Commercial Weight & Balance

pilotjww

New Member
When does a commercial pilot apply the temperature of the fuel in the weight & balance?
I’m thinking it’s when you pump the fuel as a volume, and need to convert it into a weight, because a pound of fuel weighs a pound, at no matter what temperature.
But a unit of fuel pumped some time prior may no longer be a gallon now, so you need to know the temperature of the fuel at the time it was pumped, right? Applying the temperature now to convert a volume of fuel pumped 10 hours or 10 days ago will not necessarily result in the correct weight now, right?
 

E_Dawg

Moderator
On a hot day a gallon of fuel will weigh less than on a cold day because of density changes with temperature.

If you fill it when it's hot and it cools off, you no longer have full tanks but you still have the same weight of full in the tanks.

If you fill it when it's cold and it warms up, you have the same amount (excess will exit the tank from vents) but it will weigh less.

Beyond this I don't know..... if we are talking small aircraft, isn't the difference negligable anyway?
 

sbav8r

New Member
[ QUOTE ]
On a hot day a gallon of fuel will weigh less than on a cold day because of density changes with temperature.



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The weight of fuel will not change with temerature only the volume of the fuel will change.

Only GA aircraft ever talk about fuel in volume, as you learn to fly larger aircraft you will always be using lbs of fuel. Therefore, they do not consider the temperature of the fuel because weight remains constant as volume and temperature varies.

In case you are confused: As temperature rises objets expand and as temperature decreases objects shrink.

When you take on fuel they give it to you in measurements of weight not volume.

Does that make sense to you now? Is that what you were asking?
 

E_Dawg

Moderator
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The weight of fuel will not change with temerature only the volume of the fuel will change.

[/ QUOTE ]

Yes, but for a given volume (like a gallon), the weight will change with temperature. That's why when you fill your car up on a cold day you actually get more gas for the money.

Also, on hot days you can see the fuel dripping from the tanks. You still have the same volume of fuel (full tanks), but it will weigh less.
 

sbav8r

New Member
The difference in volume with temperature does not mean your getting more gas for your money. Your engines need a given amount of fuel molecules to acheive a given performance. If the fuel is cold than it requires less fuel in volume than hot fuel but the weight will remain the same.

Your engines could care less how hot or cold the fuel is, all it wants is a specific amount of gas molecules to achieve combustion. If it is hot than it will require a larger volume of fuel in each cylinder to achieve the same amount of power as cold fuel. Both cases use the same amount of molecules.

Think of this like air, when your in a high altitude enviroment your body needs larger breathes to get the same amount of oxygen as when at lower more dense altitudes. Either way your using the same amount of air molecules even though the volume of air may vary.

Weight is independent of temperature. No matter what the volume of fuel for a given weight of fuel the fuel burned will remain constant for the same amount of power.

[ QUOTE ]
Also, on hot days you can see the fuel dripping from the tanks. You still have the same volume of fuel (full tanks), but it will weigh less.

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The case of fuel spilling onto the ramp is just a waste of fuel. It is not the same amount of fuel for purposes of fuel burn on a trip. Once again the weight of fuel for a given power will remain constant the volume will vary. In this case at the same engine power you would not get as far as if your fuel had not spilled onto the ramp.
 

Tired

New Member
Huh? If you top off from the truck in Phoenix and then read off the weight from the fuel gagues, then fly to Gander and top off again there you'll see that you got a better load in the cold weather. Since you paid by volume in both Phoenix and Gander you would get more fuel for your money when taking on cold fuel.
 

sbav8r

New Member
If your purchasing fuel by volume than yes, you are getting more fuel for your money when it is cold. Aircraft use measurments of weight when loading and do not need to account for this.

Regardless of temperature your fuel burn in weight will remain the same. I was addressing the original poster who asked about commercial aviation. I assume he means larger passenger service aircraft that always purchase and calculate fuel in weight not volume.

In this instance no matter what the temp. you will not get more fuel for your money.
 

Tired

New Member
I wasn't aware of anyone purchasing fuel by weight. I guess to answer the first posters question, you don't have to worry about the conversion, in the jets I'm familar with the gauge reads in pounds.
 

Alchemy

Well-Known Member
The aircraft gauges may work in pounds, but the fuel trucks certainly do not keep their fuel at a regulated temperature, and the gauges on the fuel trunk pumps measure in gallons, not pounds. I have no idea if pilots account for temperature when telling us (in gallons) how much fuel they'd like us to pump into their aircraft.

