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I have some questions on fuel that I haven't been able to answer via google, manuels, or logic. If anyone even wants to speculate on the answers, I would love to hear it!

My plane has active perf. So, it predicts fuel on destination and reads FOB (fuel on board).

I understand the fuel reading on the MFD in lbs lists the quantity of fuel based on the capacitive sensors. The sensors measure volume. There are no sensors on the plane that measure the weight of the fuel.

As we all know if it is very hot out (e.g. 35 degrees C) you can end up with as little as 5025 lbs of fuel when FULL. At colder temperatures I've filled to as much as 5250.

As far as I know the volume of the fuel tanks doesn't change with temperature. So, there must be some temperature factor that explains the variation in the amount of fuel that can be taken on board. And, since the fuel isn't weighed, this must be a factor applied in software.

I also understand that FOB (fuel on board) is calculated by measuring the amount of fuel that leaves the tank using the fuel totalizer. I've been watching the difference between these to and I've noticed that FOB on a long flight is typically 200 lbs lower than the fuel number presented on the MFD as measured by the capacitive sensors. (In straight and level flight.)

Question 1: Is the MFD fuel quantity corrected for temperature? If so, what temperature is used to correct converting volume to lbs? Also, how often is this value read and applied to the display? E.g. does the fuel panel use a fuel temp probe to get the temperature while filling and applies a density correction? After the plane is filled is the temperature of the fuel assumed not to change?

I'm having trouble looking up good data that show how fuel density varies in the -20 to +35 degrees C range. If anyone has a link, please provide.

Question 2: Does anyone know why the FOB number deviates significantly from the MFD fuel qty over the duration of the flight? Has anyone seen the FOB number greater than the MFD number?

Question 3: Should the MFD Fuel number be considered correct in regard that the energy on board (e.g. less dense fuel = less energy) varies between the energy of 5050 lbs of fuel and 5250 lbs of fuel?

Question 4: Code 7700 says that jet fuel density varies based on the region it is purchased. Highest in california, lowest in asia. Does anyone know if this is true? It seems that Jet A-1 is the international standard and it specifies both a specific weight and energy density for the fuel. Does anyone have any data as to how the density of fuel varies by region (or supplier) and if that correlates to energy density? Is higher density fuel always more energy dense?

Question 5: Does the FADEC control the engine N1 based on a target throttle setting and then adjust the fuel flow to meet that N1 number? Or, does the FADEC meter out a fixed amount of fuel based on the throttle setting? Said another way, will fuel with a lower energy density flow faster into the engine at max cruise than higher energy density fuel?

Question 6: I've been flying in Dubai, India, and Indonesia. A part of the world where you have to check that you are getting what you paid for. I've noticed that the gallons of fuel delivered to my plane is consistently less than my fuel receipt amount. See image. Can anyone explain this?

[Username Protected]

I'll address what probably will help you answer all of your questions. FOB is sync'd at the beginning of the flight and is a measurement at that point in time of what's on board. The system (FOB or FOD) is not looking for live readings anymore since you sync'd at the beginning your fuel load. I assume its just then looking at fuel used and subtracting from that and whatever other performance calculation for the rest of your flight it does.

That is why you may get an hour into the flight and notice the fuel readings are different than what you originally sync'd because of temp or other items

[Username Protected]

Ok, FOB has nothing to do with fuel totalizer and is entirely calculated after sync? There is nothing in the documentation on FOB other than sync.

[Username Protected]

Ok, FOB has nothing to do with fuel totalizer and is entirely calculated after sync? There is nothing in the documentation on FOB other than sync.

I also tried to look through Garmin's documentation and it is especially vague on the calculation. I did find in another G1000 manual the following:

FUEL CALCULATIONSNOTE: Fuel calculations do not use the aircraft fuel quantity indicatorsand are calculated from the last time the fuel was reset.Fuel used (GAL Used) and range (in nautical miles) are calculated basedon the displayed fuel remaining (GAL REM) and the fuel flow totalizer. Thecalculated range also takes into account the aircraft’s heading and the winddirection and speed.

Naturally that is talking about gallons, but everything I have understood about the system is consistent with this. On your next flight, try not sync'ing the FOB at first and go see what it tells you for FOD once you are at cruise. I believe you will find it uses the last reported fuel sync and even though your planning gave you enough fuel and you can see the indicators, the FOD will likely show you don't have enough.

[Username Protected]

(Username Protected) - partial answers (as best I can) embedded within your post -

I have some questions on fuel that I haven't been able to answer via google, manuels, or logic. If anyone even wants to speculate on the answers, I would love to hear it!

My plane has active perf. So, it predicts fuel on destination and reads FOB (fuel on board).

I understand the fuel reading on the MFD in lbs lists the quantity of fuel based on the capacitive sensors. The sensors measure volume. There are no sensors on the plane that measure the weight of the fuel.
I would say you are correct - nothing physically is measuring weight; only calculated by the Electronic Fuel Control Unit (EFCU) using [I believe] the electrical resistance of the fuel.

As we all know if it is very hot out (e.g. 35 degrees C) you can end up with as little as 5025 lbs of fuel when FULL. At colder temperatures I've filled to as much as 5250.

