDC from the SDC except during refueling. During refueling the
FSC operates on 24 VDC from the battery. The fuel cells act as the sensing capacitor for the system. The
capacitive value is determined by the level of fuel between the inner and outer tubes of the sensor, density of fuel,
and the shape of the inner tube. The FSC derives the operating voltages from an internal power supply. A 6 KHz
oscillator provides a LO–Z voltage across the fuel tanks. The amount of current the cell passes from the LO–Z
unshielded line to the HI–Z shielded line is directly proportional to the mass of fuel in the tank. The forward and aft
FSCs amplify and rectify the HI–Z signals, producing 0 to 5 VDC to the fuel indicators. Internal empty adjustment
is made at the transformer centertaps at the oscillator. Internal full adjustment is made in the dc amplifier. The
FSC contains built–in–test (BIT) circuitry. BIT provides a known signal to functionally check the FSC by full cell
simulation. BIT is activated by an external switch on the FSC.
FWD SIGNAL
CONDITIONER
OSCILLATOR
AFT SIGNAL
CONDITIONER
AFT FUEL
CELL SENSOR
FWD FUEL
CELL SENSOR
POWER SUPPLY
28 / 24 VDC
INPUT
OPERATING
VOLTAGES
0 - 5 VDC
0 - 5 VDC
TO FUEL QTY
INDICATOR
TO REFUEL
INDICATOR
TO FUEL QTY
INDICATOR
TO REFUEL
INDICATOR
HI - Z
M68-132
FSC
FSC
Figure 8–25.
FSC Functional Block Diagram
(3) The pilot OAT indicator (fig. 8–26) contains a sensing element which contracts or expands as
temperature changes. This movement is mechanically coupled to the pointer, which moves to indicate current
outside air temperature.
