An addendum to my last.
The reason we currently underestimate mileage is because when we say TRACK miles few of us have taken account of the wind and so the figure given is really AIRmiles.
As was pointed out by Bright-ling we have always just added up in separation plus catchup with no serious allowance for the wind. SASS takes full account and even corrects IAS for TAS in its calculations. Last week's trials showed an insignificant difference between the SASS estimate and the actual mileage flown. If it seems more appropriate to the crews to give airmiles it is easily done, its just that the discrepancy has been picked up by BA before. It seems likely to me that if we give you true track miles more aircraft will level off.
[ 20 August 2001: Message edited by: 120.4 ]