GTN GPS in Lieu of DME questions...

[lrgabriel]

I'm trying to get smart on the use of GPS in lieu of DME.

How do you actually (legally?) display distance to a tuned navaid? The only way I've figured out so far is to have it as the active waypoint in my GTN (basically, Direct To the navaid)...which means I can't also use the GTN with the approach I'm flying activated.

If it matters for this discussion, my GTN is interfaced to a SkyView Touch, which will show valid CDI data, but no distance info to the tuned navaid. I have to look over at the GTN for Distance to Waypoint.

For a practical example, take a look at KBOI VOR/DME or TACAN RWY 10L. I have the BOI VOR (113.3) tuned in the NAV radio with valid CDI info displayed on the EFIS. I arrive at HMISH, and would like to fly the 12 DME arc. If I have the approach active in the GTN, I don't have distance to the BOI VOR displayed anywhere--any distance info from the GTN is ALONG the arc to the next waypoint, not actual DME to the VOR. For that, I have to do some sort of Direct To and type in BOI, and therefore lose the active approach in the GTN.

Am I doing this wrong? Thanks for your time!

[Mark Dickens]

Maybe I'm oversimplifying this but if you're using the 650 for the approach and you (or the a/p) are following the the guidance from the 650, why do you need the dme distance? It's baked into the approach procedure so you're legal without having to watch the dme. You can always fly the arc the old fashioned way with the 650 in vloc mode (since the VOR would be your direct to waypoint), but why would anyone do that?

[lrgabriel]

I agree that the arc baked into the procedure should be the way...but I've run across two flies in the ointment:

-When you activate the approach, the GTN itself says "GPS guidance is for monitoring only."

-The Instrument Flying Handbook talks about using GPS in lieu of DME, and says under "To Fly a DME Arc" on p9-27 to "Select from the airborne database the facility providing the DME arc as the active GPS WP. The only acceptable facility is the DME facility on which the arc is based."

What's more, AC 90-108 p6 says you have to extract waypoints, NAVAIDs, and fixes by name from the database--no mention of using a built-in approach.

Maybe I'm looking at this wrong; is there any guidance in the AFM supplement or from Garmin that expressly allows using the baked in procedure?

Thanks again!

[Reformed SeaSnake]

I am guessing (and its only a guess as I do not use the GTN series navigator) that it is not possible with a single GTN navigator but the question is; when would you need that functionality? You mention the VOR/DME 10L at KBOI but there are published RNAV approaches there with lower mins. Is there an airport you?re looking at which has a VOR/DME but not an RNAV approach? I have a separate GPS navigator and NAV/ILS receiver so I could shoot the approach you reference but it would not be my preferred approach. I would much rather shoot one of the RNAV or ILS approaches as they would have a reduced workload (with my setup I would still not get a CDI input for the 12 DME arc. That would be manual).

[Mark Dickens]

After doing some more reading, I think you're right. Unless you can monitor DME to the station with a certified receiver (GPS or DME), you can't fly that procedure. It's been so long since I've flown an arc, the question just hadn't made it on my radar. I suppose these approaches are in the GTN database in case you also have a second GPS or an old DME receiver.

[RV8iator]

Select NEAREST, VOR and select the VOR you are using and it displays the bearing/distance to that station.

[jeffkersey]

The NRST button works but you have to click to another page to see it. You can set on of the user field on the default Navigation page to show the tuned VOR and it will give radial and distance....

[Auburntsts]

Can't speak to the GTN's "Only for monitoring" annunciation, but the AIM specially allows you to fly the approach with GPS as a DME substitution without monitoring the underlining navaid:

1−2−3. Use of Suitable Area Navigation
(RNAV) Systems on Conventional
Procedures and Routes

2. Use of a suitable RNAV system as an
Alternate Means of Navigation when a VOR, DME,
VORTAC, VOR/DME, TACAN, NDB, or compass
locator facility including locator outer marker and
locator middle marker is operational and the
respective aircraft is equipped with operational
navigation equipment that is compatible with
conventional navaids. For example, if equipped with
a suitable RNAV system, a pilot may fly a procedure
or route based on operational VOR using that RNAV
system without monitoring the VOR.

c. Uses of Suitable RNAV Systems. Subject to
the operating requirements, operators may use a
suitable RNAV system in the following ways.
1. Determine aircraft position relative to, or
distance from a VOR (see NOTE 6 below), TACAN,
NDB, compass locator, DME fix; or a named fix
defined by a VOR radial, TACAN course, NDB
bearing, or compass locator bearing intersecting a
VOR or localizer course.
2. Navigate to or from a VOR, TACAN, NDB,
or compass locator.
3. Hold over a VOR, TACAN, NDB, compass
locator, or DME fix.
4. Fly an arc based upon DME.

[Mark Dickens]

Originally Posted by Auburntsts

Can't speak to the GTN's "Only for monitoring" annunciation, but the AIM specially allows you to fly the approach with GPS as a DME substitution without monitoring the underlining navaid:

1−2−3. Use of Suitable Area Navigation
(RNAV) Systems on Conventional
Procedures and Routes

2. Use of a suitable RNAV system as an
Alternate Means of Navigation when a VOR, DME,
VORTAC, VOR/DME, TACAN, NDB, or compass
locator facility including locator outer marker and
locator middle marker is operational and the
respective aircraft is equipped with operational
navigation equipment that is compatible with
conventional navaids. For example, if equipped with
a suitable RNAV system, a pilot may fly a procedure
or route based on operational VOR using that RNAV
system without monitoring the VOR.

c. Uses of Suitable RNAV Systems. Subject to
the operating requirements, operators may use a
suitable RNAV system in the following ways.
1. Determine aircraft position relative to, or
distance from a VOR (see NOTE 6 below), TACAN,
NDB, compass locator, DME fix; or a named fix
defined by a VOR radial, TACAN course, NDB
bearing, or compass locator bearing intersecting a
VOR or localizer course.
2. Navigate to or from a VOR, TACAN, NDB,
or compass locator.
3. Hold over a VOR, TACAN, NDB, compass
locator, or DME fix.
4. Fly an arc based upon DME.

That seems pretty clear...thanks for the reference!

[Mark H]

The Advisory Circular 90-108 addresses the legality of using an RNAV as alternate means of navigation. The GPS may be used as the means of navigation except for the final approach segment or as a substitute for a LOC. You can legally fly the DME arc using only the GPS until you get inbound at the FAF. You could always do it the hard way by simply displaying the NAVAID info as described in the post above and fly the VOR signal. This is great for training and checkrides but from a practical standpoint, if my GPS is working then I would fly the RNAV approach.