Fuel Temp

[CarlBB]

Hi,

Based on my observations which may not be relevant to the 744 / aviation in general - the fuel temp seems to fall quicker in flight to near TAT than I would expect. Likewise on descent it seems to warm up quick.

e.g., lets say I place a bottle of water that is at room temp into a freezer - the water will of course cool and then freeze. This will take many hours.

100 tons of fuel though in PSX loaded at say 16c will cool down to below freezing in the first half hour of flight.

I do appreciate the two aren't the same, but would expect it to cool a lot slower?

Thanks

Carl

[Will]

I can see it going either way. There's certainly a lot of air moving about the aircraft, probably enough to cool the fuel in mere moments. (Think of a fuel tank sitting on the ground, and what would happen if you used a fan to blow massive amounts of air on it, at -55 degrees C, at hundreds of miles per hour! It seems that there's ample opportunity for enough heat exchange to achieve quick cooling.)

What would slow things down is insulation. But insulation is heavy, and unnecessary, so I'm sure whatever insulating effects that there are, occur accidentally, from the shape of the tanks inside the plane. The fuel tanks are enclosed inside the aircraft, so there's insulation in the form of the surrounding air in the empty spaces... That air won't be completely still, but it certianly won't be moving at high velocity, either. Of course, in many of the tanks there wouldn't be very much airspace between the fuel and the slipstream.

Should anyone want to slow down the heat exhange further, they could wrap the tanks in a quarter inch of styrofoam.

But insulating the tanks would keep the fuel cold on descent, too, and that might be bad, because of the danger of ice as the aircraft flies down into warmer and wetter weather. Perhaps from that standpoint it's actually advantageous to have the fuel temperature stay as close to the ambient air temperature as possible.

[DougSnow]

From the B744 FCTM:

The rate of cooling of the fuel is approximately 3° C per hour, with a maximum of 12° C per hour possible under the most extreme conditions.

And

It takes from 15 minutes to one hour to stabilize the fuel temperature. In most
cases, the required descent would be 3,000 to 5,000 feet below optimum altitude.
In more severe cases, descent to altitudes of 25,000 feet to 30,000 feet might be
required. An increase of 0.01 Mach results in an increase of 0.5° to 0.7° C total air
temperature.

PFPX does not have the fuel temp prediction algorithms in it.  Boeing writes a type-specific Fuel Temp Prediction Program (FTPP) and airlines that fly polar have it as a part of their flight planning system.

If you want to know the painful details on fuel temps, fuel temp monitoring, etc., http://www.boeing.com/commercial/aeromagazine/aero_16/polar_story.html

[Britjet]

The fuel cools at about 3 degrees per hour towards the TAT.

For the benefit of anyone not familar...

The aircraft experiences dynamic heating due to air friction. This is proportional to the TAT (Total Air Temperature) which is shown on the upper EICAS. It could be -50C SAT, (or Static Air Temperature - found on Progress Page 2) outside but the TAT would be considerably warmer, and will vary according to the speed of the aircraft and the SAT.

The fuel will get colder as the flight continues, but nothing like as fast as you might think.
It will never get colder than about 5 degrees warmer than the TAT. The fuel tanks also actually have an insulation element which is the boundary layer of the airfoil.

The fuel temperature should never be allowed to get within 3 degrees of the fuel freeze point. North America and a few other places have fuel with a freeze point of -40C. Most other countries use -47C fuel.

On a long flight in cold air, such as an extreme North or South route where the temperature can get as low as -70C for quite a while, there can still be a problem. Flight planning at a lower and warmer level is a good option, and many airlines will routinely include this in their planning.

If you really do get caught out, and it can happen, then there is a Boeing checklist procedure that suggests flying at a higher Mach Number, or finding warmer air.

If the fuel does freeze, (it actually becomes waxy, and clogs the fuel lines), then the engines stop. That way you are sure of finding warmer air!

Brit

[Hardy Heinlin]

From the B744 FCTM:

The rate of cooling of the fuel is approximately 3° C per hour, with a maximum of 12° C per hour possible under the most extreme conditions.

In my current model the goal is 12°C per hour. However, however ... I just noticed I put a decimal wrong in my code! It's 120°C per hour. Sorry!

But that's fixed in a second ...

It will never get colder than about 5 degrees warmer than the TAT.

In my current code this depends on the thrust. At CRZ thrust it's +3, at idle it's +1.

I can easily increase this as well.


I think the rate should not be linear, neither at 12°C per hour nor at 3°C per hour. It should be exponential to get a range of 3 to 12.


Regards,

|-|ardy

[Hardy Heinlin]

Fixed in beta 3.

[CarlBB]

Hi

Thanks all for the extra information.

I think the rate should not be linear, neither at 12°C per hour nor at 3°C per hour. It should be exponential to get a range of 3 to 12.


Regards,

|-|ardy

Are you thinking of an effect where it reaches 12 degrees per hour under extreme conditions?

