8. Operation (Fig. 2)
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A. Functional Description
(1)
The fuel quantity indicating system is powered in-flight by the dc bus No. 3 and dc bus No. 4. On the ground, the system is powered by the ground handling bus or hot battery bus, depending on whether external power or battery power is available.
(2)
When power is supplied to the fuel quantity indicating system, the processor performs a power-up BITE routine for approximately 30 seconds. Failure of any of the individual fuel quantity channel (IFQC) or input/output (I/O) cards is evident by loss of total fuel quantity from the main EICAS display, by loss of total fuel quantity and affected tank quantity(s) from fueling quantity indicators at the refueling control panel P42, by the generation of fault message(s) in the CMCS. The LED indicator lights on the processor are illuminated red if the failure is severe enough to prevent completion of a software cycle.
(3)
Each IFQC card treats the total tank volume as the summation of the volumes associated with each tank unit. The card excites the outer tube of each tank unit in its tank and measures the capacitive and resistive current generated by the tank unit. The data from each tank unit is analyzed by the IFQC card for out-of-range or other error conditions.
(4)
The IFQC card excites the fuel compensator in each tank, with the return signal proportional to the dielectric constant of the fuel. If the compensator capaticance or resistance is out-of-range or missing, dielectric data derived from the densitometer is used. If densitometer data is not available nominal values stored on the IFQC card is substituted for use in fuel quantity calculations and error data is transmitted to the CMCS for use in trouble shooting. During refueling operation, the IFQC card uses the capacitance signal from a "monitor" tank unit, instead of the compensator, to calculate the dielectric constant when fuel covers the monitor tank unit completely.
(5)
The IFQC card also excites the densitometer, where installed, with the return signal proportional to the fuel density. If fuel density data is missing or out-of-range, density is derived from compensator data. If the compensator is failed or uncovered a nominal density is substituted for fuel quantity calculations.
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A
747-400
MAINTENANCE MANUAL
ENERGIZES WHEN P42 POWER SELECT R331 REFUELING N302 THRU N307
SWITCH IS IN BATT PWR CONT FUELING QUANTITY
(P180) 1 INDICATORS (P42)
Fuel Quantity Indicating System Simplified Schematic
Figure 2 (Sheet 1)
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D16711
A
747-400
MAINTENANCE MANUAL
M7860 FQIS PROCESSOR (E6)
Fuel Quantity Indicating System Simplified Schematic
Figure 2 (Sheet 2)
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B44968
A
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MAINTENANCE MANUAL
(6)
The card uses the inputs from each tank unit, the compensator, and the densitometer (where applicable) to compute fuel volume and mass associated with each tank unit. The volumes and masses are summed to provide total fuel quantity for the tank.
(7)
The fuel tank quantities, in-tank component data, and error messages produced by each IFQC card are received by the I/O cards. The I/O card sums the fuel tank quantities to determine total fuel quantity.
(8)
Each I/O card formats and transmits fuel quantity data to the EIUs via two ARINC 429 data buses. The data is displayed in the flight deck on EICAS in a variety of formats.
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