Ron Levy, why can't you fly DME arc with GPS?

[gismo]

Ron, you (or someone using your name) wrote to IFR magazine saying that "You are not permitted to use a GSP as a DME substitute to fly a DME arc on a VOR/DME approach unless there's a GPS overlay for that approach and you have an approach GPS..." The AIM seems to disagree (see excerpts below). While there is the disclaimer:

​

NOTE-
This approval does not alter the conditions and requirements for use of GPS to fly existing nonprecision instrument approach procedures as defined in the GPS approach overlay program.

...it doesn't seem consistent with the rest of what the AIM has to say on the subject to say you can't fly a DME arc with GPS.
​

​

1-1-19.Global Positioning System (GPS)

f. Use of GPS in lieu of ADF and DME

1. Subject to the restrictions below, operators in the U.S. NAS are authorized to use GPS equipment certified for IFR operations in place of ADF and/or DME equipment for en route and terminal operations. For some operations there is no requirement for the aircraft to be equipped with an ADF or DME receiver, see subparagraphs f6(g) and (h) below. The ground-based NDB or DME facility may be temporarily out of service during these operations. Charting will not change to support these operations.​

(a) Operations allowed:

(2) Flying a DME arc.

(c) Guidance. The following provides general guidance which is not specific to any particular aircraft GPS system. For specific system guidance refer to the AFM, AFM supplement, pilot's guide, or contact the manufacturer of your system.

(2) To fly a DME arc: ​

[a] Verify aircraft GPS system integrity monitoring is functioning properly and indicates satisfactory integrity.​

You must select, from the airborne database, the facility providing the DME arc as the active GPS WP. ​

NOTE-
The only acceptable facility is the DME facility on which the arc is based. If this facility is not in your airborne database, you are not authorized to perform this operation.​

[c] Maintain position on the arc by reference to the GPS distance in lieu of a DME readout.​

​

​

Now I know this letter mentions the KMTN VOR/DME (or TACAN) 15 approach which is unusual in that the DME arc leads to the MAP/runway rather than a VOR final approach course segment but certainly on an approach like the BWG VOR/DME 21 it must be legal to fly with a GPS (and VOR) and no DME. And I can't really see why you couldn't legally fly the MTN VOR/DME 15 with a GPS and VOR either, but I admit I'm not so sure about that.

http://download.aopa.org/iap/20051027/SE-1/bwg_vor_dme_rwy_21.pdf

​

 

[jdwatson]

My interpretation is that you can use the GPS in lieu of the DME portion, meaning distance information, but not for lateral guidance. So, VOR + GPS (distance info only) = Legal approach.

Again, my interpretation.

 

[gismo]

My interpretation is that you can use the GPS in lieu of the DME portion, meaning distance information, but not for lateral guidance. So, VOR + GPS (distance info only) = Legal approach.

Again, my interpretation.

First, where does anything from the FAA say that, and second how you do you resolve that with the AIM that gives a procedure for flying a DME arc?

 

[jdwatson]

First, where does anything from the FAA say that, and second how you do you resolve that with the AIM that gives a procedure for flying a DME arc?

To me, I am following the AIM procedure. I'm using the GPS "tuned" to the facility providing the DME, in this case the VOR. I guess what I was trying to distinguish, abeit poorly, that GPS is for range and whatever other navaid will provide guidance. You can't use the GPS to fly the needles for VOR-DME.

How do you interpret the AIM for a VOR-DME, as might be used in the the example here KMTV ?

I'm sure the FAA would never is the imprecise language that I used here. No quoting here. I was merely stating my boiled down, unofficial, non-binding, non-professional, non-CFII, personal interpretation of what I read in the AIM. I have no authritative voice on this, just my humble interpretation.

 

[poadeleted20]

First, where does anything from the FAA say that,

It says that in an email from Leslie Smith in AFS-400 dated Aug 5, 2005.

"The AIM language, 'Charted requirements for ADF and/or DME can be met using the GPS system, except for the use as the principal instrument approach navigation source.', was intended to mean that GPS could not be used as a substitute for the 'principal instrument approach navigation source' used for lateral guidance."
​

and second how you do you resolve that with the AIM that gives a procedure for flying a DME arc?

You can fly a DME arc with GPS, but not on an SIAP.

 

[kath]

Would it be fair to make this statement:

If the DME is telling you when you can step down, when you've arrived at a fix, or when you should go missed, etc... then you can substitute your GPS.

If the DME is telling you whether to turn left or right, then you can't. That's what you mean by "lateral guidance", yes?


