GPS Distance Intead of DME

[ARFlyer]

I been taught that you can use the distance information on our IFR GPS in lieu of DME. Also as a secondary way to id a point on a approach plate. However I have discovered this is a very gray topic among pilots. I assume you can but what about slant distance problems? As we all know DME measures in slant range while GPS measures in a straight line. So wouldn't this cause a problem very close and low on an approach?

 

[gismo]

I been taught that you can use the distance information on our IFR GPS in lieu of DME. Also as a secondary way to id a point on a approach plate. However I have discovered this is a very gray topic among pilots. I assume you can but what about slant distance problems? As we all know DME measures in slant range while GPS measures in a straight line. So wouldn't this cause a problem very close and low on an approach?

Slant range is generally a non-issue. When you're high enough that it might result in a significant error, the consequences of being off by a few tenths of a NM are zilch.

 

[ARFlyer]

Slant range is generally a non-issue. When you're high enough that it might result in a significant error, the consequences of being off by a few tenths of a NM are zilch.

Do I have it backwards? I thought the closer you are to the DME point, aka VOR/DME, the more error in the distance. I.E. DME will stay at 1NM if your ALT is 1NM.

We do not have DME in our aircraft. So my DME knowledge is a gathering from classes and stuff I read.

 

[denverpilot]

I thought only a named fix was substitutable, not just the distance itself.

Example: CASSE can be identified by multiple methods in this Approach.

- 8.1 DME from the ILS/LOC frequency pairing.
- GPS fix named CASSE.
- ADF station passage.
- Co-located Marker Beacon.

Moving my chart zoom a little further north, you can also see there's a cross-radial off of FQF that can be useful.

But not by a GPS distance from something. Not that I'm aware of anyway.

Can you give a specific example of where you're attempting to substitute a DME *distance* and not a specific fix with GPS?

I'm curious and new enough at this that I'm quite likely to be wrong.

Perhaps one of the CFIIs or brilliant people will be along to correct me on this one.

 

[ARFlyer]

An example would be DME ARC for the PBF VOR/DME 36. Using the GPS distance as the DME for the arc. Or to id ZAPLE IAF on the HOT VOR 5. I would post a screenshot but I do not know how to.

 

[denverpilot]

Do I have it backwards? I taught the closer you are to the DME point, aka VOR/DME, the more error in the distance. I.E. DME will stay at 1NM if your ALT is 1NM.

We do not have DME in our aircraft. So my DME knowledge is a gathering from classes and stuff I read.

This part is definitely correct. You'll also often see wonky groundspeed information on the DME at that point.

Typically if you're high enough to cross the fix 5000' above it, you're not worried too much about the slant range error saying you're 1.0 mile away since you're usually looking to pass a fix and turn, up that high.

If the DME says 1.0 when you're over the station and the VOR flag flips (and the DME starts counting back up), you're past the fix and should have already started the turn. The VOR flag wins.

The DME counting up is backup/supporting information confirming it.

The "cone of confusion" directly overhead a VOR station is covered very well in texts, but little discussion is made in the books about the fact that DME antennas also have a very small and very sharp null directly above them if they're a typical vertical UHF antenna. Both the ground station and the aircraft antenna have these nulls.

You can sometimes see a DME "freak out" directly over the station too, similar to the VOR receiver. Not mentioned much in the books.

Note also that DME antennas are almost always mounted on the bottom of the aircraft for this same reason.

On top there'd be a big dead null caused by the body of the airframe and/or the wings when directly over the DME ground station.

 

[denverpilot]

An example would be DME ARC for the PBF VOR/DME 36. Using the GPS distance as the DME for the arc. Or to id ZAPLE IAF on the HOT VOR 5. I would post a screenshot but I do not know how to.

Ahh yeah.

That far out the error becomes negligible and you're not looking for 0.1 mile precision like you are once closer in on an approach, say for a missed approach point.

How much protected airspace for terrain and obstructions is allowed either side of a DME arc?

