DME question.

[xcrmckenna]

I have an IFD440 but no DME. Can I use the certified gps to substitute for a DME so I can fly a DME arc for an approach? I’m using King’s School for my IR ground training and just watched a video of Martha say you can substitute a DME with a certified ifr gps if the VOR/DME is out of service. But didn’t say it could be substituted if you just don’t have a DME. Or will I need to have a DME receiver to be able to accept a VOR/DME approach?

 

 

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[gsxrpilot]

Yes you can fly the DME arc's with the IFD440.

This video shows it being done on the GNS430 which is pretty close.

 

[Steve W]

And for the regulatory guidance, check out AC 90-108 https://www.faa.gov/documentLibrary/media/Advisory_Circular/90-108.pdf  Basically it can substitute for the DME or ADF in any phase of flight, unless the DME is the primary lateral guidance on the final approach segment(I think the only one that would qualify is the Martin State VOR/DME approach all the others only use DME to identify fixes for step-downs and procedure turns.)

[xcrmckenna]

Thank you guys.


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[carusoam]

Holly Cow, Steve beat me too it and supplied the AC!!!...

 

The rest of this just got old...

Somewhere is a rule that got set allowing for GPSi in place of DME...

Might need to be a CFII to be able to quote the regulation.

It might be good to understand the mathematics of the slant range vs gps distance, right angle geometry...

Or think of it this way...  at 12k’ over a target like the Airport, the DME will indicate 2 miles.  The GPS will say you are there....  

Realistically, you won't approach the Airport from 12k’ overhead... the actual difference in distance will be quite small...

PP thoughts only not a CFII...

Best regards,

-a-

 

[Steve W]

6 minutes ago, carusoam said:

Might need to be a CFII to be able to quote the regulation.

 

I've barely started working on the CFI yet(stupid boring FOI studying...)

[xcrmckenna]

Holly Cow, Steve beat me too it and supplied the AC!!!...
 
The rest of this just got old...
Somewhere is a rule that got set allowing for GPSi in place of DME...
Might need to be a CFII to be able to quote the regulation.
It might be good to understand the mathematics of the slant range vs gps distance, right angle geometry...
Or think of it this way...  at 12k’ over a target like the Airport, the DME will indicate 2 miles.  The GPS will say you are there....  
Realistically, you won't approach the Airport from 12k’ overhead... the actual difference in distance will be quite small...
PP thoughts only not a CFII...
Best regards,
-a-
 

I thought slant range might have been the reason for not being able to use it. Since in the approach the closer you get the lower you are and you can’t really be 1,000’ agl for every mile away from the fix to negate the error and the gps would have you closer.


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[xcrmckenna]

Yes you can fly the DME arc's with the IFD440.
This video shows it being done on the GNS430 which is pretty close.
 

I was hoping you were going to tell me that the 540 was allowed but not that 440 so I could justify the install to the better half....


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[isaacpr7]

I have an IFD440 but no DME. Can I use the certified gps to substitute for a DME so I can fly a DME arc for an approach? I’m using King’s School for my IR ground training and just watched a video of Martha say you can substitute a DME with a certified ifr gps if the VOR/DME is out of service. But didn’t say it could be substituted if you just don’t have a DME. Or will I need to have a DME receiver to be able to accept a VOR/DME approach?
 
 
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Yes you can. The answer is in chapter 1 of the AIM

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[kortopates]

On 1/21/2018 at 12:10 PM, Steve W said:

And for the regulatory guidance, check out AC 90-108 https://www.faa.gov/documentLibrary/media/Advisory_Circular/90-108.pdf  Basically it can substitute for the DME or ADF in any phase of flight, unless the DME is the primary lateral guidance on the final approach segment(I think the only one that would qualify is the Martin State VOR/DME approach all the others only use DME to identify fixes for step-downs and procedure turns.)

Very good - you are well on your way to Ace'ing your CFII - but that circular, although the original source, is a bit dated and these days and it's better covered by the AIM in section 1-2-3 (which actually references 90-108).

