(21)
 Any corrosion or damage to the airplane wiring harness or the tank wiring harness can result in unknown accuracy fault for the applicable fuel tank. This can cause the fuel quantity indicator displays for the applicable tank and total to go off.

 3.  Operation (Fig. 10)_________
 A. Functional Description
 (1)  
Fuel Quantity Processor Unit Operation

 (a)  
The processor operates whenever the system has power. No flight crew operator actions are necessary for the processor to calculate and show fuel quantity.

 (b)  
Power requirements for the processor, 28 volt dc, normally comes from the battery bus, and the right main bus. Alternate power sources for the processor come from the ground handling bus and the hot battery bus.

 (c)  
The battery bus normally supplies power to power supply 1 in the processor, and the right main bus supplies power to power supply 2.

 (d)  
The processor will operate even with only one power source available.

 (2)  
Tank Unit Signals and Programming

 (a)  
Strategically located tank units measure the fuel quantity in each fuel tank. Each fuel tank also contains a compensator, which measures the dielectric of the fuel. Both the tank units and the compensator are used to calculate the fuel quantity.

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MAINTENANCE MANUAL

FUELING CONTROL PANEL FQIS PROCESSOR
Fuel Quantity Indicating System (FQIS) Schematic Figure 10
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 ú  Sep 28/00 

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MAINTENANCE MANUAL

 (b)  With power applied to the processor, an excitation signal is sent to the tank units to measure their capacitance value. This is accomplished by comparing the tank unit against a rebalance capacitor of known value, which increments or de-increments until a balance is reached. This operation is carried out for each tank unit, and a computation of tank volume made by the microcomputer as follows: 1) The computer program divides each tank unit into equal
 sections, and assigns a value proportional to tank shape. Wetted sections of the tank units are summed, and the value of unusable fuel kept in the computer memory is subtracted from the total. Fuel quantity indicators show the quantity of usable fuel.
　
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