As far as I know the volume of the fuel tanks doesn't change with temperature. So, there must be some temperature factor that explains the variation in the amount of fuel that can be taken on board. And, since the fuel isn't weighed, this must be a factor applied in software
Fuel density (which temperature is a part of). Part 121 operators measure fuel density -- at my carrier, it is measured twice a day at all stations.

I also understand that FOB (fuel on board) is calculated by measuring the amount of fuel that leaves the tank using the fuel totalizer. I've been watching the difference between these to and I've noticed that FOB on a long flight is typically 200 lbs lower than the fuel number presented on the MFD as measured by the capacitive sensors. (In straight and level flight.
Slight difference - I understand that the FOB / FOD is a calculated based on the "sync" as (Username Protected) says, but then is calculated based on fuel flow - no longer using the capacitive sensors (they only supply the quantity on the synoptics and "fuel gauge").

Question 1: Is the MFD fuel quantity corrected for temperature? If so, what temperature is used to correct converting volume to lbs? Also, how often is this value read and applied to the display? E.g. does the fuel panel use a fuel temp probe to get the temperature while filling and applies a density correction? After the plane is filled is the temperature of the fuel assumed not to change?
The POH says fuel temperature is supplied to the EFCU (which also processes the fuel quantity indication). Kind of repeating here, but I understand the density is a bigger factor than temperature; of course temperature is a component of density.

I'm having trouble looking up good data that show how fuel density varies in the -20 to +35 degrees C range. If anyone has a link, please provide.

Question 2: Does anyone know why the FOB number deviates significantly from the MFD fuel qty over the duration of the flight? Has anyone seen the FOB number greater than the MFD number?
Sorry, I usually have FOD up and don't often look at FOB (probably only when I fly with you LOL).

Question 3: Should the MFD Fuel number be considered correct in regard that the energy on board (e.g. less dense fuel = less energy) varies between the energy of 5050 lbs of fuel and 5250 lbs of fue
That seems to be a fair statement.

Question 4: Code 7700 says that jet fuel density varies based on the region it is purchased. Highest in california, lowest in asia. Does anyone know if this is true? It seems that Jet A-1 is the international standard and it specifies both a specific weight and energy density for the fuel. Does anyone have any data as to how the density of fuel varies by region (or supplier) and if that correlates to energy density? Is higher density fuel always more energy dense?
Density does vary - I will verify that based on experience; part of my duty as an FO was "fuel verification" - the fuel service form our carrier uses has a "planned density" - 6.75 lb/gal and a "station density" as measured at the station. Using that difference they derive an allowable tolerance which also considers the capacity of the tanks. It was not unusual in Europe to either be out of tolerance because of low density, or to have the fueler arrive to the cockpit to report the tanks were full to the brim and he couldn't get the requested gallons on board.

I don't have examples of density from different suppliers/regions, but here is a quote from our FOM:

"The energy contained in jet fuel is directly related to the weight of the fuel. The number of gallons of fuel necessary to achieve a specific weight may vary, based on refining process and temperature. Low-density fuel restricts a fuel tank's capability to be loaded to the maximum weight capacity. Variations in fuel density impact the fuel calculations necessary to ensure sufficient fuel is boarded. Errors in fuel density may cause discrepancies in calculated and actual fuel loaded in terms of gallons and/or pounds."

I have had flights departing Europe for the USA that for legality we had to dispatch to "refuel" airport like KBOS then "redispatch" inflight once we knew we had sufficient fuel to proceed to the actual destination. (Of course we never told that to the passengers...and I personally never had to stop short for fuel.)
Question 5: Does the FADEC control the engine N1 based on a target throttle setting and then adjust the fuel flow to meet that N1 number? Or, does the FADEC meter out a fixed amount of fuel based on the throttle setting? Said another way, will fuel with a lower energy density flow faster into the engine at max cruise than higher energy density
I can't answer that--be a good question for the engineers (good luck asking contact center...but maybe? or Pratt?) My hunch is the former, based on observation at different altitudes.
Question 6: I've been flying in Dubai, India, and Indonesia. A part of the world where you have to check that you are getting what you paid for. I've noticed that the gallons of fuel delivered to my plane is consistently less than my fuel receipt amount. See image. Can anyone explain this?
I am a little confused by your table. Fuel Used / Gallons based on logbook = X yields wildly different values. I'm deducing you were not servicing to full so kind of hard to tell. What quantity are you requesting? In pounds or US gallons (make sure to not get Imperial gallons mixed in there!). Then you can ask them for the fuel density - not sure if they will know? That question has a lot of variables that are possible.... not to mention faulty meters, or 'padding'....

take care,
(Username Protected)

https://www.(Username Protected)aviation.com

[Username Protected]

Hey all. I got a response from contact center that clarifies quite a bit:


1. Can you tell me how the fuel quantity displayed on the MFD in green is calculated? E.g. does the fuel panel measure the capacitance of the fuel and then apply a temperature correction (with an assumed density) to display the quantity of fuel on board? If so, is there any documentation of the temperature calculation? Also, if there is a temperature calculation, does this also get applied during flight? Or, during flight is the totalizer used to compute the remaining fuel?