--Kath

 

[gismo]

It says that in an email from Leslie Smith in AFS-400 dated Aug 5, 2005.

"The AIM language, 'Charted requirements for ADF and/or DME can be met using the GPS system, except for the use as the principal instrument approach navigation source.', was intended to mean that GPS could not be used as a substitute for the 'principal instrument approach navigation source' used for lateral guidance."​

​

Would the DME providing distance info for an arc leading to a VOR based final approach course be the "principal instrument approach navigation source"?

You can fly a DME arc with GPS, but not on an SIAP.

What other DME arcs are there?

 

[jdwatson]

If the DME is telling you whether to turn left or right, then you can't. That's what you mean by "lateral guidance", yes?

That what I meant, non-authoritatively of course.

[/SIZE][/I][/INDENT]Would the DME providing distance info for an arc leading to a VOR based final approach course be the "principal instrument approach navigation source"?

Lance, my assumption was the VOR and DME were either co-located or the source of the DME is in the GPS database. The quoted e-mail does seem to clarify that we can't use GPS for lateral guidance unless GPS/RNAV is a part of the approach.

When I fly an DME arc. I put the DME source in the KLN-94, then tune NAV1 to the same source. Then "OBS" my way around the arc keeping the distance on the KLN-94.

Does this agree with your understanding on how flying DME arcs is done with GPS in lieu of DME ?

 

[MauleSkinner]

When I fly an DME arc. I put the DME source in the KLN-94, then tune NAV1 to the same source. Then "OBS" my way around the arc keeping the distance on the KLN-94.

Does this agree with your understanding on how flying DME arcs is done with GPS in lieu of DME ?

IMO, that would be using GPS in lieu of DME for lateral guidance, which you agree is not allowed...you're on the SIAP, so the arc would have to be part of the approach in the GPS database, with course guidance for the arc available to your HSI/CDI.

I've never flown a DME arc using a KLN94...I'll have to check that out next time I'm in our Baron, but I've done it with Garmin 530s...switch to GPS Steering, engage the autopilot, and watch the airplane fly a really nice arc.

Fly safe!

David

 

[jdwatson]

IMO, that would be using GPS in lieu of DME for lateral guidance, which you agree is not allowed...you're on the SIAP, so the arc would have to be part of the approach in the GPS database, with course guidance for the arc available to your HSI/CDI.

Drat... and english is my first language. I was trying to say that one I found the DME from the VOR, I would be using NAV #1 OBS to navigate around the arc while keeping Xnm from the VOR via the KLN-94 displaying the distance. To me, the DME arc approach is an odd bird in that lateral guidance is a blend of distance and path.

I think I should become one with silence for a while. Most of my attempts to explain haven't had much clarity.

 

[MauleSkinner]

Drat... and english is my first language.

Well, they say that you're really comfortable with a second language when you start to dream in that language. My dreams didn't have any dialogue at all until I was in my 30's...just pictures.

Does that mean that English is my second language?

Fly safe!

David

 

[poadeleted20]

Would it be fair to make this statement:

If the DME is telling you when you can step down, when you've arrived at a fix, or when you should go missed, etc... then you can substitute your GPS.

If the DME is telling you whether to turn left or right, then you can't. That's what you mean by "lateral guidance", yes?

Yes, I think that's a fair translation of what AFS-400 said.

[/size][/i][/indent]Would the DME providing distance info for an arc leading to a VOR based final approach course be the "principal instrument approach navigation source"?

Yes. You are not allowed to use IFR GPS to fly DME arcs on a SIAP.

What other DME arcs are there?

I suppose one might be asked to fly a DME arc in an enroute or terminal phase of flight, although I can't point to any specific examples.

IMO, that would be using GPS in lieu of DME for lateral guidance, which you agree is not allowed...

OTOH, in a plane with GPS but no DME, there's no reason not to do this as a training exercise for flying DME arcs on a practice approach under VFR.

 

[ejensen]

I suppose one might be asked to fly a DME arc in an enroute or terminal phase of flight, although I can't point to any specific examples.

I was once assigned a 30 mile DME arc while enroute. KC135/C130s were refueling over the VOR. It was that or climb to 15000.

 

[jdwatson]

Ok...
I'm trying to wrap my thick skull around this... Let's take the KMTN VOR DME RWY 15 approach as an example. I'm south-east bound on V268. I put the BAL VOR in the KLN-94, and switch NAV1 from EMI to BAL. My CDI is in NAV mode. At 14.7nm on the 334 radial of BAL, we're at SLOAF. I turn left, twist the OBS from 334 to 340. At the 340 radial, I notice the KLN-94 reports my distance from BAL is 13.7nm. I turn slightly left away from BAL, twist the OBS to 350... and so on.

