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Fuel quantity in each tank
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Total fuel quantity.
The FQPU shows fuel quantity data on the EICAS display and on the integrated refuel panel (IRP) indicators.
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FUEL INDICATING -FQIS -GENERAL DESCRIPTION
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FUEL INDICATING -FQIS -FUEL TANK SENSORS
General
These are the sensors in the fuel tanks:
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Tank units
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Densitometers
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Water detectors
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Temperature sensor.
Tank Units
The tank units measure fuel height. There are 52 tank units in the three tanks: 20 in each main tank and 12 in the center tank.
Each tank unit position has a number. In the main tanks, tank unit 1 is at the inboard end of the tank. Tank unit 20 is at the outboard end of the tank. In each side of the center tank, the tank unit numbers are from 1 to 6. The left tank unit 1 is at the left outboard end of the center tank. The right tank unit 1 is at the right outboard end of the center tank.
Each tank unit has an ultrasonic transmitter/receiver and a stillwell. Eight of the tank units also have calibration targets.
The fuel quantity processor unit (FQPU) sends a signal to the ultrasonic transmitter/receiver. This causes a sonic pulse to go through the fuel in the stillwell to the fuel surface, and back to the receiver. The FQPU measures the travel time of the pulse and uses it to find the fuel height.
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The sonic pulse also reflects from the targets back to the receiver. The FQPU uses these pulses to measure the velocity of sound (VOS)in the fuel.
Densitometers
Each fuel tank has one densitometer. Densitometers use a vibrating cylinder transducer to measure the fuel density in each tank. The frequency of the vibrations is proportional to the fuel density. The densitometers are energized and monitored by the FQPU.
Water Detectors
Water detectors are at the low point in each fuel tank in the tank sump area. The FQPU uses water detector signals to see if there is water in the tank sump area. Each fuel tank has one water detector. The fuel quantity maintenance page shows a message when there is water in the fuel tank.
The water detectors are ultrasonic devices like the tank units. The FQPU sends a signal to the water detectors. The water detectors send a pulse to the bottom of the tank. If there is water in the bottom of the tank, the pulse reflects off the interface between the water and fuel.
Temperature Sensor
There is only one fuel temperature sensor. It measures the fuel temperature in the left main tank. The temperature sensor is a resistance-type sensor that fits inside a sleeve.
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FUEL INDICATING -FQIS -FUEL TANK SENSORS
The sleeve is on the rear spar of the left wing, aft of the fifth fuel tank access door (between ribs 9 and 10).
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777 AIRCRAFT MAINTENANCE MANUAL
FUEL INDICATING -FQIS -FUEL TANK SENSORS
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FUEL INDICATING -FQIS -WIRING HARNESS
General
Tank wiring includes internal harnesses, external harnesses, and connectors. Internal harnesses connect the fuel tank sensors to connectors on the spars. External harnesses go from the spar connectors to the FQPU.
Internal Harness
Each main tank has two internal harnesses. The inboard harness connects eight tank units, one densitometer, and one water detector to one connector on the rear spar. The inboard harness connector is aft of the fifth fuel tank access door (between ribs 9 and 10). The outboard harness (not shown) connects twelve tank units to another connector on the rear spar. The outboard harness connector is aft of the sixth fuel tank access door (between ribs 10 and 11).
The center tank has two internal harnesses, one for each section. Each center tank harness connects six tank units and one water detector to connectors on the front spar. The center tank spar connectors are between ribs 2 and 3. The harness on the left side of the center tank also has a connection for the densitometer.
The internal wire harnesses connect to fuel tank sensors with Faston connectors. The Faston connectors have a terminal and insulator on the wire, and a tab on the sensor. The insulator is around the terminal.
External Harness
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