Most of the time, the pilots will give us their fuel order in gallons, but sometimes they give it to us in pounds. In those cases, we just divide by 6.7 with no adjustment for temperature. It is conceivable that a pilot who orders XXXX gallons Jet-A in minneapolis during February will recieve perceptibly more fuel by weight than someone who makes the same order in Phoenix during August.

The original poster may have a point, but someone would have to dig up the equations to determine if the differences between surface high and low temperatures has any significant effect on the volume of fuel. I'm inclined to say that the volumetric difference between fuel pumped on a zero degree day a 100 degree is probably less than .5 %, otherwise we'd probably be correcting for this phenomenon on a more regular basis.
 

sbav8r

New Member
I would have to agree with you that the difference would be so minimal it wouldn't matter. I've never worked the fuel trucks so I just assumed they worked in lbs, since that is the way it is planned and purchased.
 

B767Driver

New Member
I'm a new registered user and was browsing through some old posts; stumbled upon this one. There's some serious misinformation running through this thread regarding fuel temp, weight, density and volume.

I fly an MD88 for a major airline...deal everyday with fuel weight, temp and volume issues.

Here's the deal. The weight of the fuel is always dependent upon the outside air temp (and this determines the density of the fuel). However, the volume (capacity) of the aircraft's fuel tank never changes (ever).

For example...let's say your fuel tank holds 100 gallons. It will always be capable of holding 100 gallons, everyday of the year, regardless of the outside air temp. The weight of the fuel will not always be 670 pounds, however (JetA = 6.7 lbs per gallon on a standard day). Some days the fuel may weigh 650 lbs, some days the fuel may weigh 690 lbs.

A jet engine fuel control unit meters fuel per volume, not weight. (Most turbine aircraft determine fuel flow based upon pounds per hour...however this is derived information based upon a mechanical measurement.)

So, to respond to the original post...temperature does affect the weight and density of fuel...but the requisite volume of the fluid is not changed.
 

Alchemy

Well-Known Member
Interesting that the engine's fuel control units measures fuel per volume. I figured that mass would be a more accurate indication of the "amount" of fuel required. Like you said, a 100 gallon fuel tank will always hold 100 gallons, but some days the weight of that volume may be 670 pounds, some days 690. Is it innacurate to say that you have "more fuel" on the day where 100 gallons weighs 690 pounds?
 

EatSleepFly

Well-Known Member
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Is it innacurate to say that you have "more fuel" on the day where 100 gallons weighs 690 pounds?


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Yes. Gallons are the quantity. Pounds is just the weight of the fuel.
 

JHines

New Member
Don't most turbine engine controls (FADEC/MEC's/HMU's) operate in a closed loop mode for all normal operations (N1 scheduling, T3/T4 limiting, etc?) Since BTU's vary with mass and not volume, I would think you would see a difference with fuel density. That is, on a cold day, don't you see more range from a fixed volume of tankage?
 

B767Driver

New Member
The point should be made that you have the same amount (volume/gallons) of fuel in the tank regardless of the outside air temp (i.e. the tank is full). The only difference the temp makes is the weight of the fuel. Temp can impact range, as it changes the density altitude of the flight...upon which all performance is based. However, the range will not be affected by the temp of the fuel in the tank, i.e. the same volume exists (full tanks) regardless of outside air temp or fuel temp.

I remember when I used to fly a King Air, the W & B section of the aircraft operating manual had several different columns for the weight of the fuel depending on temp and density. For any given Volume (gallons), the weight would vary due to density variations. However, 100 gallons of gas was always 100 gallons irrespective of its weight/density.

Hope this helps.
 

pilotjww

New Member
I understand MD88's point about a hundred gallons is a hundred gallons, but my original questions was more about partial fueling. MD88, do you ever really completely fill the tanks? I suspect big jets (like 747) do not - and there are too many variables like people and packages to ever have exactly the same amount of fuel.

I actually got started thinking about this when I realized that Mooney did a creative marketing thing by using 5.82 lbs/gal for a gallon of 100LL, instead of the FAA-inspired 6 lbs/gal. My Mooney TLS needs all the lightening it can get, since it is what you might call "weight-challenged." A full load (89 gallons) of fuel weighs 16 pounds less with the lower 5.82 number. This doesn't seem like a lot, but it may be what marketing would.