I think I'm getting my lateral guidance from the NAV1's CDI. I use the DME to figure if I need to change my course but the guidance, meaning to me the instrument (source) I'm using to navigate with, is coming from the VOR and not the GPS.

Please pound into my head how the DME is providing lateral guidance... pretty please ? I'm coming from a mindset that the CDI selector on a late model 172R is how I choose what source I'm receiving guidance from. My choices are NAV or GPS on that button.

I'm trying to keep the example simple as I can. I'm not trying to be argumentative. I want to understand this. I agree that DME arcs are getting rarer all the time. Just help me understand.

The quoted e-mail tried to clarify the point by saying:

snip ...was intended to mean that GPS could not be used as a substitute for the 'principal instrument approach navigation source' used for lateral guidance."

 

[Bill]

I think I'm getting my lateral guidance from the NAV1's CDI. I use the DME to figure if I need to change my course but the guidance, meaning to me the instrument (source) I'm using to navigate with, is coming from the VOR and not the GPS.

Please pound into my head how the DME is providing lateral guidance... pretty please ? I'm coming from a mindset that the CDI selector on a late model 172R is how I choose what source I'm receiving guidance from. My choices are NAV or GPS on that button.

I'm just a dumb instrument student, but the way I see it, your OBS radial off the VOR is providing longitudenal guidance, ie., where you are along the arc. The DME is providing lateral guidance, ie., are you left/right/center with respect to the arc. (Or in other words, inside/outside/on the arc)

 

[jdwatson]

I'm just a dumb instrument student, but the way I see it, your OBS radial off the VOR is providing longitudenal guidance, ie., where you are along the arc. The DME is providing lateral guidance, ie., are you left/right/center with respect to the arc. (Or in other words, inside/outside/on the arc)

Bill, I don't disagree with you. We navigate primarily by the CDI, the GPS isn't driving the CDI so it's not the primary navigational instrument. Therefore, we can use it for DME !

Another way to look at it, is the DME number on an arc doesn't tell you when you are at a particular point, the radial on that arc does. DME only tells you which arc you are navigating along. So in terms of primary navigation, the VOR is what you are using to get some place.

Hence, my confusion. BTW, how's the final polish coming along for your 9-Dec checkride ?

 

[MauleSkinner]

Another way to look at it, is the DME number on an arc doesn't tell you when you are at a particular point, the radial on that arc does. DME only tells you which arc you are navigating along. So in terms of primary navigation, the VOR is what you are using to get some place.

You are not navigating "a particular point". You are navigating an arc, which is defined by distance from a navaid. The entry and exit points of the arc are important, but the primary source of navigation is the arc defined by DME.

Consider the same situation for a straight flight along a VOR radial. The CDI doesn't tell you where you are along the line, it tells you whether you are ON the line. This is primary navigation. You have to have cross-radials or DME to tell you at what point you are on that line, but the cross-radials and DME are not primary navigation.

Now, back to arcs...Here's how I fly them, which may give a little perspective to the above comments...

I navigate to the arc using whatever means is prescribed. I turn onto the arc, and use ONLY DME to fly the arc. When the DME increases to the desired arc distance, I turn 10 degrees. The DME reading increases maybe another tenth or two, then decreases a couple of tenths below the arc distance, then increases again. When it increases to the arc distance, I turn another 10 degrees and repeat the process.

Meanwhile, my HSI or CDI is turned to the final approach course (or lead-in radial, if published), and I wait for it to come in. That's the ONLY radial I care about, and until that needle moves off the peg, DME is the primary source of course guidance.

Fly safe!

David

 

[Bill]

Hence, my confusion. BTW, how's the final polish coming along for your 9-Dec checkride ?

My wife has been on a business trip this week, so I've been Mr Mom all week. Chasing kids, haven't had time to study or fly, but starting this weekend, back at it!

Meanwhile, my HSI or CDI is turned to the final approach course (or lead-in radial, if published), and I wait for it to come in. That's the ONLY radial I care about,

Interesting. My II wants me to be tracking the radial I am on using nav #2, while leaving nav #1 set to the final course or LR. Just different ways I suppose. He says your course to fly (to stay on the arc) should be + or - 100 degrees or so from the radial you are on.

In reality, I kind of use a combination of both, use the radial to suggest the course, and modify it by the actual DME reading.

 

[jdwatson]

I've been sufficiently convinced offline that if DME is in the title, use DME not GPS. Good thing these kind of approaches are becoming more rare.

