What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

GTN GPS in Lieu of DME questions...

L

lrgabriel

Active Member
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!
 
Last edited:
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?
 
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!
 
Why?

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).
 
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.
 
You can see it

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

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....
 
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.
 
Auburntsts said:
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.
Click to expand...
That seems pretty clear...thanks for the reference!
 
AC 90-108

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.
 
Mark H said:
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.
Click to expand...

The AIM also has verbiage to that affect:

4. Pilots may not substitute for the NAVAID (for example,
a VOR or NDB) providing lateral guidance for the final
approach segment. This restriction does not refer to
instrument approach procedures with ?or GPS? in the title
when using GPS or WAAS. These allowances do not apply
to procedures that are identified as not authorized (NA)
without exception by a NOTAM, as other conditions may
still exist and result in a procedure not being available. For
example, these allowances do not apply to a procedure
associated with an expired or unsatisfactory flight
inspection, or is based upon a recently decommissioned
NAVAID.

The Martin State VOR/DME RWY 15 is a good example of an IAP where you can't sub GPS: https://aeronav.faa.gov/d-tpp/2001/05222VDTZ15.PDF
 
RV8iator said:
Select NEAREST, VOR and select the VOR you are using and it displays the bearing/distance to that station.
Click to expand...

You may wish to verify that with your local FSDO or other knowledgeable authority. I am NOT one, most of my instrument knowledge is decades old and I don't fly single engine IFR anymore.

This issue came up decades ago. A DME arc at 30,000 feet is a tighter circle than a DME arc at 3,000 feet, because the DME measures line of sight distance to the beacon. GPS distance to a fix, even if it is a VOR/DME, gives you the LATERAL distance to the fix - so up high your arc may be well outside what was intended by TERPS, and as a result may put you INTO airspace the arc was meant to avoid.

My recollection is that if you are not equipped with the correct type of equipment for the approach, you cannot fly it. Pick an equivalent GPS approach if one is available, otherwise request vectors to an approach point for a GPS approach that does not include the arc.

There was talk of equipping GPS receivers with a calculated "Virtual DME" reading, but I've never seen that materialize on any of the Garmins I've flown with.
 
Auburntsts said:
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.
Click to expand...

Similar to my response to the above concerning flying a DME arc - GPS is not listed in your post as an "alternate means" of navigating DME approaches. GPS gives you lateral distance, not orthoganal distance - and at 25,000' can put your arc so much further from the fix that you end up INSIDE an airspace meant to be avoided by the arc.

It may be that the GPS actually displays the correct arc, but if you don't want to bust a checkride verify whether the substitution is legal with someone smarter than me.
 
breister said:
Similar to my response to the above concerning flying a DME arc - GPS is not listed in your post as an "alternate means" of navigating DME approaches. GPS gives you lateral distance, not orthoganal distance - and at 25,000' can put your arc so much further from the fix that you end up INSIDE an airspace meant to be avoided by the arc.

It may be that the GPS actually displays the correct arc, but if you don't want to bust a checkride verify whether the substitution is legal with someone smarter than me.
Click to expand...

It is perfectly legal to fly a DME Arc IAP with an IFR GPS. I didn’t mention the relevant paras for GPS substitution because I thought it was understood that TSO’d GPS systems were an approved RNAV alternate means and I didn’t want to end up quoting lots of AIM paragraphs. But here it is:

b. Types of RNAV Systems that Qualify as a Suitable RNAV System. When installed in accordance with appropriate airworthiness installation requirements and operated in accordance with applicable operational guidance (for example, aircraft flight manual and Advisory Circular material), the following systems qualify as a suitable RNAV system:

