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Hello,

monitoring fuel temps at northernly flights there is not a true °C indication, however depending on TAT the values cycle upward every 5 sec. At FL309 e.g. with TAT -22, SAT -54°C values begin at -22° upwards to -5°, at SAT -49° from -17 to -1°, and so on. Not a bug for sure, but Maybe there is a slight glitch?

Kind regards

Fritz B. Boecker

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4 hours ago, Pegaso said:

Hello,

monitoring fuel temps at northernly flights there is not a true °C indication, however depending on TAT the values cycle upward every 5 sec. At FL309 e.g. with TAT -22, SAT -54°C values begin at -22° upwards to -5°, at SAT -49° from -17 to -1°, and so on. Not a bug for sure, but Maybe there is a slight glitch?

Kind regards

Fritz B. Boecker

Almost understand what you are asking, but not quite.  You shouldn't have dropped what the numbers meant.  Do you mean at SAT -54 C and TAT -22C that the fuel temperature changes from -22C to -5C?  Giving a SAT alone doesn't mean much since the fuel is only aware of the TAT which is the effective temperature of the wing skin, or tank surface.  Also, I'm not sure if you are trying to convey that at TAT -22C the fuel temperature starts a -22C and warms to -5C, or if it starts at -5C and cools to -22C.

The FCOM description of the fuel system reveals that the temperature is measured at Tank 1 only, so it will be normal to see variations and different fuel configurations take place.  I'm sure I can be of more help if I understood the question.

 

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16 hours ago, downscc said:

Almost understand what you are asking, but not quite.  You shouldn't have dropped what the numbers meant.  Do you mean at SAT -54 C and TAT -22C that the fuel temperature changes from -22C to -5C?  Giving a SAT alone doesn't mean much since the fuel is only aware of the TAT which is the effective temperature of the wing skin, or tank surface.  Also, I'm not sure if you are trying to convey that at TAT -22C the fuel temperature starts a -22C and warms to -5C, or if it starts at -5C and cools to -22C.

The FCOM description of the fuel system reveals that the temperature is measured at Tank 1 only, so it will be normal to see variations and different fuel configurations take place.  I'm sure I can be of more help if I understood the question.

 

Dan, thanks for replying. The odd thing is not that fuel temperature changes (of course it should to some degree), but the indication is just an endless slope: it decreases from -22° e.g. up 1° every 5 seconds till it reaches -5° (or -1 or +2°). The next step is then -22° again and so on, not an ondulation between values. Cheers,

Fritz

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8 minutes ago, Pegaso said:

not an ondulation between values

Why would there be...?

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1 hour ago, Pegaso said:

it decreases from -22° e.g. up 1° every 5 seconds till it reaches -5°

Fritz, full names in the PMDG forum please.

A change from -22C to -5C is an increase not a decrease... and I still don't understand what you are trying to ask.  Are CTW involved, RES? What is the fuel load and TAT when you observe -22C and again when you observe the -5C later in the same flight?  Need more information on the situation before it can be explained.  The fuel system is complex, but it is modeled as a system with hydraulic and thermodynamic engineering principles applied and not just a gauge value in the cockpit.

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Dan and Kyle, thank you for your patience. Probably i hadn't described it clearly enough. What i want to point out is that -in my case only?- fuel temps do not show a real value, but in a fixed time interval during long level flight the numbers keep counting down (without any change of external temps). That means

-22, -21, -20, ..., -7, -6, -5, -22, -21, ....

The lowest and max values change a bit over time, but they are like an endless loop (always back from the highest indicated value to the lowest temp over and over again). Hope this will be a more clearly described.

Fritz B. Boecker 

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Hi Fritz, this is exactly the correct behavior.  The fuel temperature after loading the fuel will be near ambient, depending on how much cold fuel was in the tanks to mix with warmer fuel from the airport fuel system.  While at cruise you are in air temperatures around -50C to -60C so there is going to be cooling.  The skin of the wing is also the container for the fuel.  However, the wing skin is not the ambient temperature but it is the TAT (total air temperature, use wikipedia) and the fuel temperature will trend towards that temperature.  The rate at which the temperature declines depends mostly on the fuel quantity and the specific heat of the fuel.  All of this is modeled in the QOTSII, even the details of how the temperature varies when the reserve fuel is added to the mains.

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