[1.0.4] Weird fuel temperature displayed

[daemotron]

Nothing that would impact functionality, just an oddity I noticed: Flying at FL370 with an OAT of -24°C, the FUEL TEMP indicator on the OHP indicated a fuel temperature of +20°C - not sure that this could not happen in RL, it just seems improbable...

 

Edited April 9 by daemotron

[JRBarrett]

Aviation jet fuel takes quite a long time to cool from whatever temperature it had when it was placed into the aircraft tanks.

Depending on the aircraft's speed, fuel that started out at +20C on the ground, could take over 2 hours of exposure to -50C temps aloft before its temperature would drop significantly below 0C.

Even then, the fuel will only drop to the TAT (True Air Temperature) "felt" by the airframe structure, which is often 10 to 20 degrees C warmer than the SAT (Static) OAT. The difference between SAT and TAT depends on the aircraft's speed.

Of course, the rate at which the fuel cools depends on many factors. Fuel loaded in the tanks in Miami in July at +40 C, will take much longer to chill down in cruise, than fuel loaded in Boston in January, which might have already been cold-soaked to -5C in the fuel truck before even going into the aircraft.

The size of the tanks plays a role too. Fuel stored in thin wings will cool faster than fuel in thick wings (more capacity). And, fuel in wing tanks cools faster than center tank fuel, for similar reasons.

In any case, seeing fuel significantly warmer than ambient OAT is perfectly normal until the aircraft has been at sustained cold temperatures at the flight levels for quite a long time.

I have seen many add-on aircraft (mainly in FSX) that get this completely wrong - where the fuel "instantly" assumes the outside air temperature in a climb - which is not what actually happens in a real aircraft.

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[Litjan]

Not much to add to JRBarrett´s reply.

Fuel temperature in wing-tanks will usually assume the roughly the TAT temperature, but it might take a long time.... I will take another look again, though, just to make sure we are in the ballpark.

Thanks, Jan

 

[JRBarrett]

Not much to add to JRBarrett´s reply.

Fuel temperature in wing-tanks will usually assume the roughly the TAT temperature, but it might take a long time.... I will take another look again, though, just to make sure we are in the ballpark.

Thanks, Jan

 

It can be difficult to find an exact value for the cooling rate, since there are so many variables. Initial temperature of the fuel, quantity in the tank, size and shape of the tank etc. The problem of Jet-A cooling to the point where freezing potentially might become an issue mainly affects aircraft on very long transoceanic flights which spend many hours in cruise.

The opposite effect happens too - once thoroughly chilled, the fuel remaining in the tanks can take quite some time to warm back up after landing. On very humid days, I've seen frost form on the underside of the wings of aircraft that have just landed from a 5-hour transcontinental flight - even though the air temperature might be over +20C!

[daemotron]

Thanks, I have to keep an eye on that. When I took the screenshot, I was about one hour into the flight. However I started at LEMD, and I have a screenshot proving OAT was at around 27°C there, so yes that could be plausible. I had the feeling seeing the fuel temp increase in cruise, but I have no screenshots or any evidence. I'll watch that (and eventually document if anything odd happens).