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B.  The measuring stick reading and measuring stick conversion document (D6-32628, inches to imperial gallons; D6-32037, inches to U.S. gallons; or D6-32616 inches to kilograms) 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 the measuring sticks.
8.  Operation
A.  The fuel quantity indicating system operates on 115-volt ac power obtained from the standby bus with one or both engines or the APU running. To energize the system without operating the engines or APU, power is obtained from ground cart through external power bus (Fig. 2), or from dc/ac inverter supplied by battery powering standby bus.
B.  The tank units function as variable capacitors, their capacitance value being determined by fuel level in the tank. The tank units are connected into a capacitance bridge so that 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 alters capacitance value of the tank units and causes unbalance in the bridge circuit which is detected by phase sensitive amplifier. The amplifier controls an induction motor which moves a rebalance potentiometer wiper on the reference side of the bridge until a null point is again established in the system. The indicator dial pointer is connected to the potentiometer wiper and shows a fuel quantity reading corresponding to the null balance point.
C.  Operation of the indicators can be tested by either of the two test switches. Actuation of the test switch on the engine instrument panel grounds the tank unit portion of the indicator circuit. This unbalances the indicator circuit and causes the fuel quantity indicator pointers to move downscale. The fueling quantity indicator pointers do not move. Upon releasing the test switch, the fuel quantity indicator pointers return to their original position. When the test switch is actuated on the fueling control panel, the fueling quantity indicator pointers move downscale and the fuel quantity indicator pointers move up scale. Upon releasing the test switch, all indicator pointers return to their original position.
NOTE:  Keying of an HF system transmitter may cause a shifting or fluctuation of the fuel quantity indicator readings when the airplane is on the ground with entry door(s) open, and with service interphone cords and grounding straps connected.
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28-41-0 Page 8  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Feb 20/87 

FUEL QUANTITY INDICATING SYSTEM - TROUBLE SHOOTING
EFFECTIVITY
Airplanes with Non-Precalibrated Indicators
1. General
A.  The following trouble shooting procedures are based on performance of system operational test and are presented in tree-type format. Test steps (heavy-line boxes) are connected by shaded arrows to show OK condition path. When all steps in OK condition path check out, system is operable.
B.  When a test step does not check out, follow the NOT OK line to box containing trouble symptom. Turn to procedure indicated and continue to follow a single line by analyzing results of each test step until required corrective action is determined. Perform specified corrective action, then repeat step at which failure was encountered.
C.  To reduce time required to trouble shoot fuel quantity indicating system problems, try the following shortcuts and check for proper response of fuel quantity indicating system. It is suggested the longer trouble shooting procedures be used if the steps below do not resolve the problem.
(1)  
Try switching same tank quantity indicators between P2 center panel and P15 fueling panel as it is unlikely both fuel (primary) and fueling (repeater) indicators will fail simultaneously.

NOTE:  No fuel quantity indicator is exactly interchangeable without having to make a final check and adjustment (Ref 28-41-0 A/T) to calibrate it to a specific subsystem. However, switching like indicators for the purpose of trouble shooting is sufficiently accurate to serve the purpose.

(2)  
Ensure fuel tank sump drains have been recently opened (at least 45 minutes after airplane has come to rest or after completion of fueling) and any water accumulation drained off. Sump drainage of approximately 5 gallons or 19 liters per tank every 24 hours is recommended in severe operating conditions such as in warm, humid climates.

(3)  
Inspect wing spar mounted volumetric top-off unit or bussing plug seals for evidence of leakage of fuel or water possibly causing electrical leakage paths between connector pins.

(4)  
Perform press-to-test operation of fuel quantity indicators. If none of the indicators respond, a power supply to the indicator problem exists. If one indicator does not respond but the others do, the trouble is usually in the inoperative indicator.
　
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