DME Required?

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bdavidn
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Joined: Mon Jan 30, 2017 6:33 pm

DME Required?

Post by bdavidn »

Was doing approaches today for my IFR currency with an instructor and while looking at the VOR RWY 19 into PRB (attached) she asked me if a DME was required. I said no, an IFR approved GPS can be used in lieu of an actual DME unit. She responded that that was what she thought as well up until a few weeks ago when a very experienced instructor told her that if the notes of the approach say, "DME required," then you must have an actual unit in the plane. I can't find any reg supporting this, so just wanted to make sure I'm not the crazy one and that an IFR-approved GPS can indeed substitute for a physical DME. My interpretation of the DME note is that the title of that approach does not mention DME, just VOR, therefore if any other equipment is needed it would be in the notes field. The DME reference would be applicable if you did not have IFR-approved GPS, since you need DME to identify the arc. But since we can identify 12.5 DME with GPS, we're good to go without DME.

It says right in the AIM that you can use an approved RNAV system to:
1. Determine aircraft position relative to, or distance from a VOR, 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.

She told me one of her instrument students has a check-ride in two weeks so kind of feel obligated to reach out and let her know this person isn't correct, but first I want to make sure I'm not the one who is wrong.

-Bryan
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zerofay32
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Re: DME Required?

Post by zerofay32 »

I'm not a double I, but I've not heard of needing a DME unit if it noted on a plate. In fact, I did an approach that has a DME arc on my IFR checkride recently (last year) with only a 430 in the plane and the DPE had no objections. I also think having VOR/DME in the approach name doesn't matter either. Hopefully one of the CFII that are on here will chime in.
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Ryan B
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Re: DME Required?

Post by Ryan B »

It was probably an old instructor....before the time of GPS haha!

I'm just a dumb ATC but I believe you don't need DME in your plane to do this.
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Zachary Beard
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Re: DME Required?

Post by Zachary Beard »

Disclaimer: I'm not a CFI, this is purely my interpretation of the AIM. I'm sure there are better references and interpretations out there.

It's important to note the difference between DME and Along Track Distance (ATD). DME has slant range, and ATD doesn't. The ATD displayed on your GPS will not usually be the same as actual DME (although likely slight, but potentially up to two miles difference based on altitude).

The AIM says: "Variations in distances may also occur since RNAV system distance−to−waypoint values are along−track distances (ATD) computed to the next waypoint and the DME values published on underlying procedures are slant−range distances measured to the station. This difference increases with aircraft altitude and proximity to the NAVAID."

It also mentions to only use non-GPS/RNAV approaches in a GPS unit as an overlay. For example, you should be using raw data/NAVAIDs while flying a non-GPS approach, while using a GPS overlay as reference only.

Flying a non-GPS approach using GPS methods doesn't ensure compliance with the approach, although it might seem like splitting hairs.

DME required = DME required... and use it.
bdavidn
Posts: 16
Joined: Mon Jan 30, 2017 6:33 pm

Re: DME Required?

Post by bdavidn »

So based on that, my interpretation would be as long as the DME position is in your GPS database and your alternate doesn't require DME for the approach, you can use GPS in-lieu of DME.

Zach - If the aircraft I was flying had DME, then I would definitely tune it in, but DME receivers are getting more and more rare so I wouldn't want to think an approach isn't available to me just because I don't have a DME unit in the plane.

-Bryan
Keith Smith
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Re: DME Required?

Post by Keith Smith »

For something with a bit more weight than just the AOPA piece that I posted, Advisory Circular 90-108: https://www.faa.gov/documentLibrary/med ... 90-108.pdf
7. USES OF SUITABLE RNAV SYSTEMS.
a. Usage of Suitable RNAV Systems. Subject to the operating requirements in this AC,
operators may use a suitable RNAV system in the following ways.
(1) Determine aircraft position relative to or distance from a VOR (see first note in
subparagraph 7b), TACAN, NDB, compass locator (see second note in subparagraph 7b), DME
fix; or a named fix defined by a VOR radial, TACAN course, NDB bearing, or compass locator
bearing intersecting a VOR or Localizer (LOC) 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.
Bold emphasis mine.

Zach, I have to respectfully disagree with your post. Yes, slant range can be slightly different than the GPS measured distance, but the level of error is trivial. At 15 miles, at 8000ft AGL, the difference between slant range and the distance over the ground is 355ft (0.0592nm). Given the tolerances involved in DME arc procedures already, it's easy to see why the FAA allowed GPS to be used in lieu of DME.
Zachary Beard
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Re: DME Required?

Post by Zachary Beard »

Awesome digging! Thanks for the links.
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