Then I consulted figure 6-10 "Density Variation of Aviation Fuel" on page 6-15 of the Aircraft Weight and Balance Handbook, FAA-H-8083-1. You can find it at the FAA site:

http://av-info.faa.gov/srchAdvancedResults.asp?SearchFor=*&cmdSubmit=Search&SearchUsing=filename&ResultsPerPage=10&SortResults=date&SortDirection=%5Bd%5D&DateRange=-100y&DateStart=mm%2Fdd%2Fyyyy&DateEnd=mm%2Fdd%2Fyyyy&Category=traininghandbook

At 15 degrees C, 100/130 weighs about 5.8 lbs/gal, the number Mooney used in the POH. The FAA number of 6 lbs/gal has to be at about minus 7 degrees C on the chart. This is only about 3% difference, so maybe not a big deal. And like I suspected, and several posters have agreed, the only time it matters is when you pump the fuel. A gallon pumped today will shrink or expand tomorrow if the temperature changes enough, and you may notice a difference if you are staking large tanks, or it is running out the vent on a hot day.

If the line crew acurately convert gallons pumped to pounds of fuel, or better yet, if you put the aircraft on scales, the weight will be the same, next week, next year...etc (assuming no evaporation) but the volume willl change if the temp changes significantly.

So this leads back to my question "Applying the temperature now to convert a volume of fuel pumped 10 hours or 10 days ago will not necessarily result in the correct weight now, right?" It doesn't look like anyone has an experience where weight was mistaken because of a temperature change converting gallons to pounds. And it may be the 3% or so difference has been too small to be an issue.

It was a good question, though, that sparked a few responses. Thanks to all with an interest.
 

B767Driver

New Member
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Hmmm....I thought it depended on the temperature of the fuel itself, not the outside air...

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You are correct. (The assumption was being made that the fuel pumped = the OAT.)
 

onthebeach

New Member
Here\'s the answer to the question you are really asking:

>So this leads back to my question "Applying the temperature now to convert a volume of fuel pumped 10 hours or 10 days ago will not necessarily result in the correct weight now, right?" It doesn't look like anyone has an experience where weight was mistaken because of a temperature change converting gallons to pounds. And it may be the 3% or so difference has been too small to be an issue.<

Here is the explanation for you (and before we start, understand that I'm not a mechanic or an engineer, so don't nitpick the theory, about which I could care less; I know how the system works from a pilot perspective, and that's what I'm going to tell you now):

Turbine aircraft (at least all the ones which I have flown) employ a fuel quantity measuring system based on measurement of capacitance; that is, electron flow between two charged plates. The beauty of the capacitance system is this: as the density of the fuel increases (i.e., as its temperature goes down) or decreases (i.e., as its temperature goes up), the capacitance of the liquid changes, thus yielding a measure of the fuel's density. The system also uses a known, or reference electrical signal and a sending unit to very accurately measure the level of the fuel in the tank, i.e., the fuel volume. The whole gizmo also incorporates an analog computer unit that uses inputs of 1) volume and 2) density, to arrive at fuel weight, which is presented on the fuel gauges. If memory serves me, the spec for capacitance measuring systems is plus or minus 3%, which is a lot more accurate than the float type gauges in most GA aircraft.

This is why you can fuel up a turbine airplane partially when the OAT is 0 C...let's say, 1/2 full, and the gauges will tell you you have X pounds. If the aircraft sits there until the temperature rises to 35 C, and long enough for the fuel to reach that same 35 C OAT, the gauges will still read X pounds.

This is because (to review), the capacitors of the system measure the decreased density (pounds per gallon) and the reference (fuel level/volume) unit of the system measures the increase in volume due to heat expansion. decreased density X increased volume = same amount of fuel; that is, same weight of fuel. Remember that the amount of matter is measured by the quantity we call "mass," and that equal masses of things at the same location in the earth's gravitational field will have equal weights.

All fuel delivery gizmos, be they updraft carburetors in a Cessna 172, fuel injectors in a Navajo, or fuel control units in a Rolls Royce RB211 high-bypass fanjet engine, deliver fuel by mass, not by volume, in order to achieve a given combustion ration, i.e., ratio of air to fuel by mass, not volume.

The mechanics of how they do this are frankly beyond me, and just as frankly, I know enough about it to get my job done. It's enough for me to know what I have told you, and unless you want to design the things (i.e., be an engineer) or fix them (i.e., be a mechanic), that's enough for you to know as well.
 
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