(5)
Pressure Fueling System Test Switch
(a)
The TEST SYS switch provides a means to check the operation of the fuel quantity processor unit, causing all refuel valves to close with fuel pressure in the manifold.
(b)
When the TEST SYS switch is pushed in, all of the indicator lights for the refuel valves will go off at approximately the same time. After the TEST SYS switch is released (or after 10 seconds) the indication lights for the refuel valves will come on. The refuel operation will start again.
(6)
Overfill Indicator Light and Reset Switch
(a)
The OVERFILL light illuminates when fuel enters a surge tank, closing its surge tank float switch. The OVERFILL RESET switch resets the fueling shutoff relay to allow continued fueling provided a surge tank float switch is not still closed.
7. Fuel Quantity Processor Unit (Fueling Functions)________________________________________________
A. The fuel quantity processor unit controls the automatic shutoff feature of the pressure fueling system. The unit is installed in the center equipment center, immediately forward of the access door.
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B. The processor consists of individual fuel quantity channel (IFQC) printed circuit cards for each fuel tank, two input/output channel (I/OC) printed circuit cards, a fueling relay module, a volumetric top-off (VTO) adjust module, and a VTO override switch.
C. Each IFQC card computes the fuel quantity in its respective tank and transmits the data to the I/OC cards. The IFQC cards also transmit the VTO values set into the VTO adjust module to the I/OC cards.
D. The I/OC cards provide the control signals for the refuel valve solenoids through the fueling control module. The cards determine the fueling schedule based on the fuel load requirements (filled to VTO level or to preselected quantity) and present quantity of fuel onboard.
E. The fueling relay module receives inputs, from the I/OC cards, which signal the module to energize or de-energize control relays for the refuel valves and the refuel valve indicator lights on the fueling control panel.
F. The VTO adjust module contains adjustment potentiometers for each wing tank and the center wing tank. There are nine potentiometers on the VTO adjust module. The potentiometer on both ends are spare. Each VTO potentiometer will default to 98% of the nominal certified tank capacity (VTO value) if adjusted beyond the high or low end of the scale of the tank capacity. This is to allow 2% fuel expansion. All potentiometers are 20 turns end to end. The potentiometers are located behind a hinged door on the front of the fuel quantity processor unit. Rotating a potentiometer changes the level of fuel at which the automatic shutoff occurs in that fuel tank. The VTO module also contains a guarded switch which overrides the VTO function, allowing fueling above the designed capacity of the fuel tanks.
8. Surge Tank Float Switch (Fig. 1)
_______________________
A. A float switch in each wing surge tank grounds the fueling shutoff relay when a tank is overfilled during pressure fueling. The switches are located at the inboard end of each surge tank, installed on the front spar.
B. Each switch consists of two housings: an inner housing containing a reed switch and electrical connectors and an vented outer housing containing a float with magnets that travels vertically along the inner housing. The switch also has a mounting flange for installation.
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C. Fuel in the surge tank enters the float switch through openings in the outer housing. As the fuel level increases, the float raises and the magnetic field generated by the float magnets causes the reed switch to close. When fuel drains from the surge tank, fuel drains from the float switch through the housing openings. The float lowers, removing the magnets from the proximity of the reed switch, and the reed switch opens. 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：747-400 AMM 飞机维护手册 燃油 FUEL 2(12)
