5C8
Feb 20/89 28-41-0 Page 101
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

D.  Malfunctions within the fuel quantity (primary) indicator, associated tank units and compensator unit, interconnecting wiring, or operating with contaminated fuel result in the indicator either failing to respond to test switch, operating sluggishly or erratically, running against empty or full stop, or providing a high or low indication of fuel quantity. Also, since fuel quantity indicator on pilots center instrument panel P2 provides signals to the performance data computer system control display unit (CDU) on pilots' forward electronic control panel P9 and fueling quantity indicator (repeater) on fueling station P15 panel, malfunctions in primary indicating circuits affect the other indicators and computer as well.
E.  Malfunctions in fueling quantity (repeater) indicator and associated wiring result in indicator either not operating or failing to provide a second indication of fuel quantity displayed by the associated fuel quantity (primary) indicator within maximum tolerance of +50 pounds (±25 kilograms) for full tank; +100 pounds (± 50 kilograms) for empty tank, and +130 pounds (± 65 kilograms) for empty integral center tank.
F.  Malfunctions in fuel summation unit on pilots' aft electronic panel P8 or associated wiring result in an inoperative or inaccurate total remaining fuel quantity display on P9 panel control display unit (CDU) or inaccurate PDCS computer parameter which uses total fuel signal. The CDU total fuel quantity display should read within 1% of sum of quantities shown by the primary indicator pointers.
G.  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 possibly 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, undertorquing of terminal screw, an improper terminal screw installed, or internal probe damage due to overtorquing terminal screw. Refer to AMM 28-41-21/401, for torque requirements.

(6)  
Short circuit between compensator high Z and low Z elements.

(7)  
Broken wires in fuel tank electrical harness can be intermittently open-circuited.

(8)  
Improper O-ring seal installation at fuel bulkhead spar receptacle or bussing plug possibly allowing 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 could possibly cause variations in system capacitance.

(10)
Indicators showing erratic readings due to indicator internal sticking from lack of, or improper lubrication.

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28-41-0 Page 102  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Jun 20/83 

H.  This table gives a summary of the most usual fault indications and their causes:
FAULT INDICATION  CAUSE OF THE FAULT
1.  
The indicator in the flight compartment
goes to the high end of the scale. Short Hi-Z to Lo-Z
The refuel indicator follows.

2.  
The indicator in the flight compartment Short Hi-Z to Ground
goes to the high end of the scale.
The refuel indicator follows.

3.  
The indicator in the flight compartment Short Lo-Z to Ground
goes to the low end of the scale. (probe hitting frame or
The refuel indicator reads the same as the plumbing in tank)
flight compartment indicator.

4.  
The indicator in the flight compartment Short compensator
reads HIGH or pegs at the top stop. Lo-Z to ground
The refuel indicator reads LOW or pegs at
the bottom stop.

5.  
The indicator in the flight compartment Short compensator Lo-Z
reads LOW. Probably at the bottom stop. to Hi-Z (water in compensator)
Depends on resistance in short. Unstable if
water is present.

6.  
The indicator in the flight compartment Open Hi-Z
　
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