True, my sentence wasn’t perfect. I was talking about what I learned from the thread about Europe. But you are right, I maybe should have said: in the USA DME may be substituted by GPS, except on the final approach segment.
At the risk of stating the obvious, you still need to find a GPS waypoint corresponding to DME=0.
ArcticChiller wrote:
But you are right, I maybe should have said: in the USA DME may be substituted by GPS, except on the final approach segment.
As I understand it, it is: “in the USA DME may be substituted by GPS, even on the final approach segment”.
I’m not saying that I know how it is, only that nowhere in the information posted here it says that you cannot substitute DME with GPS on final approach, and that my CFII specifically told me that you can. The plane I rented recently doesn’t even have a DME, and it regularly flies the ILS approach at its home base which states “DME or Radar required”. Even for checkrides etc. All this tells me that there is a high chance that it is allowed. But I am actually trying to find out how it really is, for the next time I am flying a plane without DME in the US.
@NCYankee, what is your take on this?
It would be good to find out, RWY20, because
The plane I rented recently doesn’t even have a DME, and it regularly flies the ILS approach at its home base which states “DME or Radar required”.
that simply means that plane has been illegal for most IFR in Europe from the day it was delivered 
I am not suggesting people should be prosecuted but it is an enduring mystery who nobody has, especially in Germany, despite the very high profile of the SR22 population.
We did this before and it could be this and right after that post you will find a great post from NCYankee.
The plane is in the US though and I am only writing about the US here.
The plane in question has WAAS GPS. So it could be the case my CFII was referring to, but as I wrote he didn’t give any conditions or restrictions in his answer. It would also be interesting to see the source for the information that you may substitute DME with a WAAS GPS on final approach, but not with a non-WAAS GPS. The precision difference between a DME and a non-WAAS GPS is not substantial in my opinion. The more fundamental problems are in how you set up your navigators. You need to compare the DME distances to the track distances in the profile on the approach plate and choose the right waypoint. We usually had the approach loaded on GPS#2 as well and chose the waypoint called “RW20” (corresponding to the threshold).
You need to bear in mind what you are using the DME for. For situational awareness and descent planning on the one hand, but most importantly to make sure you intercepted the right GS lobe of the ILS. If your distance is 0.1 nm off, that still works.
I prefer to have the FPL page up, which shows you all the waypoints to the threshold and onwards, so you can see all legs and distances to that point. Navigation is by GPS anyway until on “base” or equivalent, so DME substitution really only comes in to check the final approach fix, and a profile check on the glidepath.
If there is a waypoint between FAF and the threshold, you will clearly see it, and you can add the distances as required. If there isn’t, you check the FAF, and distance thereafter.
The real problem are DMEs that are not zero-ranged at the threshold, hence DME substitution is something that still requires more attention than ADF or VOR substitution. The only reason it rarely kills on an ILS is that you have the glideslope, and if you follow a sensible arrival route you won’t have a problem even without any DME at all, it provides a cross check for failure modes that are very rare.
It can become a killer on a non precision approach, though.
Yes, and we’ve had a few threads here where it was evident there are pilots, apparently with an IR, who don’t know this.
However all this comes under the category of “reading the plate carefully”. I have seen NP approaches with SDFs which are defined by DME to some distant VOR/DME, so the whole approach is flown without the DME ever getting anywhere near zero.
ArcticChiller wrote:
True, my sentence wasn’t perfect. I was talking about what I learned from the thread about Europe. But you are right, I maybe should have said: in the USA DME may be substituted by GPS, except on the final approach segment.
This is still not correct. The only prohibition on using GPS to substitute for VOR, DME, or ADF on the final approach segment is if the VOR, DME, or ADF is providing the lateral guidance for that segment. There are only 2 approaches out of over 18,000 published that I am aware of in the US, where DME is used for lateral guidance on the final approach segment and therefore it may not be substituted on the final approach segment. One is the VOR/DME or TACAN Z RWY 15 at KMTN. The final approach course is a continuation of the 14.7 DME arc from the FAF to the MAP.
If the DME is simply used to provide an along track fix on the final approach course, then there is no prohibition on using GPS for substitution. The same is true for a cross radial from a VOR or an NDB bearing used to define a fix on the final approach course. Only the lateral guidance along the final approach course is not permitted for substitution of GPS for VOR, ADF, or DME.
Ahhh, now I got it.
AIM 1-2-3
“4. Pilots may not substitute for the NAVAID (for example, a VOR or NDB) providing lateral guidance for the final approach segment.”
I overlooked that bit. Very interesting, thanks for the example with the DME arc.
Cobalt wrote:
The real problem are DMEs that are not zero-ranged at the threshold, hence DME substitution is something that still requires more attention than ADF or VOR substitution. The only reason it rarely kills on an ILS is that you have the glideslope, and if you follow a sensible arrival route you won’t have a problem even without any DME at all, it provides a cross check for failure modes that are very rare.It can become a killer on a non precision approach, though.
In the US, DME are not zero ranged at the threshold. On the final approach segment, when a GS is available, the DME is not used, so substitution is not an issue. DME is only used on a Localizer approach to define FAF, stepdowns, and MAP. DME substitution is most difficult if the procedure has stepdown fixes on the final approach course. One has two choices, a do the math based on distances from the MAP if you are loading an ILS approach into a typical GPS Nav system such as a GNS530W or enter the DME station location into the GPS as the active fix and fly the approach the old fashioned way of loading the localizer frequency into the Nav receiver. The latter method does not show the path for SA, but the distances will match the charted DME distances. If you have dual systems, load the approach on the number 1 system for SA and on the number 2 system use the DME station ID for distances.
If there are no stepdown fixes on the final approach segment, then any case where the DME is used to locate the FAF and MAP, particularly where the MAP is at the threshold, a single system loading the approach provides all the information needed to fly the localizer version.