The answer to that will make it clear as to why it works "out there" but not as well in close.

 

[ARFlyer]

I did not know about the null area for DME antennas.

I been told that if your GPS is IFR approved and current you can do any approach that requires a DME. Titled DME or stated in the plan view. BUT that you CANNOT do an NDB approach with only a GPS.

 

[denverpilot]

Note that once you're inbound from the arc on that PBF plate, everything is named and is a real waypoint that you can put in the GPS.

At the distance and altitude you're on the arc itself, the error is negligible. Inside the arc you're using named fixes.

I like your ILS plate there. That looks fun!

 

[ARFlyer]

Note that once you're inbound from the arc on that PBF plate, everything is named and is a real waypoint that you can put in the GPS.

I like your ILS plate there. That looks fun!

Ok that make sense. The protected area is four miles on a DME ARC? Haha thankfully I have never had to fly that approach. We use HOT ILS 5 since our airport is M89. It gets fun when your doing VOR 5 outbound and some one is HOT ILS 5 inbound. The VOR is located on the field and basically you are back tracking the extended centerline.

 

[denverpilot]

Ok that make sense. The protected area is four miles on a DME ARC? Haha thankfully I have never had to fly that approach. We use HOT ILS 5 since our airport is M89. It gets fun when your doing VOR 5 outbound and some one is HOT ILS 5 inbound. The VOR is located on the field and basically you are back tracking the extended centerline.

You got it! Being 0.1 off the arc because of slant range error (I didn't do the math on the error at that altitude/distance but it's small), isn't gonna put you in any danger, therefore the substitution is allowed.

It's too late at night for Trigonometry.

There's a lot of those "we're outbound climbing, you're inbound descending" setups happen regularly in the training environment VFR, doing practice approaches, I've noticed. Real IFR, they wouldn't do it without a solid altitude separation until the targets had passed each other.

Jesse wisely broke off one of those once on one of our approaches in Nebraska when we ended up head to head with someone. "Turn left 30 degrees for traffic."

It's also a great way to set up the question, "What's your minimum safe altitude here and where are you exactly? How far can you fly this direction?" Etc. Ahh distractions.

Which will make you fumble around and look at the MSA ring on the chart if you forgot to brief it. And twiddle with one of the Nav heads. And realize you didn't start a timer. And then thank the evil Instructor angels that this particular approach has DME and it's on the field, so at least you know how far out you are on your mystery vector.

(Yep. Ask me how I know this! Grin!)

And then the next part, "Okay, we're clear of traffic, Skylane 79M you're cleared via own navigation to the [insert approach here] and cleared for the approach."

And you figure out how to get back...

We also had one where we were northwest of KLNK VFR and KLNK approach cleared someone else for the VOR 18 underneath us while we were given "make straight in for 17".

We were both eyeballs-locked on the aircraft only a few hundred feet below us, converging, roughly the same speed once Jesse spotted him first.

We even had enough time to talk about what we'd do if he went missed early and climbed into us for some reason.

Even once we'd swapped sides, his published missed would turn him back into us if he'd have done it early.

We thought that was a bit of a strange situation to put us both in. Us for the left runway, him for the right, crossing, us without an altitude restriction to stay higher at first.

We made up our own restriction since we had been flying that approach and knew what altitude he was *supposed* to be at.

It was night and he was fairly visible after Jesse picked him out of the lights of Lincoln.

With nearly matching speeds there wasn't much "movement" for the eye to pick out. It was more like seeing him because his lights weren't moving and the background lights were. Eerie.

Converging targets at night. Not the smartest thing I've ever seen. If I hadn't been flying that approach and the VOR 17 all week and was just tooling along VFR, I might have sauntered down to his altitude or a lot closer to it by thinking I should be something near pattern altitude a couple miles out.

I'm sure the controller would have straightened it out, but one missed radio call... and enough airtime blocked with "Say again?" would've made that into a close call.

 

[Palmpilot]

I thought only a named fix was substitutable, not just the distance itself.