1. An RNAV system with TSO−C129/ −C145/−C146 equipment, installed in accordance with AC 20−138, Airworthiness Approval of Global Positioning System (GPS) Navigation Equipment for Use as a VFR and IFR Supplemental Navigation System, or AC 20−130A, Airworthiness Approval of Navigation or Flight Management Systems Integrat- ing Multiple Navigation Sensors, and authorized for instrument flight rules (IFR) en route and terminal operations (including those systems previously qualified for “GPS in lieu of ADF or DME” operations), or
2. An RNAV system with DME/DME/IRU inputs that is compliant with the equipment provisions of AC 90−100A, U.S. Terminal and En Route Area Navigation (RNAV) Operations, for RNAV routes. A table of compliant equipment is available at the following website: http://www.faa.gov/about/office_org/ headquarters_offices/avs/offices/afs/afs400/afs47 0/policy_guidance/
NOTE−
Approved RNAV systems using DME/DME/IRU, without GPS/WAAS position input, may only be used as a substitute means of navigation when specifically authorized by a Notice to Airmen (NOTAM) or other FAA guidance for a specific procedure. The NOTAM or other FAA guidance authorizing the use of DME/DME/IRU systems will also identify any required DME facilities based on an FAA assessment of the DME navigation infrastructure.
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.
NOTE−
1. The allowances described in this section apply even when a facility is identified as required on a procedure (for example, “Note ADF required”).
2. These operations do not include lateral navigation on localizer−based courses (including localizer back−course guidance) without reference to raw localizer data.
3. Unless otherwise specified, a suitable RNAV system cannot be used for navigation on procedures that are identified as not authorized (“NA”) without exception by a NOTAM. For example, an operator may not use a RNAV system to navigate on a procedure affected by an expired or unsatisfactory flight inspection, or a procedure that is based upon a recently decommissioned NAVAID.
4. Pilots may not substitute for the NAVAID (for example, a VOR or NDB) providing lateral guidance for the final approach segment. This restriction does not refer to instrument approach procedures with “or GPS” in the title when using GPS or WAAS. These allowances do not apply to procedures that are identified as not authorized (NA) without exception by a NOTAM, as other conditions may still exist and result in a procedure not being available. For example, these allowances do not apply to a procedure associated with an expired or unsatisfactory flight inspection, or is based upon a recently decommissioned NAVAID.
5. Use of a suitable RNAV system as a means to navigate on the final approach segment of an instrument approach procedure based on a VOR, TACAN or NDB signal, is allowable. The underlying NAVAID must be operational and the NAVAID monitored for final segment course alignment.
6. For the purpose of paragraph c,“VOR”includesVOR, VOR/DME, and VORTAC facilities and “compass locator” includes locator outer marker and locator middle marker.
 
Last edited:
breister said:
You may wish to verify that with your local FSDO or other knowledgeable authority. I am NOT one, most of my instrument knowledge is decades old and I don't fly single engine IFR anymore.

This issue came up decades ago. A DME arc at 30,000 feet is a tighter circle than a DME arc at 3,000 feet, because the DME measures line of sight distance to the beacon. GPS distance to a fix, even if it is a VOR/DME, gives you the LATERAL distance to the fix - so up high your arc may be well outside what was intended by TERPS, and as a result may put you INTO airspace the arc was meant to avoid.

My recollection is that if you are not equipped with the correct type of equipment for the approach, you cannot fly it. Pick an equivalent GPS approach if one is available, otherwise request vectors to an approach point for a GPS approach that does not include the arc.

There was talk of equipping GPS receivers with a calculated "Virtual DME" reading, but I've never seen that materialize on any of the Garmins I've flown with.
Click to expand...

Certified (TSO?d) gps navigators are certainly legal to use until the final segment on VOR approaches.

Second comment...show me an arc approach you would fly at 30?000 feet and worry about TERPS.

All the pertinent info about legally using GPS is shown in a post above quoting the AIM.
 
Auburntsts said:
It is perfectly legal to fly a DME Arc IAP with an IFR GPS. I didn?t mention the relevant paras for GPS substitution because I thought it was understood that TSO?d GPS systems were an approved RNAV alternate means and I didn?t want to end up quoting lots of AIM paragraphs. But here it is:

b. Types of RNAV Systems that Qualify as a Suitable RNAV System. When installed in accordance with appropriate airworthiness installation requirements and operated in accordance with applicable operational guidance (for example, aircraft flight manual and Advisory Circular material), the following systems qualify as a suitable RNAV system:

