DME missed approach point (MAP) using GPS

[sixpack]

GPS Overlays of LOC, VOR and NDB approaches often have the step-down waypoints, but not the missed approach point. Anybody know why this is.

Example: LOC BAK at KSLE using Garmin 430.
The step-down at D1.9 off the Localizer is shown in the garmin as 19LOC.
The MAP is at D0.6 off the Localizer, but doesn't have a waypoint.
After crossing 19LOC, the GPS sequences to the runway, rather than the Localizer. Hence, it is difficult to determine the MAP from the GPS.

Anyone know why the MAP would be left off of the GPS for overlays?

[Tired]

When shooting the approach with the GPS that runway waypoint would be the MAP. Is there any chance you could post a copy of the chart?

[sixpack]

PDF file of example approach:
http://www.myairplane.com/databases/.../SLE_lbr13.pdf

Note, the MAP is 0.6nm from the localizer, and 0.8nm from the runway. I do not believe it is authorized to use 0.8 from the runway.

[Tired]

According to that chart the approach does not have a GPS overlay. When you load that appoach into the Garmin does it have GPS written after the name of the approach in small letters?

[sixpack]

Oops. Ignore where I used the term "gps overlay". It isn't an overlay.
It's simply a non-GPS approach which exists in the GPS database (situational awareness).

Anyway, these approaches which exists in the database, make it easy to use GPS for DME for most fixes, but (like in this case), not the DME distance for the missed approach point.

[Tired]

Ok, if it isn't an overlay approach you cannot use the GPS legally to identify the MAP. You can use the GPS for situational awareness but do not navigate using it. If I was setting the GPS up for this approach I would do the following...

1. Hit Direct To, and then set the waypoint to ISLE
2. Hit OBS, and set the course to 130
3. Verifty the CDI mode is set to VLOC

This will give you a line on the display showing approx where the final segment of the approach is and give you DME from ISLE.

[mtsu_av8er]

[ QUOTE ]
Ok, if it isn't an overlay approach you cannot use the GPS legally to identify the MAP.

[/ QUOTE ]

I don't think that's completely accurate. You CAN use GPS in lieu of DME. Since the MAP is identified by the .6 DME from the localizer, you could indeed set I-SLE in the GPS, and use that distance to identify the MAP, just as if you were using DME.

[Tired]

Yes, as I posted above in the method I described. Read in context that was supposed to mean if you load the approach into the GPS when it is not an actual GPS approach, as can be done in the Garmin 430, you cannot use the GPS to identify the MAP.

If you load the approach, you won't get DME from ISLE.

[mtsu_av8er]

I'm with you...we're both saying the same thing....

[sixpack]

I do add-on ratings, so I typically only fly 3-8 hours with each student. Cant' show them too much GPS in that time. I show my students the ISLE method more often then not, and if they become comfortable with the GPS I show them the OBS/CDI.

So back to my original question...
Why do you think that Garmin elected to include the step-down waypoint (19LOC), but not the MAP (06LOC)?

[Ralgha]

I could be a limitation in the firmware, all GPS approach MAWPs are RW##, so it may not be flexible enough to accept a non-RW## waypoint as a MAWP.

Don't really know though.

I see the RNAV 31 at Salem is a WAAS precision approach, time for me to go get a CNX-80. Too bad GNS-430s won't be upgradable for another year, I like those better.

Here's another interesting question: Why is the VNAV DH higher than the LNAV MDA on the RNAV 31 at Salem?