Per AC 90-108, Paragraph 7a(1), you can use an IFR GPS (or any "suitable RNAV system") to locate a DME fix or a named fix. Per Paragraph 9b, the location of the navaid (i.e., the DME station in this case) has to be in your GPS database.

As far as I know, the only things you can't substitute for in general are procedures notamed "NA," the lateral guidance on a final approach segment, or lateral guidance on any portion of a localizer-based course. (See section 8.)

 

[flyingmoose]

I been taught that you can use the distance information on our IFR GPS in lieu of DME. Also as a secondary way to id a point on a approach plate. However I have discovered this is a very gray topic among pilots. I assume you can but what about slant distance problems? As we all know DME measures in slant range while GPS measures in a straight line. So wouldn't this cause a problem very close and low on an approach?

You will have more error when you are "Close and High" not "Close and Low" Like others said, if you are on the approch segment then the slant range errors will not be an issue because you are so low. Hope that helps

 

[poadeleted20]

Per AC 90-108, Parag raph 7a(1), you can use an IFR GPS (or any "suitable RNAV system") to locate a DME fix or a named fix. Per Paragraph 9b, the location of the navaid (i.e., the DME station in this case) has to be in your GPS database.

You can still sub without the DME station being in your database, e.g., on a LOC/DME approach, leaving your non-approach GPS on the FAF and counting up.

As far as I know, the only things you can't substitute for in general are procedures notamed "NA," the lateral guidance on a final approach segment, or lateral guidance on any portion of a localizer-based course. (See section 8.)

That's pretty much it in a nutshell.

 

[roncachamp]

I been taught that you can use the distance information on our IFR GPS in lieu of DME. Also as a secondary way to id a point on a approach plate. However I have discovered this is a very gray topic among pilots. I assume you can but what about slant distance problems? As we all know DME measures in slant range while GPS measures in a straight line. So wouldn't this cause a problem very close and low on an approach?

Not really, close and high is when it becomes significant. Let's look at JUKOB on the LOC BC RWY 24 approach at KGRB as an example. The actual distance from the DME antenna to JUKOB is 2998'. The MDA is 1100 MSL, the elevation of the DME antenna is 702 MSL, so the height above the antenna is 398'. Pythagoras tells us the slant distance is 3024', just 26' more than the actual distance.

 

[COFlyBoy]

Page 7-29 of the Instrument Flying Handbook describes the acceptable substitution of GPS for DME and ADF.

http://www.faa.gov/library/manuals/aviation/instrument_flying_handbook/

Basically, they do not allow the substitution for anything inside a FAF, so that any delta is negligible.

 

[TMetzinger]

Page 7-29 of the Instrument Flying Handbook describes the acceptable substitution of GPS for DME and ADF.

http://www.faa.gov/library/manuals/aviation/instrument_flying_handbook/

Basically, they do not allow the substitution for anything inside a FAF, so that any delta is negligible.

No, they don't allow it for lateral navigation, but you can certainly use it for DME substitution, on an VOR/LOC/ILS approach for example, inside the FAF.

 

[roncachamp]

Do I have it backwards? I thought the closer you are to the DME point, aka VOR/DME, the more error in the distance. I.E. DME will stay at 1NM if your ALT is 1NM.

Your theory is correct, but you just don't find DME fixes close enough to the source for the error to be significant. IIRC the closest DME fixes on Victor airways are seven miles, approach fixes less than a mile from the source exist but aircraft are quite low at that point. At one mile from and 1000' above the DME source the slant range error is just 82'. With approach fixes rounded off to the nearest tenth of a mile it's a non-issue.

 

[bobmrg]

I did not know about the null area for DME antennas.

I been told that if your GPS is IFR approved and current you can do any approach that requires a DME. Titled DME or stated in the plan view. BUT that you CANNOT do an NDB approach with only a GPS.

I'm a little disturbed by your reliance on what you have been taught and what you have been told. Not knocking the idea of believing your instructor, but the idea of accepting what he/she says without question. ALWAYS ask for documentation when a subject like this comes up. Get it in writing.

