IO540, - Except when taking off or landing
Yep, that is what it says in the ANO under Instrument Flight Rules.
However, think of it like this when dealing with GPS -
One can not complete an approach that is not in the databse. There are no approaches in the UK.
Thus when in the UK, most approved GPS units are limited to BRNAV which has an RNP of 5.
Since the unit is only by virtue of the RNP5 certified to be accurate to within 5nm of the indicated position for 95% of the time, you must considder all obstacles within that 5nm unless you can avoid them visually.
Remember that RNP5 is acheived on the ATS routes where the VORs are not more than 100nm appart!!!!...........so when it comes to using the equipment in accordance with the approvals within the UK you are just as good using a VOR 50nm away!!!
Also considder what it says in the AIP about required visual reference before descending below the applicable height that provides the required obstacle clearance.
GPS is very accurate. So is my flying..........but I can't just use that as the basis for flying CATIIIC approaches..........unless the aircraft is approved and the ILS/airfield capable.
Regards,
DFC