I thought about this again and came up with this as a possible..... .At lower temps a pressure alt of say 40 000ft is physically closer to surface than at warmer temp (because warm air is less dense and therefore pressure drop with height will take place more gradually).. .1. The A/C at lower temp has less potential energy for a given mass.. .2. The A/C at lower temp will infact have a shallower angle of descent than that calculated using the simple 3Nm per 1000ft ball park 3degree calculation.. .How this affects vertical speed would I think depend on what you do with the thrust levers(I mean delay descent or keep more thrust on during descent).. .Is this any help or am I just rambling to my self? Or talking Bull....!