Change Forum... Civil Aviation Travel, Polls & Prefs Tech/Ops Aviation Hobby Aviation Photography Photography Feedback Trip Reports Military Av & Space Non-Aviation Site Related LIVE Chat My Starred Topics | Profile | New Topic | Forum Index | Help | Search
 How Do Fuel Gauges Work In An Aircraft?
 AirPacific747 From Denmark, joined May 2008, 3129 posts, RR: 25Posted Mon Jan 23 2006 17:00:17 UTC (10 years 4 months 4 days 3 hours ago) and read 29012 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]
 AirPacific747 From Denmark, joined May 2008, 3129 posts, RR: 25 Reply 1, posted Mon Jan 23 2006 17:03:41 UTC (10 years 4 months 4 days 3 hours ago) and read 29020 times:

 sorry I realise that this thread should probably be moved to the tech/ops forum
 A319XFW From , joined Dec 1969, posts, RR: Reply 2, posted Mon Jan 23 2006 18:49:42 UTC (10 years 4 months 4 days 1 hour ago) and read 28929 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!
 Charliecossie From Germany, joined Oct 2001, 480 posts, RR: 9 Reply 3, posted Mon Jan 23 2006 19:00:55 UTC (10 years 4 months 4 days 1 hour ago) and read 28921 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"
 AirPacific747 From Denmark, joined May 2008, 3129 posts, RR: 25 Reply 4, posted Mon Jan 23 2006 19:20:03 UTC (10 years 4 months 4 days ago) and read 28905 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!
 Jspitfire From Canada, joined Feb 2005, 309 posts, RR: 1 Reply 5, posted Mon Jan 23 2006 19:20:04 UTC (10 years 4 months 4 days ago) and read 28905 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
 Liedetectors From United States of America, joined Jul 2005, 360 posts, RR: 0 Reply 6, posted Tue Jan 24 2006 03:09:42 UTC (10 years 4 months 3 days 16 hours ago) and read 28744 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.
 CRJonBeez From United States of America, joined Nov 2004, 317 posts, RR: 3 Reply 7, posted Tue Jan 24 2006 07:29:05 UTC (10 years 4 months 3 days 12 hours ago) and read 28703 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
 Planefreakaa From United States of America, joined Jan 2005, 113 posts, RR: 0 Reply 8, posted Wed Jan 25 2006 16:59:04 UTC (10 years 4 months 2 days 3 hours ago) and read 28543 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..
 A/c train From United Kingdom, joined Jun 2001, 501 posts, RR: 3 Reply 9, posted Wed Jan 25 2006 17:19:42 UTC (10 years 4 months 2 days 2 hours ago) and read 28537 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
 HAWK21M From India, joined Jan 2001, 31875 posts, RR: 54 Reply 10, posted Wed Jan 25 2006 17:28:19 UTC (10 years 4 months 2 days 2 hours ago) and read 28534 times:

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!
 A/c train From United Kingdom, joined Jun 2001, 501 posts, RR: 3 Reply 11, posted Wed Jan 25 2006 17:39:44 UTC (10 years 4 months 2 days 2 hours ago) and read 28525 times:

 Put em all in kg, cant be doing with metric bananas!!
 Danman From United Kingdom, joined Jul 2002, 39 posts, RR: 0 Reply 12, posted Thu Jan 26 2006 00:54:33 UTC (10 years 4 months 1 day 19 hours ago) and read 28461 times:

 707s had sticks in inches and you ALWAYS fuelled on the sticks and converted to Lbs. Fun
 Planefreakaa From United States of America, joined Jan 2005, 113 posts, RR: 0 Reply 13, posted Thu Jan 26 2006 07:03:45 UTC (10 years 4 months 1 day 13 hours ago) and read 28409 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...

 Jetlagged From United Kingdom, joined Jan 2005, 2620 posts, RR: 25 Reply 14, posted Thu Jan 26 2006 07:13:23 UTC (10 years 4 months 1 day 12 hours ago) and read 28406 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.
 Top Of Page Change Forum... Civil Aviation Travel, Polls & Prefs Tech/Ops Aviation Hobby Aviation Photography Photography Feedback Trip Reports Military Av & Space Non-Aviation Site Related LIVE Chat Forum Index

Reply To This Topic How Do Fuel Gauges Work In An Aircraft?
• Tech/Ops related posts only!
• Not Tech/Ops related? Use the other forums
• No adverts of any kind. This includes web pages.
• No hostile language or criticizing of others.
• Do not post copyright protected material.
• Use relevant and describing topics.
• DETAILED RULES

 Similar topics: More similar topics...
New Plane:new Engine: How Do Engine Makers Get In? posted Mon Dec 26 2011 10:36:54 by VC10er
How Do Modern Reversers Work? posted Sun Feb 14 2010 06:39:58 by peteschiller
How Do Jet Engines Work?: For School Project posted Fri Mar 9 2007 21:31:10 by YYZflyer
How Do You Change A Wheel On An Airliner? posted Sat May 7 2005 15:47:53 by Ajet
How Do Pilots Rotate Shifts In The Cockpit? posted Sun Feb 27 2005 23:50:56 by JumboJim747
How Do Flight Trackers Work? posted Thu Apr 1 2004 19:25:58 by Bruce
How Do Jetliner Brakes Work? posted Sun Jun 22 2003 21:55:29 by William
Pilots: How Do The Airplanes Perform In Comparison posted Wed Feb 20 2002 17:06:40 by Seven37s
Last Two Digits In An Aircraft's Name posted Tue Apr 17 2001 01:45:38 by US A333 PIT
How Do Hush Kitts Work posted Thu Feb 1 2001 15:11:21 by Chopper