Is IFR with a single GPS (or single radio) legal?

From here

Peter wrote:

and for private flying (now called “NCO”) there is today no requirement for a backup. Exceptions exist e.g. that infamous STC for installing two GNS boxes, which EASA would not approve unless a (3rd) COM radio was installed.

When the GNS430 and similar systems were approved for in Europe BRNav/RNAV5, the basis of aircraft-level certification was EASA AMC20-4 (now replaced with EASA CS-ACNS subsection C

The back-up requirement exists in both documents. This was AMC20-4:

4.4.2 Limitations on the Use of Navigation Systems
The following navigation systems, although offering an RNAV capability, have limitations for their use
in Basic RNAV operations.

4.4.2.3 GPS
The use of GPS to perform Basic RNAV operations is limited to equipment approved to ETSO-C129a,
ETSO-C 145, or ETSO-C 146 and which include the minimum system functions specified in paragraph
4.2.1. Integrity should be provided by Receiver Autonomous Integrity Monitoring (RAIM) or an
equivalent means within a multi-sensor navigation system. The equipment should be approved in
accordance with the AMC 20-5. In addition, GPS stand-alone equipment should include the following
functions:
AMC 20-4
5
(a) Pseudorange step detection
(b) Health word checking.
These two additional functions are required to be implemented in accordance with ETSO-C129a
criteria.
Traditional navigation equipment (e.g. VOR, DME and ADF) will need to be installed and be
serviceable, so as to provide an alternative means of navigation

Interesting. How is that reconciled with previous posts re part-NCO?

I know that historically Europe had all sorts of equipment carriage requirements e.g. an ADF and DME for all IFR in CAS, etc.

Peter wrote:

It appears that the US still requires a VOR-capable backup, if a pre-WAAS navigator is the only one.

Peter, I have this in writing from the UK CAA that if I want to fly IFR and are using GNSS to do that, I still need some backup – one of the VOR/ADF should suffice.

Peter wrote:

Interesting. How is that reconciled with previous posts re part-NCO?

This is from the current Part NCO:

Two parts stand out:
(b) requires sufficient navigation equipment in case one item fails
(d) requires that the aircraft meets the certification requirements for the appropriate nav spec – so you have to meet AMC20-4 or CS-ACNS section C.

I have this in writing from the UK CAA that if I want to fly IFR and are using GNSS to do that, I still need some backup – one of the VOR/ADF should suffice.

That, I do not doubt, but the UK CAA has almost no smart people left. The last time they were able to give a good reply to a technical query was about 15 years ago.

I am sure the answer is out there somewhere – e.g. a search like this might dig out some references – but I can’t do it right now.

This stuff is hard to find. EASA regs are hard to find anyway (so many old versions are online) and in this case you are looking for a stipulative text, not a prohibitive text. It should be a well worn path though.

In practice, there won’t be anybody flying with a GNS430 who has no VOR backup because a GNS430 has COM+NAV. It does not have ADF+DME which used to be another IFR equipment carriage requirement. And I have never known anybody in Europe who bought the stripped-down GNS400 boxes.

This is from the current Part NCO:

OK, so how do we understand this post ?

Peter wrote:

OK, so how do we understand this post ?

I’ve provided the evidence above from Part NCO to support my understanding. That’s why my STCs have the requirements for back-up nav sources. Why others feel no back-up equipment is needed is down to them to justify.

I’ve provided the evidence above from Part NCO to support my understanding

I don’t doubt you at all. I am not a resident expert on everything-EASA; I am a mod/admin here and am trying to keep things straight and informative. The “problem” is that the “no duplication” thing has been “known and all over the place” for a number of years now. It started with the 8.33 radio business, where lots of avionics shops went around saying you needed two of them; that was BS. Lots of threads here with the references for that.

Maybe navigation is exempted from the no-duplication rule, but that’s a new one.

I also recall your 2×GNS STC (you never posted the design details/justification to EASA; those were apparently confidential) required a 3rd radio for comms, not for nav. I went looking for your post(s) but on a quick look could not find it.

Peter wrote:

I also recall your 2×GNS STC (you never posted the design details/justification to EASA; those were apparently confidential) required a 3rd radio for comms,

None of my dual GNS STCs required a third Comm – a;though some of my competitors did get stick with this. The problem was EASA’s determination of a Common Mode Failure condition. I argued that the transponder was an acceptable means of communication (you can squawk Radio Failure) and so the AFMS required that the transponder has to be serviceable on dispatch.