Info needed!

[Homo Ludens]

Could some of you, gentlemen, enlighten me on why the max fuel temperature of 49 degrees C on B777?
I have two possible explanations, but was unable to confirm nighter of them.
Thanks in advance!
Cheers!
H.L.

[shaun.s]

My (admittedly very basic) grasp of these things makes me think that it's for cooling purposes- items like the fuel boost pumps, for example, are cooled by immersion in fuel.

[Homo Ludens]

Thanks, Shaun! Much appreciated!
Cheers!

[rudderrudderrat]

Hi,

Sorry about the table lay out - but I can't figure out how to display it.

From my FCOM, the minimum temperature (i.e. waxing temp) of various fuels is listed and also the Max 54°C

For JET B & JP 4 (wide cut fuel = higher petroleum content), the Max temp is listed as 49 °C.

So I don't think it's for fuel pump cooling purposes on the Airbus - I suspect it's something to do with vapour pressure.


FUEL TEMPERATURE


JET A1/JP 8/ ... JET A .... JP 5 .... RT .... TS-1
MINI - 43°C... -36°C (1) - 42°C - 45°C - 45°C
MAXI 54°C



...... Jet B ...... JP 4
MINI - 46°C - 54°C
MAXI 49 °C


(1) : For JET A only, if TAT reaches - 34°C, monitor (on the ECAM FUEL page) that fuel temperature remains higher than - 36°C.

[STBYRUD]

Vaporisation is the right lead - I believe the fuel flash point is the most critical factor here - pure paraffin oil's flash point is as low as 38°C - while the JET-A flash point should be well above this maybe the 49°C ensure a certain margin also with regard to JET-B or other alternate fuels with lower flash points. (Oh, and as to the specific value of 49°C - I recon thats a typical Boeing conversion of US customary units - 49°C=120°F)

[rudderrudderrat]

Hi,

My old copy of FCOM mentions problems with JP 4 or JET B with various FLs and Fuel temperatures.

"Fuel in the center tank is to be regarded as unusable if the wing fuel temperature exceeds the following values before engine start and if the given FL is exceeded before the center tank fuel has been used :

+30 degs C not above FL 350
+40 degs C not above FL 300
+49 degs C not above FL 250

Reason : At high altitude with high fuel temperature, the pressure delivered by the center tank pumps becomes lower that the pressure delivered by the wing tank pumps."

So I think the blanket 49 degs C limit is much simpler.

Edit: Is it because the vapour pressure at high temperature and low atmospheric pressure can lead to cavitation on the suck side of the fuel pump with a low fuel head height? Not a problem on the wing tank pumps - because we don't plan to run those tanks dry.

[Homo Ludens]

Thank you, guys, for all the input! Great stuff!
But it's all may-be's. No hard info, anyone?
Cheers!
H.L.

[fredgrav]

Hi Homo Ludens,
things stay as follows, without "maybes":
- fuel temperature min limit: is based on fuel freezing point, engine margin, overall temperature accuracy error.
- fuel temperature max limit: one of the main causes for high fuel temperature is its use as means for cooling the oil, hydraulic fluid etc. As a consequence in some flight regimes, its temperature may rise above 100° C or more. Temperatures changes are manifested in modifications of fluid viscosity, dissolved gas concentration and other complex stuff. Above its maximum operating temperature, the cooling ability of fuel is greatly reduced thus leading to high oil and hyd fluid temp: this modifies the direction and rate of friction forces, hence considerably increasing wear processes. Every limitation, as reported by the FCOM, is strictly related to the engine type, fluid type and system type. The final value you get by the manufacturer (eg. 54° C for Airbus) takes into account all of these variables. That's it !

Hope this helps,
fredgrav

[Homo Ludens]

Thanks, Fred, for the great post!
Hope one day to be able to return the favour!
Cheers!
H.L.