captjetblast
Topic Author
Posts: 288
Joined: Thu Aug 30, 2001 5:59 am

### Remaining Fuel Quantity

Is the remaining fuel quantity measured or calculated (initial quantity - drain)?

Guest

### RE: Spoilers On Takeoff

Hola Che...
xxx
Generally not, unless we would have a reason to want to know "exactly" what amount of fuel remains after a flight. Fuel gages are extremely accurate... But there are checks performed for fueling if required, to verify the accuracy of a fuel load if necessary... We can also depart with a fuel tank gage inoperative, but in that case, we would measure the fuel in the tank with dripsticks, under the wings or under the center wing tank in the belly...
xxx
Happy contrails
(s) Skipper

captjetblast
Topic Author
Posts: 288
Joined: Thu Aug 30, 2001 5:59 am

### RE: Remaining Fuel Quantity

What I want to know is the way the remaining fuel is calculated. How does the fuel gauge work? Is it constantly measuring the quantity or, knowing the starting quantity, calculating the fuel flow and time of flight?

And for dripsticks, remember the Gimli Glider accident. The fuel gauge was inoperative, they mismatched the volume units (metric-english) and loaded half the fuel they needed for the B767 flight.

The result, many of you know, was a B767 turned to a glider, with no fuel for its engines.

avioniker
Posts: 1100
Joined: Tue Jan 01, 2002 5:38 am

### RE: Remaining Fuel Quantity

The fuel is measured in real time by the change in dielectric value in the tank.

Simply put, there are a number of capacitors (probes) connected in series in each tank. The total capacitive value is represented as a quantity of fuel.

Fuel and air have different dielectric constants so if you know the capacitive value of an empty tank vs. the value of a full tank you can devise a system that'll read proportionately with the quantity of fuel in a tank.

The system reads in pounds because the volume will change with temperature but the weight and BTU value remains constant. The weight is important for balance and CG computations and the BTU's for propulsion.
One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533

Guest

### RE: Remaining Fuel Quantity

There are actually three independent ways to measure the fuel remaining and fuel consumed during flight...
1. Fuel gauge indications. As mentioned above, the fuel gauges are very accurate and reliable.
2. FMS Fuel Calculations. The aircraft's FMSes derive fuel consumed and fuel remaining by taking input from the aircraft fuel flow sensors. The crew inputs the initial fuel on board value while initializing the FMSes.
3. The "old fashioned" elapsed time method. Simply put, fuel burn is very predictable based on elapsed time. The crew simply multiplies the elapsed flight time by the 1st hour fuel burn, 2nd hour fuel burn, 3rd hour fuel burn etc.

Slcpilot
Posts: 594
Joined: Thu Aug 07, 2003 3:32 am

### RE: Remaining Fuel Quantity

For B747skipper..

"Fuel gauges are extremely accurate... "

At what level of aircraft do you start to put more weight on the accuracy of the fuel gauges? This is "Airliners.net" and not "GeneralAviation.net" but I have always taught my students to NOT trust fuel indications and lend more weight to "We started with X, and we burn Y per hour" after dipping the tanks.

Come to think of it, after having pumped fuel for 6 years, I don't know if I have ever seen a crewmember check the tank levels manually in a turbine aircraft.

SLCPilot
I don't like to be fueled by anger, I don't like to be fooled by lust...

AAR90
Posts: 3140
Joined: Fri Jan 21, 2000 11:51 am

### RE: Remaining Fuel Quantity

At what level of aircraft do you start to put more weight on the accuracy of the fuel gauges?

An airliner's fuel quantity guages are very accurate in all but the most extreme FULL or EMPTY ranges. i.e. the last few hundred pounds of an 8,000+ pound capacity tank.

This is "Airliners.net" and not "GeneralAviation.net" but I have always taught my students to NOT trust fuel indications and lend more weight to "We started with X, and we burn Y per hour" after dipping the tanks.

Most light civil aircraft fuel quantity systems operate very similar to an automobile system... a simple float. Only accurate when initially installed and with the plane perfectly level and not moving. Airliners are much more complex and much more accurate.
*NO CARRIER* -- A Naval Aviator's worst nightmare!

Guest

### RE: Remaining Fuel Quantity

Let me put it this way... for some of you...
xxx
Many airplanes (the case of the 747) have gages with fuel burn (associated with the fuel flow system indicators). On the 747, we can read at any time the amount of fuel burned so far, since engine start... provided you zeroed the fuel counters before start...
xxx
If we know "exactly" how much fuel we have in the tanks (say, measured with the dripsticks) we know at any time the fuel remaining in flight... our gages read in kilos, in the USA, they are generally calibrated in pounds...
xxx
In a worst case scenario you "could fly without any fuel quantity gages"...
But... not in my MEL, fortunately...
xxx
Happy contrails
(s) Skipper

[Edited 2003-10-31 19:03:08]

Guest

### RE: Remaining Fuel Quantity

Further, to tell our general aviation friends how extremely accurate the gages are in an airliner...
xxx
If you have a "float" quantity indicator (like in a car, or a Cessna)... the gage, if accurate, measures merely the VOLUME of the fuel in the tank... That is not good enough for airlines. With the capacitance probes used in airliners, you actually read the WEIGHT of the fuel, regardless of its density, and temperature. You all know that volume changes with temperature, because of density...
xxx
Jet engines are only interessed in burning kilos or pounds of fuel, not liters or gallons...
xxx
In addition, there are generally two probes (or more) per tank, so as to take an average and accurate reading regardless of attitude of the airplane.
xxx
Happy contrails
(s) Skipper

CX Flyboy
Posts: 6079
Joined: Sun Dec 26, 1999 6:10 pm

### RE: Remaining Fuel Quantity

We get both Totalizer and Calculated figures and will compare them both, but normally any critical fuel planning will go on the lower of the two figures. i.e. when to divert when we get to min fuel.

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