Regarding your first question, there are 8 fuel quantity probes in the electrical fuel quantity indicating sub-subsystem. The probes provide capacitance values to the Electronic Fuel Conditioning Unit (EFCU) for fuel quantity indication. The tank contents are computed using a measurement algorithm based on the tank geometry and the immersed height of each probe. Each probe reading is compensated by correcting, for fuel temperature, the values of permittivity and density.

Please, refer to AMM SDS - 28-41-00 - ELECTRICAL FUEL-QUANTITY INDICATING for more information.


2. Can you tell me how the FOB is calculated for active perf. I see that over the course of a flight the FOB is lower than the green MFD fuel amount by about 200 lbs. Is the FOB calculated based on active perf, or is it calculated using a fuel totalizer that measures flow?
The Fuel On Board (FOB) should be initialized (FOB SYNC) on ground (ramp). The Weight and Balance computes FOB based on initialized FOB and fuel flow over time, it is not recommended to synchronize FOB after the airplane is airborne, which would avoid the function to provide FOB computations independent from the fuel quantity sensors.

Please, refer to POH 4-40, page 4 for more details

POH 4-40 is the following, which has been discussed.

here is the snippet from the POH



Does anyone have access to the AMM document that is referenced?

(Username Protected) - regarding the fuel numbers. This got me also. The fuel filled on the plane is the fuel from the start of the current flight minus the end of the PREVIOUS flight. Yes, that does mean you can't repeat my calculation for the first flight. End fuel for the previous flight is not available.

[Username Protected]

<Does anyone have access to the AMM document that is referenced?>

(Username Protected), I emailed it to you.

[Username Protected]

AMM SDS-28-41-00 is an overview of the capacitive fuel measurement system. Some important points:

- Accuracy of fuel measurement is +/- 1.0% of the maximum capacity plus +/- 2.0% of the reading at normal attitudes.
- Normal attitudes on the ground -2.2 to + 0.5 degrees pitch and +/- 1.0 degree roll.
- Normal attitudes in the air are +1.0 to +3.0 degrees pitch and +/- 1.0 degree roll.
- There is no explanation of temperature correction for weight.

I found a chart for fuel density table for Jet A.

Using this I found the equation for density

density (lbs/gal) = -6.38E-03 * Temp + 6.94

I expect to get 5150 lbs of fuel at 20 degrees C. Using that as a reference I get the following table for lbs of fuel vs. temperature.

Temp Deg C	Density	Expected Fuel
-20	7.06	5343
-15	7.03	5319
-10	7.00	5295
-5	6.97	5271
0	6.94	5246
5	6.90	5222
10	6.87	5198
15	6.84	5174
20	6.81	5150	reference
25	6.78	5126
30	6.74	5102
35	6.71	5078
40	6.68	5054

This correlates well with the number of lbs of fuel I expect to get when the tank is full. This tells me we would need to chill the fuel truck below -20oC to get the mythical 5350 lbs of fuel the plane is rated to.

So, at 40oC one would expect 4% lower fuel density than at -5oC.

From Embraer, we know that FOB is calculated based on totalizer flow.

If FOB is not equal to the fuel measurement the question is why? Is this an error in the totalizer? Perhaps the totalizer is not correcting for temperature and just using a fixed density such as 6.75 lbs/gal?

I understand that density of Jet A directly relates to energy. From this table density at a given temperature seems to vary ~8% depending on the supply.

So, I would conclude that there is an 8% difference in energy loaded on the plane based on the fuel supplier. There is a 4% additional difference based on temperature.

If you have a bad day - headwinds significantly worse than forecast, fuel the plane on a hot day, and get a low density of fuel you could face a 20% fuel deficit from your plan.

I still can't explain why the volume of fuel delivered by the fuel truck is consistently > 6% more than measured on the fuel gauge.

[Username Protected]

Some more information that is useful.

Thanks to (Username Protected) for sending me a challenger presentation that states, "Jet A or A1 fuel density is controlled to 6.75 lbs/gallon in the USA. International specs for Jet A or A1 fuel are 6.46 - 7.00 lbs/gallon."


From Chevron's website, "Generally, less dense jet fuels have a higher gravimetric energy content, and more dense jet fuels have a higher volumetric energy content."


By my calcs that says you can get +/- 4.3% range out of the fuel depending on where you take on the fuel outside of the USA.


On a 4000 lb trip that's +/- 172 lbs depending on fuel density (excluding temperature factors).


Interesting that Bharat Petroleum, a major supplier in India, quotes 775 to 840 (kg/m^3) range for density. That corresponds to the 6.46 to 7.00 lbs/gallon.

Jet A-1 fuel in the United Arab Emirates adheres to ASTM D-1655 to that guarantees 6.75 lbs/gallon.

So, looks like a good rule of thumb is that there is a 12% range difference between (a) 7.00 lbs/gal fuel (outside the USA) at -20 degrees C vs. (b) 6.46 lbs/gal fuel at 40 degrees C.

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