Originally Posted by
awayIgo
As already said, there is cold outside air and hot engine air. It doesn't cost anything.
It is actually a common practice for airlines to ask flight/cabin crews to adjust cabin temps in order to save fuel, most notably by turning off one or more a/c units as this article points out:
http://www.nytimes.com/2004/09/03/st...trqa_ed3_.html