XXXX10
Topic Author
Posts: 706
Joined: Mon Jan 03, 2000 7:10 am

### Fuel On Board Indicators

I have a question on regading fuel on board indicators.

Does anyone know how they calculate the weight of the fuel on board, when they can only know the quantity?

Is I understand it, the weight depends on the quantity and the temparature. Is there some sort of fuel temprature sensor? or do the pilots do the calculation and somehow enter the weight ?

Many thanks

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

### RE: Fuel On Board Indicators

Capacitance probes,using Air-Fuel & Fuel as the Di-electric medium.
Compensator assys to adjust for Changes in Fuel quality.
regds
MEL
I may not win often, but I damn well never lose!!!

A319XFW
Posts: 1519
Joined: Thu Nov 03, 2005 5:41 am

### RE: Fuel On Board Indicators

You might find some answers here:

How Do Fuel Gauges Work In An Aircraft?

[Edited 2006-04-06 19:44:41]

Skyslave
Posts: 44
Joined: Sat Oct 22, 2005 12:52 pm

### RE: Fuel On Board Indicators

 Quoting XXXX10 (Thread starter):s I understand it, the weight depends on the quantity and the temparature

Actually, I dont think temperature has anything to do with the weight. Its true that fuel expands under heat, but I dont think it gains any mass. As the guy above me said, its a probe that can detect the amount of resistance in the tank. It calculates that resistance into weight. I learned about it in The Turbine Pilot's Flight Manual. If you're like me and want to know how things work in airliners, you should pick up this book. It answered ALOT of my questions, and breaks things down nicely.

Starlionblue
Posts: 18084
Joined: Fri Feb 27, 2004 9:54 pm

### RE: Fuel On Board Indicators

 Quoting Skyslave (Reply 3):Its true that fuel expands under heat, but I dont think it gains any mass.

I would hope it doesn't gain mass, or the laws of physics would be in trouble.

By the same token, it does not gain or lose energy, so you only really need to know the weight to know how far you can go. However, tanks are measured in volume units, so you need to know volume and how it relates to weight and temperature for some things.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

troubleshooter
Posts: 420
Joined: Mon Feb 07, 2005 5:22 am

### RE: Fuel On Board Indicators

The volume changes with the temperature as the density changes. The weight always relates to the local gravity. The mass is always constant. Thats why the fuel mass is the only reliable indication in aviation. If the fuel quantity indication is calibrated in lbs instead of kg they assume a constant gravity.
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Starlionblue
Posts: 18084
Joined: Fri Feb 27, 2004 9:54 pm

### RE: Fuel On Board Indicators

 Quoting Troubleshooter (Reply 5):If the fuel quantity indication is calibrated in lbs instead of kg they assume a constant gravity.

Well, gravity does change with altitude (decreases with the square of distance from the gravitating mass) but not very much
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

troubleshooter
Posts: 420
Joined: Mon Feb 07, 2005 5:22 am

### RE: Fuel On Board Indicators

 Quoting Starlionblue (Reply 6):Well, gravity does change with altitude (decreases with the square of distance from the gravitating mass) but not very much

To be honest, I don´t know if (and how) the fuel quantity computers take this into account.
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Starlionblue
Posts: 18084
Joined: Fri Feb 27, 2004 9:54 pm

### RE: Fuel On Board Indicators

 Quoting Troubleshooter (Reply 7):Quoting Starlionblue (Reply 6): Well, gravity does change with altitude (decreases with the square of distance from the gravitating mass) but not very much To be honest, I donï¿½t know if (and how) the fuel quantity computers take this into account.

I will venture a guess and say they don't at all. The gradient is tiny in the atmosphere (what's 10km more on a 12700km diameter globe?) and as you say mass is what counts, not weight.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

Skyslave
Posts: 44
Joined: Sat Oct 22, 2005 12:52 pm

### RE: Fuel On Board Indicators

 Quoting Starlionblue (Reply 8):I will venture a guess and say they don't at all. The gradient is tiny in the atmosphere (what's 10km more on a 12700km diameter globe?) and as you say mass is what counts, not weight.

