Martin wrote:
Because there was no NDB involved. Or are you saying you could fly an approach with a missed approach procedure based on an NDB without an ADF?
Well we all know you can perfectly safely. But the legal position is still not allowing this at least in the UK.
There is no GPS substitution concession in Europe.
In the USA they have this in various forms. And a “W” GPS can be used for more. @NCYankee is the expert here 
Why the UK insists that some GPS approaches have an NDB in the MA segment (e.g. EGKA) but not others (e.g. EGMD) is a mystery. I recall reading some explanations; I believe some were posted here. However the CAA rarely discusses why they object to this or that, and the airports in question don’t talk about it either. For example there are reasons why Shoreham EGKA still doesn’t have its LPV approaches but they aren’t publicly discussed (they are to do with procedure design issues).
However you still do need a DME for IFR in Europe, generally, and a decent DME costs a lot of money too. AFAICS most of the SR22s that never had an ADF also never had a DME.
Peter wrote:
However you still do need a DME for IFR in Europe, generally, and a decent DME costs a lot of money too. AFAICS most of the SR22s that never had an ADF also never had a DME.
I agree you need a DME in Europe but practically (rather than from a regulatory perspective) you don’t need ADF.
The FAA never approved substitution of ADF, VOR, or Localizer for the final approach segment for part 91 operators. Early on before there were stand alone GPS, and later RNAV approaches. Many of the NDB and VOR procedures were designated as overlay and were named with “or GPS” in the approach title, for example NDB or GPS RWY 2; VOR or GPS RWY 20. If the approach name did not have “or GPS” in the title, the final approach segment could not be flown using GPS and the primary navigation system needed to be displayed for guidance. As RNAV approaches were added, the overlay was eliminated, so that now there are very few overlay approaches left. Of course there are no RNAV approaches that use ground based navaids in the US as far as I am aware, including in the missed approach procedures
Certificated operators such as the airlines could be approved to use their FMS systems to fly VOR and NDB procedures according to OpSpec C300.
Peter wrote:
There is no GPS substitution concession in Europe.
That is not exactly true. E.g. France allows substituting ADF with GNSS for en-route navigation (B-RNAV) and in terminal areas (P-RNAV). And AFAIK they can authorize you to substitute ADF with GNSS even for missed approaches. That’s not the whole story, just the gist of it.
@NCYankee Exactly. You can substitute with GNSS/ GPS on the “or GPS” approaches. AIUI those are official overlays and I guess they actually tested them. France doesn’t (or didn’t have) have any official overlays (they aren’t in the AIP and weren’t tested) so substitution is not allowed (at least wasn’t). Autopilot can fly it, you just have to monitor the ADF.
PS: Which means to me that authorities could give us overlays if they wanted.
So what is the difference between a gps and a rnav approach?
Commander wrote:
So what is the difference between a gps and a rnav approach?
RNAV is an umbrella term for systems that allow navigation on arbitrary course, not just from/ to a beacon. GNSS is one kind of RNAV. That’s why it should be RNAV(GNSS) or RNAV(GPS) in the US.
NCYankee wrote:
Early on before there were stand alone GPS, and later RNAV approaches. Many of the NDB and VOR procedures were designated as overlay and were named with “or GPS” in the approach title, for example NDB or GPS RWY 2; VOR or GPS RWY 20. If the approach name did not have “or GPS” in the title, the final approach segment could not be flown using GPS and the primary navigation system needed to be displayed for guidance.
NCY, are you aware of the criteria and level of scrutiny that was applied before adding “or GPS” to existing conventional IAPs? And what proportion of approaches turned out to be unsuitable for GPS overlay?
The problem in Europe is that the funding for procedure design comes from airports and ANSPs, not from authorities through central funds. Hence the process of adding “or GPS” is potentially expensive.
Bathman wrote:
Did the FAA allow NDB approaches to be flown by GPS in overlay mode in about 1990 and then 10 years later say you can now fly them using GPS with no adf in the aircraft.
There was a period in the FAA overlay program when the underlying conventional navaids (airborne and ground) needed to be available and operational. It was a period of months, I believe, not years, in 1994, before it was changed so that the underlying conventional navaids were no longer required.
There were, of course, a number issues discovered with “or GPS” overlay procedures, and as NCY says, most have been replaced by standalone procedures that use GPS more efficiently. However, if a conventional NPA scores a 1 for safety, and a standalone GPS scores a 10, I would have thought that on average an overlay “or GPS” procedure get an 8 or 9. The FAA’s forward thinking overlay program made a huge contribution to IFR safety in the US, and Europe lags literally decades behind.