It is amazing (and frightening) how much bad information comes from the instructor ranks, and the cause is usually because the instructor is just parroting what s/he has been told without doing any independent research.

Bob Gardner

 

[Artiom]

I'm a little disturbed by your reliance on what you have been taught and what you have been told. Not knocking the idea of believing your instructor, but the idea of accepting what he/she says without question. ALWAYS ask for documentation when a subject like this comes up. Get it in writing.

It is amazing (and frightening) how much bad information comes from the instructor ranks, and the cause is usually because the instructor is just parroting what s/he has been told without doing any independent research.

Bob Gardner

Bob, I agree with your comment in general, however this particular subject (using GPS in lieu of DME) is kind of gray area.
I looked at FAR/AIM and didn't find direct answer to this question so I had to discuss it with my CFII to figure it out.

 

[ARFlyer]

Page 7-29 of the Instrument Flying Handbook describes the acceptable substitution of GPS for DME and ADF.

http://www.faa.gov/library/manuals/aviation/instrument_flying_handbook/

Basically, they do not allow the substitution for anything inside a FAF, so that any delta is negligible.

I will look at this page as I have that book in print.

I'm a little disturbed by your reliance on what you have been taught and what you have been told. Not knocking the idea of believing your instructor, but the idea of accepting what he/she says without question. ALWAYS ask for documentation when a subject like this comes up. Get it in writing.

It is amazing (and frightening) how much bad information comes from the instructor ranks, and the cause is usually because the instructor is just parroting what s/he has been told without doing any independent research.

Thats the reason behind me starting this thread. As my flight hours and knowledge increases I have become aware that there is stuff I have been taught wrong.

I have had five CFIs over the past two years. Three for Private and Two for Instrument. Every one of them always corrects me on what the other taught me. So I am having to change my style and procedures for every CFI just to keep them quite in the right seat.

I agree with your comment in general, however this particular subject (using GPS in lieu of DME) is kind of gray area.

Everyone I have asked this at my flight school gives me a different answer and a different place to find it. So I agree with you that this is a gray area.

 

[Jaybird180]

I'm a little disturbed by your reliance on what you have been taught and what you have been told. Not knocking the idea of believing your instructor, but the idea of accepting what he/she says without question. ALWAYS ask for documentation when a subject like this comes up. Get it in writing.

It is amazing (and frightening) how much bad information comes from the instructor ranks, and the cause is usually because the instructor is just parroting what s/he has been told without doing any independent research.

Bob Gardner

....and the correct answer is....?

 

[olasek]

BUT that you CANNOT do an NDB approach with only a GPS.

I think if you look carefully you will see that great majority of NDB approaches in the US are in fact named NDB (or GPS) so you can in fact use an approach certified GPS instead of NDB.

 

[Ghery]

I think if you look carefully you will see that great majority of NDB approaches in the US are in fact named NDB (or GPS) so you can in fact use an approach certified GPS instead of NDB.

And when they aren't, don't use the GPS. NDB 35 to TIW comes to mind.

 

[John Collins]

I think if you look carefully you will see that great majority of NDB approaches in the US are in fact named NDB (or GPS) so you can in fact use an approach certified GPS instead of NDB.

That used to be the case where most NDB approaches were also overlay approaches, that is, they had a title of "NDB or GPS", but it is no longer true. I just did a search on approaches with the title "NDB or GPS" and there are 121 in the inventory out of the 876 NDB approaches still around. As RNAV approaches have been added to the inventory, the GPS overlays have been deleted. They have also gone away whenever the NDB was decommissioned which is happening a lot these days. They won't be around much longer.

 

[John Collins]

Thats the reason behind me starting this thread. As my flight hours and knowledge increases I have become aware that there is stuff I have been taught wrong.

I have had five CFIs over the past two years. Three for Private and Two for Instrument. Every one of them always corrects me on what the other taught me. So I am having to change my style and procedures for every CFI just to keep them quite in the right seat.

