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D. Erratic fuel quantity indicator readings can be experienced more often than other fuel quantity indicating system malfunctions. The following are some of the more common causes of erratic readings.
(1)
Accumulation of excessive amounts of water in fuel tanks causing shorting of tank units and compensator units.
(2)
Clogged drain holes in wing tank spanwise stringers preventing water from draining to sumps.
(3)
Microbial growth and contamination in fuel tank internal structure and on tank units and compensator units.
(4)
Fuel tank unit grounding due to contact with structure, tubing, or bonding straps inside fuel tank.
(5)
Fuel tank unit electrical signal output being open or intermittent due to missing terminal screw, under-torquing of terminal screw, an improper terminal screw installed, or internal probe damage due to overtorquing terminal screw. Refer to 28-41-21 R/I, for torque requirements.
(6)
Short circuit between compensator high Z and low Z elements.
(7)
Broken wire in fuel tank electrical harness intermittently open-circuit.
(8)
Improper O-ring seal installation at fuel bulkhead spar receptacle or bussing plug allows fuel to leak from tank onto face of connector and/or moisture to enter onto face of connector. Either condition can create a leakage path between connector pins and vary system capacitance characteristics.
(9)
Fuel penetration through fuel tank wire harness to face of fuel bulkhead spar receptacle causing variations in system capacitance.
(10)
On airplanes with analog indicators, indicators showing erratic readings due to indicator internal sticking from lack of, or improper, lubrication.
2. Prepare for Trouble Shooting
A. Provide electrical power (Ref 24-22-0 MP).
B. Ensure EP GRD FUELING, fuel QTY, and CAPTAINS AND CTR PANEL AND INSTR LIGHT circuit breakers on P6 panel and PDC circuit breaker on P18 panel are closed.
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EFFECTIVITY
AIRPLANES WITH DIGITAL INDICATORS
5C8
Feb 20/89 28-41-05 Page 103
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EFFECTIVITY
AIRPLANES WITH DIGITAL INDICATORS
5C8 28-41-05 Feb 20/89 Page 104
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EFFECTIVITY
AIRPLANES WITH ANALOG INDICATORS
5C8
Feb 20/89 28-41-05 Page 105
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
EFFECTIVITY
AIRPLANES WITH ANALOG INDICATORS
5C8 28-41-05 Feb 20/89 Page 106
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
EFFECTIVITY
AIRPLANES WITH DIGITAL INDICATORS
5C8
Feb 20/89 28-41-05 Page 107
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
EFFECTIVITY
AIRPLANES WITH ANALOG INDICATORS
5C8 28-41-05 Feb 20/89 Page 108
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
5C8
Feb 20/89 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 28-41-05 Page 109
FUEL QUANTITY INDICATING SYSTEM -ADJUSTMENT/TEST
EFFECTIVITY
Airplanes with Precalibrated Indicators
1. General
A. When a fuel quantity indicator is changed, an adjustment of the fuel quantity indicating system is not required.
B. After a compensator, tank unit, wiring harness or capacitance trimmer is changed, check P2 fuel quantity empty indication. Trim or adjustment may be required per par. 3 or 4, depending on empty reading as indicated in following table.
NOTE: Ensure applicable tank(s) is defueled with sump(s) drained (Ref 28-23-0 MP) prior to taking empty reading. Momentarily press P2 fuel quantity PUSH-TO-TEST switch prior to checking P2 fuel quantity empty indication.
TANKS ACTION REQUIRED
NONE TRIM (Ref Par 3) ADJUSTMENT (Ref Par 4)
1 or 2 +50 kg +50 to +100 kg over 100 kg
CENTER +75 kg +75 to +150 kg over 150 kg
C. When trim adjustment is required, perform system trim per par. 3.
D. When a complete adjustment is required, perform system adjustment per par. 4.
E. When the fuel summation unit is replaced, an adjustment of the unit is not required.
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