DME and IFR

fiveoboy01

fiveoboy01

Pattern Altitude
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Apr 21, 2013
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Madison, WI
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Dirty B
I'm at a decision point with my DME. Options are to leave it, fix it, replace with a new/used unit, or remove it altogether.

I know in some(all?) cases if there's a WAAS GPS onboard, distance information can be used from it in lieu of a DME. I'm not sure if that's all the time, or in just some instances/approaches.

My question is, am I giving up any ability to do anything if I ditch the DME? Is it wise to keep it for redundancy?

Not that it matters but the issue is that the display, whilst not burned out, is dying. Some digits are barely readable.

Airplane is equipped with a 430W.
 
fiveoboy01 said:
I'm at a decision point with my DME. Options are to leave it, fix it, replace with a new/used unit, or remove it altogether.

I know in some(all?) cases if there's a WAAS GPS onboard, distance information can be used from it in lieu of a DME. I'm not sure if that's all the time, or in just some instances/approaches.

My question is, am I giving up any ability to do anything if I ditch the DME? Is it wise to keep it for redundancy?

Not that it matters but the issue is that the display, whilst not burned out, is dying. Some digits are barely readable.

Airplane is equipped with a 430W.
Click to expand...

If you ditch the DME you give up the ability to fly the VOR/DME RWY 15 approach at Martin State Airport in Baltimore.
 
I've tried "DME required" approaches with a GTN650, both with and without a real DME.

There are two (fairly small) things you give up:

1. You have to program the GPS specially (with "direct-to") if you want to just read out the DME. Or you can overlay the approach, but then you have to do the math for announcing distance to the airport and planning descents. It's real nice to have both. There is also the issue that DMEs read slant range distance (and that's how a XXX/DME approach is labeled), but the GPS reads 2D distance. That might differ close to the navaid if you're at high altitude.

2. With a LOC/DME, you can't use the GPS to give you DME directly if the DME is from the localizer. Localizers aren't in the Garmin database. You can, however, use the overlay approach and rely on named waypoints.

There are ways to do everything, but it isn't always the simplest. It's nice to just have a unit whose job it is to give you distance.
 
fiveoboy01 said:
I'm at a decision point with my DME. Options are to leave it, fix it, replace with a new/used unit, or remove it altogether.

I know in some(all?) cases if there's a WAAS GPS onboard, distance information can be used from it in lieu of a DME. I'm not sure if that's all the time, or in just some instances/approaches.

My question is, am I giving up any ability to do anything if I ditch the DME? Is it wise to keep it for redundancy?

Not that it matters but the issue is that the display, whilst not burned out, is dying. Some digits are barely readable.

Airplane is equipped with a 430W.
Click to expand...

I think the display fix was something like $250 on my 62A done about 6 years ago. I didn't have a panel mount GPS at the time so it was a no-brainer then. I still use it since it helps me keep track of where I am under DEN's class B shelves. When I'm not under the Bravo I generally turn it off.
 
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MAKG1 said:
I've tried "DME required" approaches with a GTN650, both with and without a real DME.

There are two (fairly small) things you give up:

1. You have to program the GPS specially (with "direct-to") if you want to just read out the DME. Or you can overlay the approach, but then you have to do the math for announcing distance to the airport and planning descents. It's real nice to have both. There is also the issue that DMEs read slant range distance (and that's how a XXX/DME approach is labeled), but the GPS reads 2D distance. That might differ close to the navaid if you're at high altitude.

2. With a LOC/DME, you can't use the GPS to give you DME directly if the DME is from the localizer. Localizers aren't in the Garmin database. You can, however, use the overlay approach and rely on named waypoints.

There are ways to do everything, but it isn't always the simplest. It's nice to just have a unit whose job it is to give you distance.
Click to expand...

#2 was my main concern. I assume that not being able to read DME off the localizer could hinder my ability to shoot an ILS/LOC approach. Some GPS approaches get you down as low if there's LPV service, but not all.
 
fiveoboy01 said:
#2 was my main concern. I assume that not being able to read DME off the localizer could hinder my ability to shoot an ILS/LOC approach. Some GPS approaches get you down as low if there's LPV service, but not all.
Click to expand...

You CAN use the overlay approach, which will name all the waypoints. Just make sure the CDI is on VLOC no later than the FAF and that you switched the standby nav frequency (and verify the GPS did actually load the correct localizer frequency).
 
I've actually done that quite a few times practicing an ILS/LOC approach but I've always had my attention focused on the DME and needles and never paid too much attention to the GPS once on the final approach course.

Guess I still have a LOT to learn lol.
 
fiveoboy01 said:
I'm at a decision point with my DME. Options are to leave it, fix it, replace with a new/used unit, or remove it altogether.
Click to expand...

