Whilst computer flight planning systems are incredibly accurate nowadays thanks to very accurate upper wind and temperature forecast data, these systems never take into account the possible last minute loading changes, which may result in the flight planned levels not being achievable. There has to be some 'flex' in at least level allocation even if there will be none for lateral tracking.
Originally Posted by
Gonzo
Once Target Time of Arrival (TTAs) are common, I'd imagine (hope) flight crews will end up refusing directs if offered and if it would take their projected ETA out of the TTA tolerance.
One issue with TTA is that the shorter the distance remaining to the fix, in turn requires more aggressive changes to the speed to achieve the TTA. My lot did a trial a few years ago and it was soon shelved, as ACCs were complaining of 'unexpected' speed changes in the cruise and this was even more than a couple of hours before landing. Could such technology only exacerbate this problem of not flying what is filed initially?