Thats right. From what I understand the fuel probe sends out a radar like signal through the fluid. It can detect the amount by how much of the signal comes back to the probe. It calculates that into a weight readout in the cockpit. Essentially, you could be in zero gravity and it would still be able to detect the amount of fuel.

Starlionblue
Posts: 18084
Joined: Fri Feb 27, 2004 9:54 pm

### RE: Fuel On Board Indicators

 Quoting Skyslave (Reply 9):Quoting Starlionblue (Reply 8): I will venture a guess and say they don't at all. The gradient is tiny in the atmosphere (what's 10km more on a 12700km diameter globe?) and as you say mass is what counts, not weight. Thats right. From what I understand the fuel probe sends out a radar like signal through the fluid. It can detect the amount by how much of the signal comes back to the probe. It calculates that into a weight readout in the cockpit. Essentially, you could be in zero gravity and it would still be able to detect the amount of fuel.

Nitpicking, but wouldn't it actually be a mass readout?
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

### RE: Fuel On Board Indicators

 Quoting Troubleshooter (Reply 5):Thats why the fuel mass is the only reliable indication in aviation. If the fuel quantity indication is calibrated in lbs instead of kg they assume a constant gravity.

You can measure mass in kgs, lbs, grams, ounces... still a mass. There's lbf to use if you measure the weight, which is the force resulting from gravity pulling on the mass.

Often misused and abused though.

I'd suggest sticking with SI units though, kg for mass and N for forces.

I often find myself working with kgin^2 and other interesting units. Eeeew.

When fuelling up aircraft, you will have to take the density of the fuel into account if you are measuring the uplift in volume units.

 Quoting Skyslave (Reply 9):Thats right. From what I understand the fuel probe sends out a radar like signal through the fluid. It can detect the amount by how much of the signal comes back to the probe.

Nothing radar-like about most fuel pickups. Capacitive sensors or floats are the most common. You basically have a capacitor with the fuel inbetween the poles. The capacitance gives you the fuel level, and if you know the shape of the tanks this can be converted into a volume which can in turn be converted into a mass if you know the density.

It only gets interesting when you get into fighters, which tend to have dozens of tanks crammed into every nook and cranny of the aircraft, with bizarre shapes. On top of that, you are interested in getting correct fuel quantity readouts even if the aircraft is manouevering, causing the fuel to slosh about...
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Skyslave
Posts: 44
Joined: Sat Oct 22, 2005 12:52 pm

### RE: Fuel On Board Indicators

 Quoting FredT (Reply 11):Nothing radar-like about most fuel pickups. Capacitive sensors or floats are the most common. You basically have a capacitor with the fuel inbetween the poles. The capacitance gives you the fuel level, and if you know the shape of the tanks this can be converted into a volume which can in turn be converted into a mass if you know the density.

Well this is word for word in the turbine pilots manual:

"Fuel quantity in large turbine aircraft is generly measured using a different system than the floats with which you may be familiar...Capacitance fuel quantity indicator systems are used to measure fuel mass (and therefore weight). These systems consist of a series of long metallic probes stretched from top to bottom across the fuel tanks. By measuring electrical capacitance across the probes, the fuel indicating system determines the amount of fuel in one or more tanks."

I supose that I should have specified that I was explaining the fuel measurment systems in large jets, sorry

dl757md
Posts: 1483
Joined: Mon May 24, 2004 9:32 am

### RE: Fuel On Board Indicators

Let me give a quick but comprehensive description of a typical fuel quantity indicating system on a commercial airliner. I will use the 767 as my example.

The fuel quantity indicating system is a microcomputer controlled system that uses the variable capacitance principle for fuel quantity measurement. The capacitance of each tank unit is proportional to the percentage of the tank unit which is immersed in fuel.

The FQIS processor is the principal component of the FQIS. It is a digital, dual-channel, microprocessor based computer. The FQIS processor unit reads signals from the tank units, compensators, and densitometers, and computes fuel volume and weight.

