AirPacific747
Topic Author
Posts: 9586
Joined: Mon May 19, 2008 9:52 am

How Do Fuel Gauges Work In An Aircraft?

Hi

I have been wondering how it is possible to give such a precise estimate of how much fuel is left in the fuel tanks of the aircraft. I have noticed that in fs, the plane can tell you exactly how many gallons you have left of fuel, and wonder if its like that in real aircraft too and how that is possible.

[Edited 2006-01-23 17:02:03]

AirPacific747
Topic Author
Posts: 9586
Joined: Mon May 19, 2008 9:52 am

RE: How Do Fuel Gauges Work In An Aircraft?

sorry I realise that this thread should probably be moved to the tech/ops forum

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

RE: How Do Fuel Gauges Work In An Aircraft?

Well I can only say for Airbus aircraft, but you have 2 ways of knowing how much fuel you have in the tank.

1) On the ground there are magnetic level indicators which are like a dipstick on a car, only they have a float which is magnetic and lifts your 'dipstick' up and down.

2) The fuel gauging system. Here you have a few (around 30 on an A320) low voltage capacitors where the fuel can go between them. At a different fuel level you will have different capacitances (i.e. capacitance proportional to the height of fuel) and therefore you know where your level is at. For the aircraft pitch and roll attitudes the fuel computer works out a plain of whee the fuel is (slightly different on different manufacturers). Additionally to that you have a density and permittivity readers in the tank to then calculate the fuel quantity (either kg or lbs).
Then you have level sensors. These have two states they know, either 'wet' or 'dry'. So they are placed at known levels in the tank (high level, low level etc) so these will trigger certain valves and warnings.

Hope this explains it somewhat!

charliecossie
Posts: 408
Joined: Wed Oct 10, 2001 5:17 am

RE: How Do Fuel Gauges Work In An Aircraft?

Basically:
The size of the tank is known precisely. To know how much fuel is in there, measure the height of the fuel at a number of points in the tank. From this, we know the precise volume in there. Next, measure the density of the fuel and compensate. That gives us the mass of fluid. Present that info to the cockpit (in Kgs or lbs).
Modern systems (744/777 for example) are accurate to well within 1%.

For more precise info, Google "fuel quantity indication accuracy"

AirPacific747
Topic Author
Posts: 9586
Joined: Mon May 19, 2008 9:52 am

RE: How Do Fuel Gauges Work In An Aircraft?

Yes this helps me understanding how it works. I was also wondering what would happen when the aircraft would be climbing/descending and now I know.

Thanks a lot guys!

jspitfire
Posts: 284
Joined: Fri Feb 18, 2005 2:50 pm

RE: How Do Fuel Gauges Work In An Aircraft?

In the Cessna 172 our fuel gauges are a bit simpler. There is a float in each tank that measures the amount of fuel. No fancy calculations for when wings aren't level or anything. Therefore the fuel gauges are not very accurate, and we don't really trust them. Dipping the tanks before every flight is by far the most accurate way to see how much fuel we have!

Jason

liedetectors
Posts: 323
Joined: Thu Jul 21, 2005 10:44 am

RE: How Do Fuel Gauges Work In An Aircraft?

i think you can look at it another way too. there is a fuel flow gauge and the computer can measure how long the fuel flows at a given rate. That will tell you how much fuel has passed by the fuel flow gauge (fuel/unit time x unit time = fuel). its easy to know how much fuel was put in the tank initially...so just subtract one from the other. doing that by hand would be a bit tedious tho.
If it was said by us, then it must be true.

CRJonBeez
Posts: 310
Joined: Mon Nov 29, 2004 8:08 am

RE: How Do Fuel Gauges Work In An Aircraft?

i don't know about all eurocopters, but i know it is possible to get a readout in the cockpit of an EC135 of not only pounds or kilograms, but also gallons or liters using the basic equation of 1 gallon of jet fuel coming out to approximately 6.7 lbs

planefreakaa
Posts: 102
Joined: Sat Jan 29, 2005 4:26 pm

RE: How Do Fuel Gauges Work In An Aircraft?

Some of the gauges are nuclear, at least radioactive though, I forget the whole operation but it is installed on many Boeing products, I believe that the fuel is passed through a densitometer where the fuel is hit with the radioactive wave and it measures the density of the fuel to get the quantity in the cockpit.....here at American we don't have too many problems with the system, which means that in my nine years of working in maintenance I haven't had to chance a single one..

A/c train
Posts: 674
Joined: Sun Jun 10, 2001 7:57 am

RE: How Do Fuel Gauges Work In An Aircraft?

Charliecossie, you'll have to englighten me on this one, kg is a mass, lb's is a weight, therefore most FOB figures in the flt deck are kg from the a/c ive worked on 320/21/330/300, 75/76 etc etc, but if you've seen a gauge which gives it in lbs please tell me! im sure you have, hope this is not a nonsensuial aviation related post?
regds a/c

HAWK21M
Posts: 30002
Joined: Fri Jan 05, 2001 10:05 pm

RE: How Do Fuel Gauges Work In An Aircraft?

We've had a few B737s with Fuel gauges in lbs,while the majority in Kgs.
1 kg approx 2.2 lbs.
And Dripsticks in lbs,kgs & units.
regds
MEL
I may not win often, but I damn well never lose!!!

A/c train
Posts: 674
Joined: Sun Jun 10, 2001 7:57 am

RE: How Do Fuel Gauges Work In An Aircraft?

Put em all in kg, cant be doing with metric bananas!!

danman
Posts: 39
Joined: Thu Jul 04, 2002 6:11 am

RE: How Do Fuel Gauges Work In An Aircraft?

707s had sticks in inches and you ALWAYS fuelled on the sticks and converted to Lbs.
Fun

planefreakaa
Posts: 102
Joined: Sat Jan 29, 2005 4:26 pm

RE: How Do Fuel Gauges Work In An Aircraft?

 Quoting A/c train (Reply 9): but if you've seen a gauge which gives it in lbs please tell me!

go to any US registered aircraft, there you will find it...

Jetlagged
Posts: 2564
Joined: Sun Jan 23, 2005 3:00 pm

RE: How Do Fuel Gauges Work In An Aircraft?

 Quoting A/c train (Reply 9):Charliecossie, you'll have to englighten me on this one, kg is a mass, lb's is a weight, therefore most FOB figures in the flt deck are kg from the a/c ive worked on 320/21/330/300, 75/76 etc etc, but if you've seen a gauge which gives it in lbs please tell me! im sure you have, hope this is not a nonsensuial aviation related post?

Err, kg and lb are both units of weight. The metric unit of mass is the Newton. It's quite common for aircraft in the non-metricated outposts of the world (like the USA) to display fuel weight in lbs.
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