The KMTN approach looks very cool. Another one that looks cool to me is JFK's Parkway Visual RWY 13L/R... Lead-In lights.

 

[MauleSkinner]

My II wants me to be tracking the radial I am on using nav #2, while leaving nav #1 set to the final course or LR. Just different ways I suppose. He says your course to fly (to stay on the arc) should be + or - 100 degrees or so from the radial you are on.

Doing math while on an approach is great, but I prefer differential calculus

Fly safe!

David

 

[ScottM]

Doing math while on an approach is great, but I prefer differential calculus

Fly safe!

David

I am more of a 3d vector calc person myself.

 

[poadeleted20]

I'm trying to wrap my thick skull around this... Let's take the KMTN VOR DME RWY 15 approach as an example.

There is no such approach. Are you referring to the VOR/DME OR TACAN Z RWY 15 approach? I'll assume so...

I'm south-east bound on V268. I put the BAL VOR in the KLN-94, and switch NAV1 from EMI to BAL. My CDI is in NAV mode. At 14.7nm on the 334 radial of BAL, we're at SLOAF. I turn left, twist the OBS from 334 to 340. At the 340 radial, I notice the KLN-94 reports my distance from BAL is 13.7nm. I turn slightly left away from BAL, twist the OBS to 350... and so on. I think I'm getting my lateral guidance from the NAV1's CDI.

No -- you're getting your lateral guidance from the GPS, as that's telling you whether you're left or right of the desired track.

I use the DME to figure if I need to change my course but the guidance, meaning to me the instrument (source) I'm using to navigate with, is coming from the VOR and not the GPS.

If you have DME, you use that as primary, not the GPS, to determine whether you're on the arc or not.

Please pound into my head how the DME is providing lateral guidance... pretty please ?

The DME tells you whether you're on the 14NM arc or not, and you turn left or right based on that information.

I'm just a dumb instrument student, but the way I see it, your OBS radial off the VOR is providing longitudenal guidance, ie., where you are along the arc. The DME is providing lateral guidance, ie., are you left/right/center with respect to the arc. (Or in other words, inside/outside/on the arc)

Exactly. And you're not "dumb" at all.

I've been sufficiently convinced offline that if DME is in the title, use DME not GPS. Good thing these kind of approaches are becoming more rare.

No reason to let this chase you off using an IFR GPS to fly an approach such as BWI's VOR/DME 15L approach if you have no DME.

 

[jdwatson]

There is no such approach. Are you referring to the VOR/DME OR TACAN Z RWY 15 approach? I'll assume so...

JeppView has VOR DME Rwy 15. KMTN/MTN Plate 23-1, 23-May-2003, Amend 0
I'm on Cycle 23-2005, 18-Nov-2005 Issue date.

It seems to be identical as the approach you reference above. I'll e-mail Jeppesen to see why they named this approach differently than the NACO chart you've referenced.

I'll e-mail Jeppesen to see why they named this approach differently than the NACO chart you've referenced.

Here's the reply from Jeppesen's Advanced Navigation and Display Support:

We do not include TACAN procedures in our manuals as few civil aircraft are TACAN azimuth equipped. Back when we did, it created some problems for arriving aircraft who found they could not accept the approach.

 

[TangoWhiskey]

I think the key this is in part of what you quoted from the AIM:

f. Use of GPS in lieu of ADF and DME

1. Subject to the restrictions below, operators in the U.S. NAS are authorized to use GPS equipment certified for IFR operations in place of ADF and/or DME equipment for en route and terminal operations. For some operations there is no requirement for the aircraft to be equipped with an ADF or DME receiver, see subparagraphs f6(g) and (h) below. The ground-based NDB or DME facility may be temporarily out of service during these operations. Charting will not change to support these operations.

(a) Operations allowed:

(2) Flying a DME arc.

As Ron said, it's not valid on a SIAP. Enroute, or terminal ops, sure. But not on an approach that doesn't have GPS named in the approach title. The instructions that follow, for flying an arc, are if the arc is flown enroute, ir in the terminal environment (feeder to an approach, for example (true?? I'm asking, not saying...)), but not for the SIAP itself.

Bill, I don't disagree with you. We navigate primarily by the CDI, the GPS isn't driving the CDI so it's not the primary navigational instrument. Therefore, we can use it for DME !

Another way to look at it, is the DME number on an arc doesn't tell you when you are at a particular point, the radial on that arc does. DME only tells you which arc you are navigating along. So in terms of primary navigation, the VOR is what you are using to get some place.

Hence, my confusion. BTW, how's the final polish coming along for your 9-Dec checkride ?