Ditch it. Ten years ago when I put the GNS480 in my plane I left the KN64 because it was relatively new and didn't need the panel space. I can tell you that I've not turned it on since I got the GPS. Yeah, I can't fly the MTN (or Wallops Island) DME arc approach, but BFD. In practice, that's not the way you're going into MTN even if you can fly it.
 
MAKG1 said:
1. You have to program the GPS specially (with "direct-to") if you want to just read out the DME. Or you can overlay the approach, but then you have to do the math for announcing distance to the airport and planning descents.
Click to expand...
The moving map gives better keys that trying to intuit closure rate on some arbitrary DME point (which usually is displaced from the DME point).
It's real nice to have both. There is also the issue that DMEs read slant range distance (and that's how a XXX/DME approach is labeled), but the GPS reads 2D distance. That might differ close to the navaid if you're at high altitude.
Click to expand...
The FAA has determined the difference is not of significance on approaches.
 
fiveoboy01 said:
#2 was my main concern. I assume that not being able to read DME off the localizer could hinder my ability to shoot an ILS/LOC approach. Some GPS approaches get you down as low if there's LPV service, but not all.
Click to expand...

I assume your concern is due to slant range. The difference between that and the actual distance at 1000' AGL and one mile from the DME source is about 85 feet.
 
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roncachamp said:
I assume your concern is due to slant range. The difference between that and the actual distance at 1000' AGL and one mile from the DME source is about 85 feet.
Click to expand...
Which is about one half second at 100 Kt.
 
Walboy said:
When I installed my WAAS GPS, I ditched my KN64 and haven't regretted it one bit. I also installed a GDL88 at the same time and someone on this forum pointed out that there COULD be RF interference between the DME and GDL88.

Bottom line though, I don't miss DME

I think this is the advisory circular that applies:

http://www.faa.gov/documentLibrary/media/Advisory_Circular/90-108.pdf
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I left a KN64 in the panel when I installed a GNS480 WAAS GPS a long time ago. I rarely use it but I think it's a good backup should the GPS system fail or get jammed.

It's legal to use the GPS on any approach that required DME except the extremely rare DME arc final like the MTN VOR/DME 15. As already mentioned this occasionally requires some simple mental math or referencing WPs on the GPS but as long as you plan ahead for this it's pretty easy.
 
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roncachamp said:
Yup. A distance error of .014 NM on procedures where fixes are designated to the nearest tenth of a mile is no cause for concern.
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That's true, but not all DME fixes on all approach plates are to an on-field navaid.

I would expect there to be some standards about just how close to the navaid a DME fix can be, but it's not quite enough to say they are all negligible automatically.
 
I should have clarified by saying the giving up the ability to do any procedure that can't be done with the other working equipment in the airplane:)
 
The bottom line from the practical point of view is that by ditching the DME and getting WAAS/GPS you lose nothing as far as being able to land at any US or Canadian airport (that I am aware of). This is what I did when I put in the 750, and I haven't regretted it (or even noticed it).
Any approach that says "DME required" allows a WAAS/GPS to be substituted for the DME (ditto for NDB, btw).
 
MAKG1 said:
That's true, but not all DME fixes on all approach plates are to an on-field navaid.
Click to expand...

True. So what?

I would expect there to be some standards about just how close to the navaid a DME fix can be, but it's not quite enough to say they are all negligible automatically.
Click to expand...
In those cases, where DME fixes cannot be established because they do not meet the requirements, the displacement error between DME and GPS is not an issue.
 
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RotorDude said:
The bottom line from the practical point of view is that by ditching the DME and getting WAAS/GPS you lose nothing as far as being able to land at any US or Canadian airport (that I am aware of). This is what I did when I put in the 750, and I haven't regretted it (or even noticed it).
Any approach that says "DME required" allows a WAAS/GPS to be substituted for the DME (ditto for NDB, btw).
Click to expand...

That there answers my question quite nicely! Thank you:)

Honestly I'd just fix the DME if I didn't just spend 6K on the panel lol!
 
fiveoboy01 said:
That there answers my question quite nicely! Thank you:)

Honestly I'd just fix the DME if I didn't just spend 6K on the panel lol!
Click to expand...

Except for Stephen's example. Inside the final approach fix, you must be using the "right" nav source. Now, his example is the only one I've ever heard of that has a DME arc as a final approach segment.

You will lose very little in terms of being able to land. It is convenient to have the extra distance display. Less screen switching to do on the GPS. And almost every airport seems to have an RNAV approach these days.
 
MAKG1 said:
Except for Stephen's example. Inside the final approach fix, you must be using the "right" nav source. Now, his example is the only one I've ever heard of that has a DME arc as a final approach segment.

You will lose very little in terms of being able to land. It is convenient to have the extra distance display. Less screen switching to do on the GPS. And almost every airport seems to have an RNAV approach these days.
Click to expand...

No, that's not an exception, since with WAAS/GPS you'd shoot the RNAV approach and get in with even lower minimums.

