RobertL18C wrote:
While you are not flying magenta, the strip information will provide accurate distance to FAF and MApT in sequence.
I don’t see how that works as for the ILS you need the distance to some DME and not the distance to FAF or MApT.
dutch_flyer wrote:
In the event I need to rely on the ILS, for example because of minimums which are sometimes lower, RobertL18C’s method works fine.
The Minimums for ILS in these cases are lower, because you use different equipment. It’s cheating yourself if you use the ILS minimums while not having the required equipment.
To the original question, two aspects have not yet been discussed:
- In many cases the DME is not immediately on the route. Therefore the only way to actually get the GTN to show the distance to the DME is to have a second GTN that is not synced with the first one and to set a direct to the DME on this one.
- Obviously all discussed methods rely on the fact that GPS is still available. In case of a GPS failure (or loss of accuracy), they are therefore not working. In an airplane w/o DME, the only fallback for a GPS failure is an Airport with a PAR approach (or a visual landing if weather is VMC). Everything else is an emergency.
I don’t see how that works as for the ILS you need the distance to some DME and not the distance to FAF or MApT.
That’s the way overlay approaches work perfectly understandable, nicht wahr?
It’s like flying an RNP approach which uses the same logic.
I don’t see how that works as for the ILS you need the distance to some DME and not the distance to FAF or MApT.
Just 3 tiny details, 1/ there is no MAPT (it’s DH) on an ILS/DME and there is no FAF (it’s FAP), on ILS/DME you fly to DH and go missed with no need for MAPT, it’s on LOC/DME where you likely need DME to define MAPT or FAF and 2/ the DME in Europe on an ILS/DME or LOC/DME is always calibrated to give 0nm distance on live threshold (not at DME or VOR/DME)
On ILS/DME, you only need DME to validate glidepath of ILS at the FAP, that can be done legally with 1/ GPS in UK even on the FAP (or in Europe if FAP = FAF = exiting fix in GPS database)
3/ GPS distance on straight-in RNP or Overlay is to the threshold, it does give the same reading as straight-in ILS/DME, there is some 0.1nm difference that is due to 3deg slant distance (+/-0.01nm due to Sunyaev–Zeldovich effects and gravitational waves 
)
PS: I have DME in the aircraft as it’s mandatory for IFR in Paris TMA (even if it’s PBN aircraft) and bloody useful for situational awareness on ILS/LOC in low visibility
What correction would you need to make to gps distance to get it to equal a DME distance.
The GPS distance is I believe a straight line to the ARP whereas a DME is an oblique (hypotenuse) line to the DME reference point which I believe is usually the landing zone.
The GPS distance is I believe a straight line to the ARP
Only when you do DCT LFFK, if you load straight-in RNP it’s to threshold…
Although at LFFK the threshold is on the numbers at many airfields it is a displaced threshold or TDZ.
Is the GPS distance a horizontal distance? Do you need to calculate an offset to take account of the DME being an oblique measurement?
The slant range issue is really negligible.
The real issue with DME-GPS substitution is finding a GPS waypoint corresponding to DME=0. Many many previous threads on this one – one search
Personally I find DME really useful, as a quick reference even if not flying an IAP. Whether I would spend the money today for a new remotely controlled DME (15k; much less on US Ebay but most people will not get that past their maint. company) is another matter.
Peter wrote:
Whether I would spend the money today for a new remotely controlled DME (15k; much less on US Ebay but most people will not get that past their maint. company) is another matter.
This is the problem. You can get a second WAAS GPS for that cost, and in practice RNP approaches are everywhere. 99% of the time flying an RNP is also simpler, especially the missed approach. There are many things I’d prefer to spend that money on. Plus, practically speaking you don’t need the DME for an ILS any more than you need it for the RNP. We can argue slant range and legalities all day long, but a VTF ILS sans DME is perfectly safe in all but the most challenging obstacle environments. Even more so if you have a GPS overlay with magenta line, or any geo-referenced charts as most people flying IFR do.
Ibra wrote:
the DME in Europe on an ILS/DME or LOC/DME is always calibrated to give 0nm distance on live threshold (not at DME or VOR/DME)
Are you positive? Source? I’m sure I’ve seen ILS/DME or LOC/DME where that has not been the case.
Although at LFFK the threshold is on the numbers at many airfields it is a displaced threshold or TDZ
I have landed with 800m RVR, I doubt TDZ vs TCH matters much above that, in GA we don’t land using PAPI the slant difference on 10nm FAF is 0.5nm, very tiny
I guess while there are load of reason why GPS differs from DME, the difference on short final when loading straight-in ILS/LOC from GPS database is close to 0.1nm otherwise Jeppsen will not publish a GPS Overlay of an ILS/DME (like they do for VOR/DME or NDB/DME with large offsets, bit of CYA !)
Are you positive? Source? I’m sure I’ve seen ILS/DME or LOC/DME where that has not been the case.
Do you have the example? surely that DME is collocated with VOR/DME or NDB/DME
If it’s ILS/DME or LOC/DME it will be calibrated to runway threshold
