As yet I haven't done much with the Cost Index, other than just set it. Has anyone conducted any trials to see what difference it makes to vary the index? If some people are having trouble with overspeed on descent, using VNav,then maybe changing the cost index might result in shallower descents.Dave

(A) The cost index value determines the speed/Mach for climb, cruise and descent. For any given cost index climb and cruise calculated speeds depend on (a) aircraft weight and (:( selected cruise altitude. The heavier the aircraft and the higher the selected cruise alt, for the same cost index, the target speed/mach will increase. For descent (an exception) the target speed (IAS) is weight independent and linked only to the cost index value.We have used over 500 measurements (raw data) for modelling and validating predicted speed. The accuracy margin with over 90% certainty is 1 knot and 0.005 Mach. Common cost index values are in the raange 20-40 but more realistically around 30-35. For cost index values greater than 70 predicted speed targets rapidly converge to 315/0.815.(:( VNAV descend and ToD accuracy depend considerably on forecast wind and ISA deviation (as they should !!). Since this might be unreliable in FS9 (even while using an external weather programme) try conservative values (e.g. if ISA deviation is +2 enter +4). This will induce descend earlier. In the end watch the green arc in the ND and act accordingly.Best,Vangelis===================================== E. M. Vaos Precision Manuals Development Group www.precisionmanuals.com=====================================