<font face="Verdana, Arial, Helvetica" size="2">You cannot use a GPS for approaches ie coming below 1000 feet above the highest object within 5 miles (10?).</font>
All I'm looking for is where it says in legislation that I cannot.
<font face="Verdana, Arial, Helvetica" size="2">However as I said earlier, in general, if you are doing a hold you are possibly in a situation that requires a working ADF whether or not you are using it at the time eg you are on airways or you are flying in Class A or D TMAs (see ANA?) or you are a BA pilot etc see (SOPs) etc.</font>
There is no doubt that an ADF is required for IFR flight in class A or D airspace. It doesn't matter if you need to use it or not. I would actually contest your observation, if I understand it correctly, that you are most likely to be asked to do an NDB hold in class A or D airspace. I think it's highly unlikely, and I can't remember a time that I have been required to hold in controlled airspace. That's because the vast majority of class A/D environments are managed by radar-equipped units and, bar the majors like Heathrow and Gatwick, most traffic is managed by vectoring. Even where holds are required at Heathrow and Gatwick, most are on VORs.
By contrast, I've frequently been asked to hold on NDBs at airports with procedural approach control for IFR, the majority of which are outside controlled airspace.
<font face="Verdana, Arial, Helvetica" size="2">As regards the discussion on how to keep in pratice, the only difference with an ADF versus VOR/DME or GPS is you don't have a miles read-out. This is only the same as a VOR with unavailable DME so you should be able to do this anyway.</font>
I'm amazed that you regard the three types of approach as similar. They have vastly different user interface demands, traps for the unwary, instrument errors and failure modes. I've yet to find a GPS with dip, or an ADF that would tell me my track over the ground.
I don't doubt that some instrument approach practice is better than none, but I do believe that if you're going to use a radio of any sort for an IAP, you did need to keep proficient in using it -- and that applied as much to GPS as anything else.
I like to fly almost every approach with as much redundancy as possible. Trouble is, if you told me I had to switch off
either the ADF
or the GPS on an NDB approach, I'd have a struggle justifying a choice to switch off the GPS on risk management grounds.