Everyone I have asked this at my flight school gives me a different answer and a different place to find it. So I agree with you that this is a gray area.

Recently, the AIM consolidated the GPS in lieu of DME in section 1-2-3 - Use of Suitable Area Navigation (RNAV) Systems on Conventional Procedures and Routes.

 

[TMetzinger]

That used to be the case where most NDB approaches were also overlay approaches, that is, they had a title of "NDB or GPS", but it is no longer true. I just did a search on approaches with the title "NDB or GPS" and there are 121 in the inventory out of the 876 NDB approaches still around. As RNAV approaches have been added to the inventory, the GPS overlays have been deleted. They have also gone away whenever the NDB was decommissioned which is happening a lot these days. They won't be around much longer.

How many of those are in the databases?

I'd swear that I've seen approaches called NDB XXX and they're in the GPS DB and can be flown using the GPS. I thought that in later phases of the program they dropped the "(or GPS)" on the plates and just coded the approach in the database.

 

[John Collins]

How many of those are in the databases?

I'd swear that I've seen approaches called NDB XXX and they're in the GPS DB and can be flown using the GPS. I thought that in later phases of the program they dropped the "(or GPS)" on the plates and just coded the approach in the database.

Actually, in the later phases of the overlay program, they added the or GPS to all the titles instead of (GPS).

NDB, ILS, Localizer, and most VOR or VOR-DME approaches that can't be flown with GPS are still in the Garmin GNS, GTN, and G1000 databases. The only ones that can be flown with the GPS have or GPS in the title on the chart and a small GPS on a diagonal at the end of the approach name in the database for the GNS series.

 

[poadeleted20]

And when they aren't, don't use the GPS. NDB 35 to TIW comes to mind.

Well, you can use the GPS, but to be legal, you have to have an ADF operating and tuned up to cross-check while you do it.

 

[TMetzinger]

Actually, in the later phases of the overlay program, they added the or GPS to all the titles instead of (GPS).

NDB, ILS, Localizer, and most VOR or VOR-DME approaches that can't be flown with GPS are still in the Garmin GNS, GTN, and G1000 databases. The only ones that can be flown with the GPS have or GPS in the title on the chart and a small GPS on a diagonal at the end of the approach name in the database for the GNS series.

OK thanks!

 

[Ghery]

Well, you can use the GPS, but to be legal, you have to have an ADF operating and tuned up to cross-check while you do it.

And to do that you have to have an ADF receiver in the panel. And we took that out when the 430W went in. So, while the approach is in the database in the 430W you can't use it in that plane. Oh well...

 

[poadeleted20]

And to do that you have to have an ADF receiver in the panel. And we took that out when the 430W went in. So, while the approach is in the database in the 430W you can't use it in that plane. Oh well...

FWIW, I don't think there's anywhere with an NDB-only approach that doesn't have a GPS approach to the same runway, usually with lower mins, so that lack of an ADF shouldn't be a hindrance.

 

[John Collins]

FWIW, I don't think there's anywhere with an NDB-only approach that doesn't have a GPS approach to the same runway, usually with lower mins, so that lack of an ADF shouldn't be a hindrance.

The only place that I know that has a NDB approach as the only approach and doesn't have a GPS overlay is at Union County, SC (35A). They are on the production schedule for January 2013 to get a pair of RNAV approaches, but my guess is that this is just a place holder. So for now, I am not equipped to fly the approach. BTW, they normally have great fuel prices for our area.

 

[bobmrg]

....and the correct answer is....?

...to ask the instructor to show you the section of the AIM (or other FAA pub) that supports his/her answer.

Bob

 

[gismo]

Well, you can use the GPS, but to be legal, you have to have an ADF operating and tuned up to cross-check while you do it.

Heck, in the pre-GPS days I often tracked my LORAN signal while flying an NDB approach (with the ADF tuned, identified, and monitored) and amazingly the ADF needle always showed us right on course.