I am talking about the lag between fuel temp and TAT while airborne not ambient temperature.Regards

The NGX is the first aircraft to accurately simulate fuel temperature changes.

You are incorrect. The PMDG MD-11X accurately simulates fuel temperature changes.Michael Cubine

I am talking about the lag between fuel temp and TAT while airborne not ambient temperature.Regards

It takes hours for fuel temperature to fully cool to outside air temp (and that's the temperature of the air over most of the wing surface, TAT is experienced at the leading edge). It would get to TAT faster than that, but 15 minutes does sound very quick.Kevin Hall

It looks like a job for... CAPTAIN NEWTON and his secret weapon, the Law of Cooling (which "works" for heating as well).http://ugrad.math.ubc.ca/coursedoc/math100/notes/diffeqs/cool.html