Staffan
Topic Author
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### How Is Fuel Amounts Measured?

Can someone tell me how the remaining amounts of fuel in the fueltanks are measured on commercial aircraft?

Thanks,

Staffan

Staffan
Topic Author
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Joined: Tue Mar 18, 2008 9:21 am

### RE: How Is Fuel Amounts Measured?

What I mean is the type of sensor that measures it and how it works!

Staffan

FredT
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Joined: Thu Feb 07, 2002 9:51 pm

### RE: How Is Fuel Amounts Measured?

There are a couple of designs of course. The one I've seen the most uses a capacitative sensor. This is basically a metal tube with a metal rod in the middle, making up a crude capacitor. This is installed going from the top to the bottom of the tank.

I suspect you're familiar with capacitors but for those who aren't, the capacitance will vary depending on the properties of the substance isolating the inner rod from the outer tube. Fuel and air have different properties in this respect, so the capacitance will be an indicator of the level of fuel in the tank.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.

Staffan
Topic Author
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### RE: How Is Fuel Amounts Measured?

Ok, thanks for the reply, but it brings up another question, isn't the fuel conductive? Or are the two metal parts isolated from direct contact with the fuel?

Staffan

FredT
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Joined: Thu Feb 07, 2002 9:51 pm

### RE: How Is Fuel Amounts Measured?

Hydrocarbons are essentially insulators. Found this through a quick google search:

Conductivity of fuels is usually expressed in conductivity units (CU), 1 CU = 1 pico Siemens/meter (1 pS/m) = 1 × 10^–12 ohm^–1 meter^–1. Kerosenes may have conductivity ranging from less than 1 CU to perhaps 20 CU. For comparison, deionized water has a conductivity of about 10 million CU.

Not a very good conductor.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.

Staffan
Topic Author
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### RE: How Is Fuel Amounts Measured?

Tack så mycket! (Thank you very much!)

Staffan

tsufang@CI
Posts: 39
Joined: Mon Feb 18, 2002 8:38 pm

### RE: How Is Fuel Amounts Measured?

on boeing 777
measure by supersonic
wave through different material will chang the speed

Staffan
Topic Author
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### RE: How Is Fuel Amounts Measured?

Tsufang, sounds interesting, can you tell more about it, or do you know where I can find more info?

Thanks,

Staffan

Ratzz
Posts: 196
Joined: Mon Sep 27, 1999 6:58 pm

### RE: How Is Fuel Amounts Measured?

Remaining fuel can also be measured by utillizing s.c "dripsticks",located in various positions in each tank,easily accessible from the bottom of each tank.
On older a/c´s such as the L-188 Electra and the Sud-Aviation Caravelle,these "dripsticks"were considered more reliable than the actual gauge readouts.

HAWK21M
Posts: 30014
Joined: Fri Jan 05, 2001 10:05 pm

### RE: How Is Fuel Amounts Measured?

Varies with the Aircraft.
Can have the Float gauge in old Small Aircrafts,where a float indicates the fuel qty.
The Electrical type where in a transmitter conveys the level of fuel to an indicator in the cockpit.
Or the common used Capacitance type,where fuel or fuel/air is used as the dielectric depending on the qty of the fuel.
These capacitance probes are evenly distributed in the fuel tank taking into account the contour & profile of the wing to give an accurate reading irrespective of aircraft attitude.There is also a comparator unit located in the tank located at the bottom most part of the tank as it requires to be submerged in fuel, it corrects the fuel qty for fuels with various dielectric.
The signals from the comparator + capicatince probes is transmitted to the Indicator to register the Fuel qty in Lbs or Kgs or any appropriate unit.
regds
HAWK.
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Jet-A gasguy
Posts: 259
Joined: Sun Dec 24, 2000 4:04 pm

### RE: How Is Fuel Amounts Measured?

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.

Jet-A gasguy
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Staffan
Topic Author
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### RE: How Is Fuel Amounts Measured?

Thank you, helped a lot!!

tsufang@CI
Posts: 39
Joined: Mon Feb 18, 2002 8:38 pm

### RE: How Is Fuel Amounts Measured?

boeing 777
meausre by supersonic wave
and a device generate this wave
this wave will reflect when it touch the surface between the fuel and air in the tank
a special sensor install in the tank caculate the time difference between the wave transmit and reflect to receive by the sensor
then the computer accord the data to show the quantity to gnd staff ,pilot or for the a/c system
that's all i know !
best regards!!

L-188
Posts: 29881
Joined: Wed Jul 07, 1999 11:27 am

### RE: How Is Fuel Amounts Measured?

Ratz is exactly right on those dipsticks on the Electra.

Without going into details I know of aircraft that the inboard tanks read out in quanity, 1/4, 1/2, 3/4 ect. and the outboard tanks read out in lbs.
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