Various fuel freezing points in tanks

[Will]

I can see the gears turning in Hardy's mind... it imagine it would be pretty easy to calculate the new freezing point if you upload 200,000 lbs of Jet A on top of 50,000 lbs of Jet A-1...

Better yet, as long as we know the freezing point of what's currently in the tanks, even if it's a mixture of different fuels, we can calculate the new freezing point any time the fuel quantity is updated. Store that in the situ, and display it on Instructor > Situation > Service.

Operators who always fly with -37C in the FMC won't see any differences, meanwhile, operators that update the FMC to actual fuel temp +3 degrees will have a tad more realism to reckon with.



Edit HH: Thread split from http://aerowinx.com/board/index.php?topic=5714.0

[Jeroen Hoppenbrouwers]

I'm not so sure. Fuel does not mix completely, due to the tank and pipe system. Some parts will be 95% A, others 95% A1. One part may be sludge way before the other part gets there. What part is near your valve, your pump?


Hoppie

[Will]

That's a good point. The fuel doesn't circulate though the tanks, does it? So only the first tanks to fill would have a mixture of fuels; the final tanks to fill would only have whatever was added last. Right?

[Britjet]

The fuel goes equally into the tanks via the manifold as selected on the wing panel.. Normally of course the only tanks with fuel on arrival would be the wing tanks.
The outboard (1&4) have much smaller capacity than the inboards, so they are the ones that get cold.
Now, a typical arrival with, say, 12 tonnes of -40C fuel remaining would have 3 tonnes in each outboard main tank. How much you then put in each tank on refuel will of course depend on the fuel load required, but if you load up to "full wings" with -47C then you are putting another 10 tonnes or so in each outboard tank, so that fuel type quickly becomes overriding in terms of freeze point. The fuel will mix in these tanks. After a couple of refuels the original fuel amount will be negligible, so we now assume that the aircraft is fully -47C.

Another point about this is that for a flight of typically 9 hours or less the fuel will never get cold enough, unless you encounter extreme conditions or start off with very cold fuel. It takes quite a while for the fuel to get to -37, if it ever does. I've only had it happen once or twice..

Peter

[John H Watson]

Prior to refuelling for long flights over/near Antarctic regions, we had to pump excess fuel in the wings  from previous flights into the centre tank using the controls on the refuelling panel on the wing. This would ensure most of the old, cold fuel was burnt off in the early (warmer) stages of the flight. Of course, we would leave sufficient fuel in the wings for APU running, prior to refuelling.   

[DougSnow]

Actually the freeze point of fuel is much more difficult to calculate. Boeing and Airbus have both written tools, intended to be used in conjunction with a flight planning system, that do the math for each specific model. Here is what I wrote for a Performance Manual for work a few years ago...

"The Boeing FTPP resides within FPS.  This program is tailored to the individual aircraft model, as its internal calculations are based on how fuel resides, and flows from, the fuel tanks. Factors such as the size and shape of the tanks, as well as how much of the tank is exposed to wing skin temperatures and the resulting TAT, and how the fuel feeds from the tanks to the engines are all used in the calculations in FTPP.  Because of this, the FPS fuel temperature prediction should never be used for an aircraft for which it was not designed. There is no Airbus fuel temperature prediction tool relevant to the Airbus aircraft we operate."

[Mariano]

To shed further light on the subject, from the Boeing ETOPS Guide, Volume III:

"Some  background facts on cold fuel  are necessary to establish a  common starting point for this discussion.  The fuel freezing point  is the temperature at which wax  crystals, (formed in jet fuel as it cools), completely disappear  when warmed.  This should not be confused with the fuel becoming cloudy upon cooling.  This condition is often the result of dissolved water in the fuel freezing to form  very fine suspended ice crystals.   Aircraft fuel systems are designed to handle water ice crystals safely.

It  should be noted that the fuel freezing point is not the temperature limit that dictates flow to the boost pumps.  This limit temperature is referred to as the pour-point temperature. The critical condition of cold fuel  in an airplane fuel tank is the ability of  the fuel to flow toward and into the boost pump inlets.  In  its travel to the  boost pump  inlets, fuel must flow through small flapper valves located on solid  tank ribs used to control fuel  slosh.  These flapper valve are next to the bottom  wing skin.  The ability of the fuel to flow through the tank to the boost pump inlets depends on the pour point of the fuel, defined as the lowest temperature at which the fuel still flows before setting up into a  semi-rigid  state.  Generally, the pour point  temperature  is about 6°C lower than the fuel freezing point.  However, the exact relationship between freezing point and pour point depends on the source of  the crude oil and the refining processes.

For flights where fuel freezing is a concern, measuring the actual freezing point of the fuel being uplifted can be a valuable element of the flight planning process.  In general, actual fuel freezing points tend to be about 4°C below the specification maximum requirement.  Details on measuring freezing point when fuel is being  uplifted are available through Boeing's Customer Support organization.

However, determining the actual freezing point of  the fuel in the tank is not quite as simple when the tank contains fuel with a different  freezing point  than the fuel that is currently being uplifted.   Refueling to  capacity with Jet A-1 fuel when the prior  flight  segment was with Jet A will not proportionately improve freezing point.  For example,  a fuel blend of 25% or more of Jet A with 75% or less Jet A-1 will have a  blend  freezing point not much better than  Jet A.  Boeing  advises that a conservative approach be used with regard  to freezing point of blends of fuels.  Using the highest freezing point of the fuels being blended  is the preferred method.  As an alternative to determining  the freezing  point of the blended  fuel load, Boeing suggests using  the highest freezing point of the fuel used in last  three fuel uplifts.  For example, if Jet A-1 was used in three consecutive refuelings,  then  a -47°C freezing  point may be used.  If fuel freezing point is projected to be critical for the next flight segment, Boeing advises transfer of wing tank fuel to the center wing tank before refueling.  Then the freezing  point of the fuel being uplifted can  be used for that flight segment."

Tried to get it Italicized but couldn't ;-)

Best regards,

Mariano