Originally Posted by
yanrair
YOU DONT KNOW THIS unless you notice your ground speed falling below where it was, or lower than flight plan.
essentially this is the very point.
The flight plan GS is the average for a leg which might be over an hour long. Plenty of times I’ve seen the GS change by 40 or 50 kts in a couple of minutes, due to wind changes - which means GS is ok as a rough confirmation, but remains a fairly blunt instrument. Which is probably why the A330 procedure is AoA-based, and doesn’t refer to GS at all.