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How Do Fuel Gauges Work In An Aircraft?  
User currently offlineAirPacific747 From Denmark, joined May 2008, 2482 posts, RR: 24
Posted (8 years 11 months 1 week 1 day 18 hours ago) and read 21783 times:

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.

thankyou in advance!

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

14 replies: All unread, jump to last
 
User currently offlineAirPacific747 From Denmark, joined May 2008, 2482 posts, RR: 24
Reply 1, posted (8 years 11 months 1 week 1 day 18 hours ago) and read 21789 times:

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

User currently offlineA319XFW From , joined Dec 1969, posts, RR:
Reply 2, posted (8 years 11 months 1 week 1 day 16 hours ago) and read 21695 times:

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!


User currently offlineCharliecossie From Germany, joined Oct 2001, 479 posts, RR: 9
Reply 3, posted (8 years 11 months 1 week 1 day 16 hours ago) and read 21689 times:

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"


User currently offlineAirPacific747 From Denmark, joined May 2008, 2482 posts, RR: 24
Reply 4, posted (8 years 11 months 1 week 1 day 16 hours ago) and read 21674 times:

thankyou for your answers!

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!


User currently offlineJspitfire From Canada, joined Feb 2005, 308 posts, RR: 2
Reply 5, posted (8 years 11 months 1 week 1 day 16 hours ago) and read 21674 times:

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


User currently offlineLiedetectors From United States of America, joined Jul 2005, 360 posts, RR: 0
Reply 6, posted (8 years 11 months 1 week 1 day 8 hours ago) and read 21513 times:

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.
User currently offlineCRJonBeez From United States of America, joined Nov 2004, 317 posts, RR: 3
Reply 7, posted (8 years 11 months 1 week 1 day 3 hours ago) and read 21472 times:

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

User currently offlinePlanefreakaa From United States of America, joined Jan 2005, 111 posts, RR: 0
Reply 8, posted (8 years 11 months 6 days 18 hours ago) and read 21312 times:

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..

User currently offlineA/c train From United Kingdom, joined Jun 2001, 501 posts, RR: 4
Reply 9, posted (8 years 11 months 6 days 18 hours ago) and read 21306 times:

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


User currently offlineHAWK21M From India, joined Jan 2001, 31712 posts, RR: 56
Reply 10, posted (8 years 11 months 6 days 17 hours ago) and read 21303 times:

Quoting A/c train (Reply 9):

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



Think of the brighter side!
User currently offlineA/c train From United Kingdom, joined Jun 2001, 501 posts, RR: 4
Reply 11, posted (8 years 11 months 6 days 17 hours ago) and read 21294 times:

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

User currently offlineDanman From United Kingdom, joined Jul 2002, 39 posts, RR: 0
Reply 12, posted (8 years 11 months 6 days 10 hours ago) and read 21230 times:

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


User currently offlinePlanefreakaa From United States of America, joined Jan 2005, 111 posts, RR: 0
Reply 13, posted (8 years 11 months 6 days 4 hours ago) and read 21178 times:

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...


User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2577 posts, RR: 25
Reply 14, posted (8 years 11 months 6 days 4 hours ago) and read 21175 times:

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.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
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