7.
8.
A
737-300/400/500MAINTENANCE MANUAL
B. The fuel quantity indicators use 115-volt ac power for the bridge circuit through the transfermer and for one phase of the induction motor. (The other motor phase is energized by amplifier output). The pointers for the fuel quantity indicators are driven by the same shaft as the potentiometer wiper of the self-balancing bridge circuit and turn with the wiper to supply continuous indication of the fuel quantity.
Refuel Quantity Indicator (Fig. 1)
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A. Refuel quantity indicators on the P15 panel show the weight of fuel in the fuel tanks. The refuel quantity indicators help the ground crew in refuel or defueling operations. The refuel quantity indicators operate only when the access door for the refuel station is open. When the access door is closed, power to the refuel quantity indicators is interrupted by a proximity switch.
Fuel Summation Unit (Fig. 1)
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A. The fuel summation unit takes the voltage signals for each fuel quantity indicator and sums them for an output voltage signal representative of the total fuel remaining. This output signal is one of several inputs to a flight management computer system (FMC) which has a control display unit (CDU). The FMC converts these inputs into a display of total fuel remaining (AMM 34-62-00/001).
B. The fuel summation unit has calibration potentiometers consisting of one EMPTY potentiometer and an individual FULL potentiometer for each fuel tank. Adjustment screws are accessible after you remove a protective cover on the top of the fuel summation unit. Calibration adjustment permits precise setting of the input to the FMC so that it will always agree with the fuel sum total of the fuel tanks. All inputs and outputs are treated as voltage ratios, thereby eliminating errors caused by variations in the bus supply voltage. The electrical connector to the fuel summation unit supplies grounding connections for all tank input not in use.
Capacitance Trimmers (Fig. 1)
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A. Capacitance trimmers are used to compensate for changes in system stray capacitance and to adjust the fuel quantity indicator. The capacitance trimmers are found on the forward left side of the control stand, above the hand mike jack.
Fuel Measuring Sticks (Fig. 1)
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A. The fuel measuring sticks are used to measure the fuel quantity in the No. 1 and No. 2 tanks by mechanical means. Each main tank has five fuel measuring sticks, calibrated in inches. These fuel measuring sticks are in the tank access doors No. 1, 4, 6, 8, and 10. The fuel measuring stick unit of calibration is marked on each stick approximately 1 inch from the end.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
B. The value on the fuel measuring stick and the applicable fuel measuring stick conversion document (D6-37852, inches to USGal; D6-38055, inches to kilograms; D6-38014, inches to Imp. gallons) help the ground crew in the refuel operations when electrical power for the fuel quantity indicators is not available. The fuel measuring sticks are also used to compare the fuel weight shown by the fuel quantity indicators to the value shown by the fuel measuring sticks to make sure the fuel quantity indicators operate correctly.
9. Operation (Fig. 2)_________
A. The fuel quantity indicating system operates on 115v ac power from the standby bus or the external power bus.
B. The tank units function as variable capacitors - their capacitance value is calculated from the fuel level in the fuel tank. The tank units are connected into a capacitance bridge. Current through the tank unit side of the bridge is compared to the current of the opposite phase in the balancing side of the bridge. A change in the fuel level changes the capacitance of the tank units and causes imbalance in the bridge circuit. The imbalance is found by a phase-sensitive amplifier. The amplifier controls an induction motor, which moves a rebalance potentiometer wiper on the reference side of the bridge until the circuit becomes balanced again. The pointer on the fuel quantity indicator face is connected to the wiper and changes position proportionally to the wiper movement.
C. Operation of the fuel quantity indicators and refuel quantity indicators can be tested by either of the two indicator test switches. Actuation of the indicator test switch on the engine instrument panel grounds the tank unit portion of the indicator circuit. This unbalances the indicator circuit and causes the pointer on the fuel quantity indicators to move downscale. When the indicator test switch is released, the pointer on the fuel quantity indicators returns to its original position. When the indicator test switch is actuated on the refuel panel, the pointer on the refuel quantity indicators moves downscale and the pointer on the fuel quantity indicators moves upscale. When the indicator test switch is released, all the fuel quantity indicators and refuel quantity indicators return to their original position.
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