Edit: at least on the 750, I see a running display of distance to the destination and distance to the next waypoint. I never got that on my DME, which only showed distance to the VOR. Bear in mind that in modern IFR /G routing (at least in my experience) you rarely go VOR to VOR.
 
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Unless you are flying to Mexico or the Caribbean without DME you should be fine.
 
MAKG1 said:
2. With a LOC/DME, you can't use the GPS to give you DME directly if the DME is from the localizer. Localizers aren't in the Garmin database. You can, however, use the overlay approach and rely on named waypoints.
Click to expand...

The localizer DME locations are in the common database used by the GNS, GTN, and G1000 systems. Example at my airport KUZA ILS or LOC Z RWY 2 is in the database as IUZA.
 
RotorDude said:
No, that's not an exception, since with WAAS/GPS you'd shoot the RNAV approach and get in with even lower minimums.

Edit: at least on the 750, I see a running display of distance to the destination and distance to the next waypoint. I never got that on my DME, which only showed distance to the VOR. Bear in mind that in modern IFR /G routing (at least in my experience) you rarely go VOR to VOR.
Click to expand...

An approach plate for a XXX/DME approach is labeled with distances to the navaid, not the destination and not the next waypoint.
 
roncachamp said:
It's an exception. Ditch the DME and you do lose the ability to fly the VOR/DME RWY 15 approach at Martin State Airport in Baltimore. Other available approaches do not change that.
Click to expand...

I meant it's not an exception to my practical point to which he was referring. And my point was that I am not aware of any airport in the US or Canada where you won't be able to land because you ditched your DME, assuming you have WAAS/GPS.
 
roncachamp said:
It's an exception. Ditch the DME and you do lose the ability to fly the VOR/DME RWY 15 approach at Martin State Airport in Baltimore. Other available approaches do not change that.
Click to expand...

Forgive my ignorance, but if you have a WAAS GPS and an independent VOR source, you do NOT give up this approach correct?

You'd use the WAAS / CDI to fly the arc since you can use it as a replacement for DME from a VOR for example. Then you'd use the second VOR for the MAP to identify the 060 radial and the GPS / VOR would give you your 14.7.

So, I guess if you had a dual VOR system, one for the GPS and the other for a VOR (SL30 for example) this is do-able?
 
Caramon13 said:
Forgive my ignorance, but if you have a WAAS GPS and an independent VOR source, you do NOT give up this approach correct?

You'd use the WAAS / CDI to fly the arc since you can use it as a replacement for DME from a VOR for example. Then you'd use the second VOR for the MAP to identify the 060 radial and the GPS / VOR would give you your 14.7.

So, I guess if you had a dual VOR system, one for the GPS and the other for a VOR (SL30 for example) this is do-able?
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Nope. I don't have the ref handy but this is one of the instances where you cannot sub a GPS for DME. It has something to do with lateral guidance IIRC. Oh and an IFR GPS doesn't have to be WAAS to sub for DME for all of the instances where it's legal to do so.

Also I don't know of any IFR GPS that can fly an unpublished DME arc.
 
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Hmm...weird..think I found it though:

b. Substitution on a Final Approach Segment. Substitution for the NAVAID for
example, a VOR or NDB providing lateral guidance for the final approach segment.

This was in AC No: 90-108. It's really interesting that you can use it for DME arc, but not if that DME arc is a FAS.

Things you can't use it for:

a. NOTAMed Procedures. Unless otherwise specified, 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.

b. Substitution on a Final Approach Segment. Substitution for the NAVAID for
example, a VOR or NDB providing lateral guidance for the final approach segment.

c. Lateral Navigation on LOC-Based Courses. Lateral navigation on LOC-based courses
(including LOC back-course guidance) without reference to raw LOC data.
 
Caramon13 said:
Forgive my ignorance, but if you have a WAAS GPS and an independent VOR source, you do NOT give up this approach correct?
Click to expand...

I forgive you, but that's not correct. GPS cannot substitute for the NAVAID providing lateral guidance for the final approach segment. On most VOR/DME approaches lateral guidance is provided by the VOR while DME provides along track distance. The VOR/DME approach at KMTN is different, lateral guidance is a DME arc and position on that arc is provided by crossing VOR radials. To my knowledge this is the only approach like that.
 
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roncachamp said:
I forgive you, but that's not correct. GPS cannot substitute for the NAVAID providing lateral guidance for the final approach segment. On most VOR/DME approaches lateral guidance is provided by the VOR and DME provides along track distance. The VOR/DME approach at KMTN is different, lateral guidance is a DME ARC and position on that arc is provided by crossing VOR radials. To my knowledge this is the only approach like that.
Click to expand...

Only one to a public use airport. NASA has one down at their Wallops Island facility on the Eastern Shore of Virginia.

http://155.178.201.160/d-tpp/1511/00639VDT10.PDF
 
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