On newer Boeing commercial aircraft, in addition to the capacitance probes and weight compensators, there are also densitomiters. Fuel Density plays a large part in the total wieght of the fuel. The FQP(Fuel Quantity Processor) takes information from all three sources and in turn provides an accurate fuel weight to the Fuel Quantity Indicators. The information from the FQIS(Fuel Quantity Indicating System) is sent via seperate channels to both the Flight Deck Gauges and to the Underwing panel indicators. Since both Flight Deck and Underwing Indicators receive input directly from the FQP, they can both be considered "Master" gauges. Although Flight Deck Indicators will always be considered the final authority if there are any discrepencies between the two. On the Boeing 777, ultrasonic waves are also used to measure Fuel quantity. This and other fuel system components can be accessed on the computer in the Flight Deck through the Fuel Quantity Maintenance Pages. Accessing this will provide:
1) Fuel Quantites
2) Fuel temperature
3) Fuel densities
4) Height of Fuel in each tank(through ultrasonic sound)
5) VTO(Volumetric Top-Off) Values
6) Water detection status.
Hope that helps.
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