Originally Posted by
PompeyPaul
So when do you switch to FL ? Or don't you ever ? Why did the books seem to suggest about 3000ft you switched to FL ?
Flight Level as a concept is more related to IFR than VFR flight.
Simplistically, once you are above the Transition altitude (which in many parts of the UK is 3000 ft - but in many parts of the South of England is higher) you must (IFR) or can (VFR) set 1013 as your reference pressure setting. This provides a simple way of insuring level separation of aircraft where the aircraft is sufficiently high that terrain separation is no longer an issue. (Although I believe there are areas of the UK where flying just above the transition altitude would not provide separation from terrain).
In the US it is much simpler, the transition level is 18000 feet and all airspace above 18000 feet (to about FL450 if I recall correctly) is Class A. This is why you are still on an altitude at 10000 feet in the US. IF your post is correct that a LHR inbound flight was cleared to 17000 feet, this seems wrong. I have never received a descent to an altitude above the Transition Level - always a descent to a FL. (If it was in the US, see previous point
)