Skytalkert,
Speeds should be flown as accurately as possible. This is what it says in the UK ATC manuals. Any one not able to fly the assigned speed (give or take one or two knots only) should inform ATC ASAP.
This is especially true on the approach side of things, where 2 aircraft on the ILS instructed to maintain 160kts to 4DME, in trail on the ILS 2.5 miles apart, will cause much trouser browning if the second decides to fly at 170kts and the first decides to fly at 150. The further out on the ILS the aircraft are, the worse it becomes as the second a/c has more time to catch the first a/c up!
Certainly at Heathrow (can't comment on others) pilots should expect at least a go-around or maybe being broken off the approach and resequenced depending on exactly how close the two aircraft get!
AI
[ 11 August 2001: Message edited by: Aluminium Importer ]