The tank units and compensators are similar in construction and installation provisions. They are capacitance type sensors but the tank units sense the depth of fuel and the compensators sense the dielectric constant of the fuel. These components each have an inner and outer concentric tube and measure the capacitance of the fuel between these tubes

The densitometer senses fuel density. Knowing the density and the volume of fuel enables the FQIS processor to calculate the mass or weight of the fuel. The densitometer has a gamma emitter and detector. That is where your "radar like signal" comes from.

It's a pretty amazing system in that the processor can accurately calculate fuel load at all normal operating attitudes even as the fuel sloshes around the tank by analyzing the height of the fuel at every tank unit and the dielectric at each compensator.

DL757Md
757 Most beautiful airliner in the sky!

FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

### RE: Fuel On Board Indicators

 Quoting Skyslave (Reply 12):Well this is word for word in the turbine pilots manual:

Uh, yeah. Capacitive sensors in large jets - which are not radar-like. You just argued my point. Stop it, you are confusing me!

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

eilennaei
Posts: 1003
Joined: Tue Nov 23, 2004 8:41 am

### RE: Fuel On Board Indicators

 Quoting Dl757md (Reply 13):compensators sense the dielectric constant of the fuel.

I had this one reviewed by the Global Nitpickers' Association, and I received the message that if something is constant, it needs not be remeasured.

Without knowing any of the specifics, I think constructing a "bridge" device where one of the identical (capacitance principle) arms is always in a "totally immersed" condition and the other in a "measuring" condition, any external influence, such as phases of the Moon, gravity, ugly & fat people in the cabin, will automatically be cancelled. The ambient temp will have to be know as it will have varied during the flight. Everyone be cleared and deconfused now!

David L
Posts: 8551
Joined: Tue May 18, 1999 2:26 am

### RE: Fuel On Board Indicators

 Quoting Eilennaei (Reply 15):I had this one reviewed by the Global Nitpickers' Association

Ooo... Starlionblue, did you see that? I assume these Nitpickers are not "mad". I feel such a fraud.

 Quoting FredT (Reply 11):You can measure mass in kgs, lbs, grams, ounces... still a mass

Thank you! Otherwise people in the US and a lot in the UK would have weight but no mass and those in the rest of Europe, Canada, etc., would have mass but no weight.

eilennaei
Posts: 1003
Joined: Tue Nov 23, 2004 8:41 am

### RE: Fuel On Board Indicators

 Quoting Eilennaei (Reply 15): any external influence,

How could I forget to mention: for the variations across different types of fuel as well. You will get the clear difference signal of fuel level, which will have to be compensated for the shape of the tank in order to be of use.

dl757md
Posts: 1483
Joined: Mon May 24, 2004 9:32 am

### RE: Fuel On Board Indicators

 Quoting Eilennaei (Reply 15):I had this one reviewed by the Global Nitpickers' Association, and I received the message that if something is constant, it needs not be remeasured. Without knowing any of the specifics, I think constructing a "bridge" device where one of the identical (capacitance principle) arms is always in a "totally immersed" condition and the other in a "measuring" condition, any external influence, such as phases of the Moon, gravity, ugly & fat people in the cabin, will automatically be cancelled. The ambient temp will have to be know as it will have varied during the flight. Everyone be cleared and deconfused now!

What I wrote was straight out of the Boeing 767-400AMM. Perhaps you should take it up with Boeings' engineers. Or perhaps you should have consulted the dictionary for the definition of dielectric constant rather than consulting the Global Nitpickers Association.

From Wikipedia, Electrically, the dielectric constant is a measure of the extent to which a substance concentrates the electrostatic lines of flux. More specifically it is the ratio of the amount of electrical energy stored in an insulator, when a static electric field is imposed across it, relative to vacuum (which has a dielectric constant of 1). Thus, the dielectric constant is also known as the static permittivity.

DL757Md
757 Most beautiful airliner in the sky!

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