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GPS distance in place of DME

P.Hendo

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Apr 28, 2002
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Just a uick query, which I'm quite sure I know the answer to...

When can I use GPS distance in place of DME distance? The reason for the post is that I have been informed by a very senior pilot whom is instructing me through my CIR that you can use GPS distance instead of DME to fly the published DME arc, but must discontinue using the GPS once established on the azimuth aid. The approach is a simple "ILS or ILS DME or LOC DME". The aircraft hasn't a DME fitted, but a serviceable IFR approved (and current) GPS.

My understanding is that unless the plate specifies "GPS/DME" then one cannot be used in place of the other. For 2 reasons:
1. the GPS reference point is (almost always) different to the DME reference point;
2. DME gives a slant distance, unlike GPS.
....... and hence required terrain separation cannot be assured.

Have read extensively through the Aussie Jepps and I can't see where I could be wrong. With my CIR test looming I'm in a sticky situation. I have questioned the pilot about it and he is adament ("black and white" ......."it is gospel") that he is right.:eek:
 

flydog

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KingAirKiddo proved me wrong so I removed my previous post. See his post for the answer.

I personally dont like using GPS in lieu of DME especially in foreign countries when in non-radar environment in mountainous terrain at night. I have seen differences as high as 3 DME on some approaches. I also think its too easy to put in the wrong facility as a lot of foreign airports use VORs and NDBs using the same identifier and they are not always collocated. But I suppose if the FAA says you can do it then go ahead and do it.
 
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Timebuilder

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When used correctly, IFR approved and current GPS distance is a legal substitute for DME distance.

As Flydog pointed out, the airport reference point and the GPS "distance to the aiport" would be a very wrong way to substitute for DME distance. ARP's are not intended to be used for IFR navigation to the airport, so if this is what you have been doing, I'd stop this practice right away.

On the other hand, a DME arc, which references an agreed-upon point on the ground, in this case the VOR, works quite well using GPS distance in lieu of DME. At the altitudes used to fly most DME arcs you will have very little slant range error. Since I regularly fly planes equipped with both GPS and DME, I can vouch for its accuracy.

Since the aproach has been test flown using the VOR radial, that indication would be your primary source of navigation information. I can't think of a reason to ignore a good GPS as a backup, though.
 

KingAirKiddo

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Straight from the AIM...

"...operators in the United States NAS are authorized to use GPS equipment certified for IFR operations in place of ADF and/or DME equipment for enroute and terminal operations..."

1. Determining the aircraft position over a DME fex. GPS satisfies the 14 CFR Section 91.205(e) requirement for DME at and above FL 240

2. Flying a DME arc.

3. Navigating to/from an NDB/compass locator.

4. Determining aircraft position over an NDB/compass locator.

5. Determining aircraft position over a fix defined by an NDB/compass locator bearing crossing a VOR/LOC course.

6. Holding over an NDB/compass locator.

Hope this helps...good luck with your training.

Of course, I just realized that you were referencing Australian airspace and regs, so this probably doesn't mean anything, but hope it helps anyway!:)
 
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Timebuilder

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Thanks for posting the list. I didn't have my Summit Aviation CD with me.
 
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