I don’t know exactly when this started, probably around SU15, but I seems to have unrealistically high temperatures on high altitudes with Live Weather turned on.
For example I am now out on the The Pacific Ocean at N41W150 at cruise FL380 and I have TAT at -18c in the 787. This seems way to high for me, right?
This happens in all the aircraft I flown recently, like the Airbus (A320v2, FBW) and Boeings (737, 777 and now 787).
I found an interesting curve at Temperature at Altitude Calculator (omnicalculator.com):
So FL380 is 11580m and it should be in the troposphere and expected temperature is around -50c to -55c.
However, if I look at the temperature at altitude 38000m (i.e. 124000ft way
up in the stratosphere), this curve actually corresponds exactly to what I see now, i.e. -26c.
Could it be a mix up here in units, so somehow the simulator is picking the temperature value at 38000m when it should actually look at 38000ft? It seems to fit very well with what I observe at the moment…
If I am the only one with this problem, it might possibly be some messed up setting somewhere that mixes up m and ft? Anyone got any ideas or experienced something similar?
I thought I ask here first before I start clearing out my Community folder for the nth time and posting a bug report…
Don’t confuse True Air Temp (TAT) with Static Air Temp (SAT). TAT takes compression into account.
From https://www.flyingmag.com/air-temperature-explained/ :
“Total air temperature is the SAT (actual OAT) plus the temperature rise associated with high-speed flight. This temperature increase is called “ram rise” and is the result of heating of the air due to compression. From a practical standpoint, TAT (or RAT) is the temperature the airplane’s skin feels, while SAT is the free air’s temperature (aka OAT). TAT is greater than SAT due to the ram rise in temperature that occurs because of dynamic heating. Ram air temperature rise is proportional to the speed of the aircraft. At 0.8 Mach or higher, a ram rise of 30 degrees C can be expected. This rise in temperature may be enough to prevent ice from forming.”
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Do not re-install anything. This is a sim issue and have seen it on many systems.
Since our 777 release, we have had reports of people running out of fuel, and it took us a while to realize it was due to the temperature issue. I have seen +2C at FL290 myself.
Mathijs Kok
PMDG
Spot on, thanks! It is of course TAT shown in the 787 and I fly M.84 so I guess a 30c+ increase from SAT (-52c) it exactly as you would expect.
I mixed up the TAT with OAT, should have looked it up a bit better 
Edit: so @MikeB54331’s explanation then explains what I see in the 787 as it is shown TAT. But according to @TheFalconOne, it may still be an issue with other aircraft, including PMDG’s, so I will still be cautious and possibly add some more fuel for any longer trips.
@TheFalconOne do you have any rule of thumb to how to calculate the extra fuel needed for the PMDG 777 to avoid running out of fuel on a long haul?
Impossible to say. If you calculate with -50c and you get +2c you burn a lot more fuel.
I’m doing a long haul flight at the moment of 6,300nm. Over the pacific I’m seeing what I’d expect, TAT at -20 (the compressed air temp) and -50 for SAT. And fuel at +3 deg C:
I suspect its the plane itself rather than the sim.
