Interesting to note the differing opinions on this. My own opinion is that one is legally required to use FM-immune ILS equipment when flying an ILS both inside or outside CAS in the UK.
The basis for my opinion is as follows:
This ORS states that, and I quote;
1) The Civil Aviation Authority, in exercise of its powers under Article 20(3) of the Air Navigation Order 2005 ("the Order") hereby directs that:
a) where an aircraft is flying in the United Kingdom in circumstances in which it is required under Schedule 5 of the Order to be provided with specified radio
navigation equipment all such required equipment shall comply with the FM immunity requirements of Annex 10 to the Chicago Convention Volume I (Fifth Edition - July 1996 Amendment 80) and Volume III (First Edition - July 1995 Amendment 80);
Under the explanatory notes that accompany the ORS, it expands thus;
For non-public transport flights under instrument flight rules (IFR) in controlled airspace in the UK, the legal requirements include one VOR receiver (which must be FM immune). To conduct an ILS approach inside or outside controlled airspace one (FM immune) ILS receiver is required.
Now we're at the same point as has already been discussed. Is this guidance material with no legal basis, as suggested, or is this a legally binding requirement?
In my mind it comes down to Article 20(3) of the Air Navigation Order, which is a Statutory Instrument and therefore legally binding. This also explains my choice of emphasis in the first quote.
Artice 20(3) states;
(3) In any particular case the CAA may direct that an aircraft registered in the United Kingdom shall carry such additional or special radio communication or radio navigation equipment as it may specify for the purpose of facilitating the navigation of the aircraft, the carrying out of search and rescue operations or the survival of the persons carried in the aircraft.
It's a matter of personal interpretation but for me at least, this is the trump card. This statements suggests -- to me at least -- that the CAA have
carte blanche to impose any additional requirements on our navigation equipment. In this instance, they specify that to fly an ILS inside or outside CAS requires FM Immune kit if performed under IFR and Article 20(3) of the ANO makes such a declaration legally binding.
This is my own interpretation, at any rate.
In summary, the rule I use for myself is that to fly an ILS anywhere in the UK under IFR requires an FM immune kit. If it ain't FM Immune, it ain't legal... unless flown under VFR, of course. That said, we are all free to draw our own conclusions.