I think it may be because the descent is normaly carried out at an approx 3 degree angle( for light A/C such as CRJ which I fly)and for a given Mach number the lower the temp the higher the TAS and therefore the higher the rate of descent required to maintain the 3 degree angle.. .If you are talking heavy jets using idle power and FMS calculated decents I am out of my depth. <img border="0" title="" alt="[Cool]" src="cool.gif" /> <img border="0" title="" alt="[Cool]" src="cool.gif" />