Overlay Approaches

[acroguy]

I'm coming late to this discussion, but let me add a couple of points.

In the US, all so-called "overlay" approaches are being discontinued. Here, an overlay is an approach that was previously labeled GPS/NDB-A, for example. When in effect, you could substitute the GPS for the NDB, even if the NDB was out of service. With the discontinuation of these approaches, the FAA has commissioned separate RNAV and, in the previous example, NDB approaches. There was an interval during which the NDB approach to my local airport was separated but the RNAV was not yet commissioned. At that point, if the NDB was OTS, one couldn't fly the approach even with GPS terminal guidance and approach wouldn't approve it. GPS is allowed to substitute for an NDB or VOR but not if it is the primary navaid for the approach.

Now, with respect to DME, GPS is allowed to substitute for DME in virtually all circumstances, at least in the US. There is a specific page in the front of the Jepps that deals with this. However, on an ILS one can only use GPS up to the FAF and must use the localizer/glideslope inside the FAF. In fact, the box will warn of this.

I think that the discussion of FMS has confused the matter, as there are multiple sensors involved. However, inasmuch as the FMS includes GPS guidance, I see no problem with using it to intercept the localizer outside the FAF on an arc with DME inop as long as the actual localizer/glideslope is tuned and used as primary guidance inside the FAF.

At least that is my understanding of the FAA's stance on the matter...I realize that individual operators have opspecs that are specific and may differ.