1. An RNAV system with TSO−C129/ −C145/−C146 equipment, installed in accordance with AC 20−138, Airworthiness Approval of Global Positioning System (GPS) Navigation Equipment for Use as a VFR and IFR Supplemental Navigation System, or AC 20−130A, Airworthiness Approval of Navigation or Flight Management Systems Integrat- ing Multiple Navigation Sensors, and authorized for instrument flight rules (IFR) en route and terminal operations (including those systems previously qualified for ?GPS in lieu of ADF or DME? operations), or
2. An RNAV system with DME/DME/IRU inputs that is compliant with the equipment provisions of AC 90−100A, U.S. Terminal and En Route Area Navigation (RNAV) Operations, for RNAV routes. A table of compliant equipment is available at the following website: http://www.faa.gov/about/office_org/ headquarters_offices/avs/offices/afs/afs400/afs47 0/policy_guidance/
NOTE−
Approved RNAV systems using DME/DME/IRU, without GPS/WAAS position input, may only be used as a substitute means of navigation when specifically authorized by a Notice to Airmen (NOTAM) or other FAA guidance for a specific procedure. The NOTAM or other FAA guidance authorizing the use of DME/DME/IRU systems will also identify any required DME facilities based on an FAA assessment of the DME navigation infrastructure.
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.
NOTE−
1. The allowances described in this section apply even when a facility is identified as required on a procedure (for example, ?Note ADF required?).
2. These operations do not include lateral navigation on localizer−based courses (including localizer back−course guidance) without reference to raw localizer data.
3. Unless otherwise specified, a suitable RNAV system cannot be used for navigation on procedures that are identified as not authorized (?NA?) without exception by a NOTAM. For example, an operator may not use a RNAV system to navigate on a procedure affected by an expired or unsatisfactory flight inspection, or a procedure that is based upon a recently decommissioned NAVAID.
4. Pilots may not substitute for the NAVAID (for example, a VOR or NDB) providing lateral guidance for the final approach segment. This restriction does not refer to instrument approach procedures with ?or GPS? in the title when using GPS or WAAS. These allowances do not apply to procedures that are identified as not authorized (NA) without exception by a NOTAM, as other conditions may still exist and result in a procedure not being available. For example, these allowances do not apply to a procedure associated with an expired or unsatisfactory flight inspection, or is based upon a recently decommissioned NAVAID.
5. Use of a suitable RNAV system as a means to navigate on the final approach segment of an instrument approach procedure based on a VOR, TACAN or NDB signal, is allowable. The underlying NAVAID must be operational and the NAVAID monitored for final segment course alignment.
6. For the purpose of paragraph c,?VOR?includesVOR, VOR/DME, and VORTAC facilities and ?compass locator? includes locator outer marker and locator middle marker.
Click to expand...

Thanks for the info. Things have certainly changed!
 
RV8iator said:
Certified (TSO?d) gps navigators are certainly legal to use until the final segment on VOR approaches.

Second comment...show me an arc approach you would fly at 30?000 feet and worry about TERPS.

All the pertinent info about legally using GPS is shown in a post above quoting the AIM.
Click to expand...

As I said, my knowledge is old and apparently has been superseded. Thanks for the update.

The 30,000' was simply an exaggerated example to illustrate the difference. I rarely got that high in Phantoms, we usually cruised around FL250.
 
And back to one of the original questions in this thread about displaying the distance to the VOR that you?re flying the ARC around on the GTN. When you display the map on a GTN there are 4 user controlled data fields? one in each corner of the map. One of the choices is ?VOR/LOC - Tuned VOR/LOC Info?. Select that, tune the applicable NAVAIDs freq into the Nav radio and blam, the radial and distance is displayed right there on the moving map while the GPS navigates to the arc?s exit waypoint.
 
Auburntsts said:
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.
Click to expand...

Yes, but still need DME and VOR equipment on the aircraft and operational.
 
RVDan said:
Yes, but still need DME and VOR equipment on the aircraft and operational.
Click to expand...

I don’t think that’s completely true. It is true non-WAAS but I think there’s an out for WAAS IFR GPS’s:

2) Aircraft using un-augmented GPS (TSO-C129() or TSO-C196()) for navigation under IFR must be equipped with an alternate approved and operational means of navigation suitable for navigating the proposed route of flight. (Examples of alternate navigation equipment include VOR or DME/DME/IRU capability). Active monitoring of alternative navigation equipment is not required when RAIM is available for integrity monitoring. Active monitoring of an alternate means of navigation is required when the GPS RAIM capability is lost.

however

2) Ground−based navigation equipment is not required to be installed and operating for en route IFR operations when using GPS/WAAS navigation systems. All operators should ensure that an alternate means of navigation is available in the unlikely event the GPS/WAAS navigation system becomes inoperative.

9. Unlike TSO−C129 avionics, which were certified as a supplement to other means of navigation, WAAS avionics are evaluated without reliance on other navigation systems. As such, installation of WAAS avionics does not require the aircraft to have other equipment appropriate to the route to be flown. (See paragraph 1−1−17 d for more information on equipment requirements.)

[Note the above paras are talking about TSO 145/146 WAAS boxes like the GTN]
 
Last edited:
I just took my Instrument Rating check ride last weekend (my plane has a G3X Touch and a GTN 650) and one of the approaches we flew was the VOR03 into KDLH, which has a DME arc.

I used GPS for the arc, let the GPS steer to the inbound course after the arc, then I switched the CDI over to VLOC to fly the rest of the approach with the VOR as the navigation source.

Works great, and the DPE was happy with the way I did it.
 
You must log in or register to reply here.
Back
Top