Thanks

Carl

[Hardy Heinlin]

Hi,

the wider the "spread", the faster the rate. If wider than 30, rate is 12; if smaller than 10, rate is 3. In between, rate is ca. 6.


Cheers,

|-|ardy

[United744]

Hi,

Does PSX simulate the mixing of fuel types (thus freezing temperatures) or does it just assume that all the fuel is the selected type?

[Hardy Heinlin]

Hi, all the fuel is the selected type.

[Britjet]

I think 3 degrees per hour is a reasonable figure. Any higher than that in normal cruise would have you searching for diversions on a long flight!

Consider a typical flight from (say) Tokyo to Europe. The flight can take 12 hours plus in the winter, with outside air temps of as low as -70C for three or four hours. Nearly everyone makes it to Europe without descending...

If you go anywhere near 12C per hour if is game over. I think this figure is probably drawn from a very lightweight aircraft flying slowly at high altitude..

Brit

[John H Watson]

I think 3 degrees per hour is a reasonable figure. Any higher than that in normal cruise would have you searching for diversions on a long flight!

So you're saying you monitor the rate (as routine), not just the temperature?

I think Hardy was trying to stress here that the rate in PSX is not linear, but eventually slows down as it nears TAT, never quite reaching it  (asymptotic?). Averaged out over a 10 hour flight, it may equal 3deg/hour.

The sensor is in one of the inboard tanks, where the wing is fattest, so I'd imagine that cooling would be slower than in the outboard tanks (no doubt convective currents would speed up the cooling process, unless quick manoeuvres and turbulence disturbed the currents).

Rgds
JHW

[Britjet]

John, yes, I have on a number of occasions kept a log of the temperature drop. When you are halfway across Siberia in Winter it seems like a good idea! In my experience it is a fairly steady reduction. I have twice had to descend for it. Often you can't speed up because there is traffic ahead. If you can't descend either for the same reason then you have a problem! That is why it is usually best to gave a howgozit so you can plan ahead.

I wouldn't think it was linear, but it is fairly close provided conditions don't change dramatically, and the logs I have kept have been within 2-5 degrees per hour, I would say, but never more than that.

There are too many variables to be empirical, which is why Boeing won't commit themselves to a definite figure. Even a change of 1 degree outside will obviously have an effect. To try and put it more accurately than that would in my opinion be playing with non-existent data.

Brit

[United744]

Maybe have an average rate and then some random fuzzing for each flight, or even each hour?

Mass of the fuel has an effect as well - an empty tank will cool more rapidly than a full one due to thermal inertia (though the effect is probably small).

[Hardy Heinlin]

I wouldn't think it was linear, but it is fairly close provided conditions don't change dramatically, and the logs I have kept have been within 2-5 degrees per hour, I would say, but never more than that.

2 to 5 is a wide range. It's like a range of 5 to 10 hours, i.e. a 5 hour variation.

In Beta 5 it takes ...

ca. 3 hours for 10° to go, that corresponds to 3° per hour.
ca. 6 hours for 30° to go, that corresponds to 5° per hour average.

I can reduce this rate a bit more to something like this:

ca. 5 hours for 10° to go, that corresponds to 2° per hour.
ca. 10 hours for 30° to go, that corresponds to 3° per hour average.


|-|

[Britjet]

That sounds reasonable, Hardy...
Nice one.

Brit

[United744]

Hi,

From the accident report into the B777, the fuel temps worked out like this:

Temps are in degrees Celsius.

@0207: Takeoff. SAT -6, TAT -6, fuel temp -2.
@0234: Cruise (FL348 ). TAT -23, fuel temp -2.
@0500: SAT -56, TAT -32, fuel temp -22.
@0930: SAT -70, TAT -47, fuel temp -32.

I know a 777 is not a 747, but I can't see them being radically different. It takes about 2 1/2 hours for the initial fall in temperatures, then a further 3 1/2 hours for the fuel to cool to the lowest recorded value of -32.

On the 777 fuel temps are taken from the left main (wing) tank. During the first part of the flight the tank held 28 tons of fuel.

[Hardy Heinlin]

Hi,

these numbers agree nicely with Britjet's and Beta 7.

[Jeroen D]

Not sure what the average length of a 744 flight is, but it is probably somewhere between the 10-15 hours or so. Looking at the various ratio's degree cooling per hour mentioned I feel a very relevant factor is at what temperature did you start. If you set of your long haul with a fuel temp of say 25-30oC it seem unlikely, even after 15 hours, you would find yourself in trouble. But you start below 5-10oC you might have a problem.

all depends of course about true air temperatures
What determines true air temperatures at cruising altitudes? I was under the impression that the air temperature on the ground is not that relevant at all?

Jeroen

[Hardy Heinlin]

TAT is Total Air Temp.

"Total" means, it includes SAT (OAT) plus the heat produced by air compression which rises with the Mach number.

Wikipedia has the formula.


|-|ardy