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LO-Z 2 Short of Open in Hi-Z NA 3 Short or Open in LO-Z NA 4 Defective Indicator NA 5 Defective Indicator Defective Indicator 6 Defective Indicator Defective Indicator 7 NA Indicated P2 Indicator Error 8 NA Signal Error Between P2 and
P15 Indicators 9 Open or Short on Compensator LO-Z NA
5.  Fuel Quantity Indicators (Airplanes with analog indicators)
A.  Precalibrated analog fuel quantity indicators on the P2 panel show the weight of fuel in the tanks. Each indicator consists of the reference side of a self-balancing capacitance bridge, a power transformer, an amplifier, a reversible two phase induction motor, gear trains, pointers, a main dial, and a vernier dial. All indicators are hermetically sealed. Electrical connections are at the rear of each indicator.
B.  The fuel quantity indicators use 115-volt ac power for the bridge circuit through the transformer and for one phase of the induction motor. (The other motor phase is energized by amplifier output). Indicator pointers are driven by the same shaft as the potentiometer wiper of the self-balancing bridge circuit and rotate with the wiper to provide continuous indication of the fuel quantity.
6.  Fuel Summation Unit
A.  The fuel summation unit (P23 panel) receives the digital fuel quantity information from each indicator and sums them for a total fuel quantity output to the PDCS system. This output signal is one of several inputs to a performance data computer system (PDCS) which has a control display unit (CDU) on the P9 panel. The performance data computer (PDC) converts these inputs into optimum airspeed and engine performance limits for climb, cruise, and descent but will also display the instantaneous total fuel remaining when the mode selector knob is positioned on STBY and key No. 6 is depressed. For information on the PDCS and CDU, (Ref Chapter 34).
5C8 
28-41-05 Page 6  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Feb 20/89 

5B8  Fuel Quantity Indicating System Schematic 
Jun 20/84  Figure 2 (Sheet 1)  28-41-05 
Page 7 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

 

Fuel Quantity Indicating System Schematic  5B8 
28-41-05  Figure 2 (Sheet 2)  Feb 20/84 
Page 8 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

7.  Fueling Quantity Indicators
A.  Fueling quantity indicators, located on the fueling control panel, show the weight of fuel in the fuel tanks (Fig. 1). The fueling quantity indicators function as repeaters of the fuel quantity indicators located in the control cabin and aid the ground crew in fueling or defueling operations. The fueling quantity indicators are interchangeable with the fuel quantity indicators. The fueling quantity indicators are operative only when the fueling station door is open. When the door is closed, a proximity switch (Fig. 2) interrupts power to the fueling quantity indicators.
8.  Trimmers
A.  Capacitance trimmers (Fig. 1), located on the forward left side of the control stand above the hand mike jack, can be adjusted to standardize the total tank capacitance for the indicator. This adjustment allows for replacement of precalibrated indicators without recalibrating the indicator gage.
9.  Measuring Sticks
A.  The measuring sticks are used to measure the fuel quantity in tanks No. 1 and 2 by mechanical means (Fig. 1).  Each main tank has five measuring sticks, calibrated in inches, located in wing fuel tank access panels No. 1, 4, 6, 8, and 10. The measuring stick unit of calibration is marked on each stick approximately one inch from end of stick.
B.  The measuring stick reading and the applicable measuring stick conversion document (D6-32037, inches to U.S. gallons; or D6-32616, inches to kilograms; or D6-32628, inches to Imperial gallons) assist the ground crew in fueling operations when electrical power for fuel quantity indicators is not available. The measuring sticks are also used to verify fuel quantity indicator operation by comparing the fuel weight indicated by the indicators to the reading obtained by measuring sticks.
10.  Operation (Airplanes with digital indicators)
A.  The fuel quantity indicating system operates on 115-volt ac power obtained from the standby bus or external power bus.
B.  The tank units function as variable capacitors, their capacitance value being determined by fuel level in the tank. The phased measurement of the current through the tank units is performed by the microcomputer system in the indicator. It measures the magnitude of the capacitance current independent of the resistive current due to leakage. The microcomputer monitors the leakage current and when it reaches a magnitude effecting indicator accuracy, an error code is displayed.
　
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