Michael wrote:
Here’s a great [counter] example of how the FAA “removes the chances of error”
There is no DME in this approach, so I do not really see how it would be a good example of DME substitution.
Peter wrote:
It would be interesting which products’ databases contain the “RWY07” type of waypoint mentioned earlier.
Proline 21 in both the Citation and King Air implementations
I purposely used a US based Example, since, from the start, my position has been the same :
- The FAA validated the practice of using GPS for DME fixes in 1998 giving us 18 years of real life experience.
- In those 18 years there has been NO notable increase in accidents resulting from this practice and , in fact, there has been a decrease but it is next to impossible to attribute this solely to using GPS in leu of DME.
A and C stated that the FAA has done something to “remove the chances of error” – what THAT is remains a mystery …
Aviathor wrote:
There is no DME in this approach, so I do not really see how it would be a good example of DME substitution.
Ok, guilty as charged, but nonetheless :
Michael wrote:
A and C stated that the FAA has done something to “remove the chances of error” – what THAT is remains a mystery
The substitution of DME=0 by the RWY waypoint works best where airfields have both RNAV and ILS/DME approaches to the same runway. A good example is the 34 approach to Lyon-Bron. With the RNAV approach loaded it is a simple matter to GOTO the RWY waypoint whilst being vectored for the ILS.
Many aircraft can accept LNAV approaches but not LPV. At Lyon-Bron the ILS 34 minima are 200’/750m whereas the LNAV minima are 510’/1500m so the ILS would be the default choice. Under these circumstances GPS substitution would be logical and safe. RNAV and ILS/DME approaches to the same runway are fairly common at larger airports
Hey, finally a post that looks it is written by somebody who has done it!
In reality, on a GNSx30 or G1000, you load the overlay procedure and fly it. It contains the step down fixes (if any). Also for localizer approaches. And for the ILS, you tend to have the FAF and a check fix in the database, so the fact that the distance to run is not the DME distance is irrelevant. You could put in a DCT to the RWxx waypoint to get it, but that t would actually be counterproductive.
At the other end of the scale – here is how you fly a NDB/DME approach where the DME is paired with the ILS (assuming ILS is defunct) if there is no approach overlay in the database. You need two GPS, one for the lateral guidance, one for the distance. If you have only one, or a G1000, tough.
1) Switch off cross-fill.
2) Select the NDB waypoint in GPS1 (DCT). Select OBS mode and set the course in the HSI / OBI to the final approach track, also set the display scaling to 1.0 or 0.3 NM, according to taste.
3) Select the ILS procedure in GPS2 (PROC, twiddle, twiddle, ENTER); go to the FPL page and go to the RWxx waypoint, pres DCT ENTER ENTER. that gives you the distance close enough to the ILS DME distance of the zero-ranged ILS.
Fly to the final approach track and intercept. Of course, getting there might need additional fiddling.
It can be done, but I wouldn’t recommend it.
So if the procedure is in the database, you are unlikely to come to grief. If not, good luck figuring that out from the approach plate when flying one-up in IMC.
In the US, if the DME is used for navigation, there is a named facility name for it. Over time all of these DME locations have been added to the database. So if you enter the charted DME name as the direct-to navigation point, it will match the DME distance on the chart. In most cases in the US, the DME fix is also a named fix or the runway threshold. In a few cases, DME fixes on an ILS are there for the Localizer procedure and not charted other than by a distance. If the distance is from a nearby DME (a VORTAC) or if it is a localizer DME (IUZA), one enters the VORTAC ID or the Localizer ID to obtain the GPS position, in which case the DME distance is the same as the GPS distance. So with a single GPS navigator such as a GNS430 or GTN system, you may not be able to determine the distance and use the GPS to load the procedure simultaneously. So you just have to tune the ILS and fly the localizer with the GPS set to navigating to the DME. AKA the old fashioned way where the moving map does not show the procedure, but just the distance to the DME location. Of course, the US does not support DME offsets, so this is not an issue in the US.
NCYankee wrote:
So you just have to tune the ILS and fly the localizer with the GPS set to navigating to the DME. AKA the old fashioned way where the moving map does not show the procedure, but just the distance to the DME location. Of course, the US does not support DME offsets, so this is not an issue in the US.
Aren’t the FAF, OM or equivalent for a glide path check, and step-down fixes (for LOC approaches) in the procedure database for the US?
The FAF is in the database and the GS elevation is normally charted if it is different than the GS intercept altitude. In some cases the FAF is defined by a DME distance, but as far as I am aware, it is always named as well, so the GPS can be used to determine position over the FAF.
That was my point earlier. If a procedure is coded like this, you can safely fly the ILS without any consideration to where DME zero range is. You have a gross error / false glideslope check against the FAF in the procedure. If no interim waypoint before the minimum is in the database, you might miss one altitude check – the likelihood of that killing you is extremely low.
The issues with substitutions only arise if you fly an approach that is not sensibly coded in the database.
