Hi Phantomwray, sorry I didn't reply sooner I was away. Yeah I guess I just thought my point on less potential energy on a cold day was obvious but I should have said it is because the A/C is physically closer to the ground.In the event of a flight idle descent being used you have less potential energy to trade for kinetic energy to achieve the planned flight path(I think).Sorry for the confusion but I'm sure you weren't losing any sleep. <img border="0" title="" alt="[Smile]" src="smile.gif" /> <img border="0" title="" alt="[Smile]" src="smile.gif" /> <img border="0" title="" alt="[Smile]" src="smile.gif" />