DME arcs are certainly covered, including the note worthy unique Martin State DME arc, where the actual Arc leads to the runway. The note about not using substitution for primary lateral guidance past the FAF on approach is referring to VOR and NDB lateral guidance - not Arc's. But even this is outdated thanks to another AIM entry added more recently that clarifies we may navigate via GPS as long as we can monitor the raw Nav signal (see 1-2-3 c) note-5)  This is something we can all do on a second CDI for VOR approaches which is nice but still does nothing legally to allow us to do NDB approaches with just GPS and no installed ADF equip. (But who really cares?) But it also means we can't fly a VOR based approach with an OTS VOR since we are unable to monitor the raw nav data (ironically even though we can fly VOR airways with an OTS VOR since we can use GPS to substitute for VORs sole source enroute and terminal navigation). 

As for the difference in DME slant range versus actual GPS distance this is made irrelevant in one of two ways. In the enroute/terminal navigation, the difference is well within the tolerance of the protected zone of the TERPS criteria for calculating the required obstacle clearance. For approaches, you'll note even though the approach plate may only list DME fixes without GPS waypoints names, the navdata procedure uses a jeppesen coded letter and number to provide an actual GPS waypoint for every DME fix needed on the approach - thus you'll always be able to use GPS waypoints for DME fixes as long as you have the approach procedure loaded and a current database.  (But the substitution rules would still allow us to use GPS distance from the VOR if we didn't have the procedure with DME waypoints, but that's pretty silly when we have the approach procedure in the DB). 

Edited January 23, 2018 by kortopates

[Piloto]

For ILS Cat II, III approaches DME, RA and ILS are required. There is no GPS approach for CAT II, III approved. 

 

Edited January 22, 2018 by Piloto

[Bob - S50]

I would add a couple cautions.

1.  Any approach you want to fly with your GPS has to be available in the GPS database.  You are NOT allowed to construct an approach by entering points into the route of flight that correspond with the approach.  So if your GPS database contains the approach with a DME arc, yes you can fly it.  If the approach is not in the database you are NOT allowed to just dial in the VOR as your destination and then fly so as to keep the distance to destination equal to the desired DME arc.

2.  Most approaches that have DME fixes also have names for those fixes so all you need to do is use the named fixes.  However, if you are flying an approach that does not have named fixes, be very careful when substituting your GPS for DME for a couple reasons:

     a.  The position of the DME equipment is most likely NOT the same point used by the GPS for distance to the airport.  The GPS is probably using something around mid-field while an ILS DME would be at the end of the runway.  On a long runway that could make a difference of up to a mile.  And my go-to example is the RNAV approach to KVUO (Vancouver, WA).  The approach is not in the database and the DME in use is located at another airport (PDX).  Another would be the ILS/DME to KOLM (Olympia, WA) where the DME is from the VOR even though you are flying an ILS/LOC approach.

     b.  In those cases you will need to do math to figure out the proper indication for each fix.

However, you might be able to cheat in some cases.  In the VUO case, the fixes have names so even though the approach is not in the database, you could load the fixes into your route of flight for the GPS.  And while you can't use them for navigation, you could fly the approach based on the VLOC and note passage of each fix.

When in doubt, err on the side of safety.

[Steve W]

1 hour ago, Bob - S50 said:

I would add a couple cautions.

1.  Any approach you want to fly with your GPS has to be available in the GPS database.  You are NOT allowed to construct an approach by entering points into the route of flight that correspond with the approach.  So if your GPS database contains the approach with a DME arc, yes you can fly it.  If the approach is not in the database you are NOT allowed to just dial in the VOR as your destination and then fly so as to keep the distance to destination equal to the desired DME arc.

 

Ok, where's the regulatory guidance on this. I see nothing that requires that you have to have any entries in a database besides the actual VOR/DME or VORTAC to fly a DME arc based on a GPS. As a matter of fact AOPAs guidance says exactly the opposite of what you said:

"DME arcs associated with instrument approaches may be flown using GPS distance provided the DME transmitter, on which the arc is based, is identified in the GPS database"

https://www.aopa.org/advocacy/advocacy-briefs/air-traffic-services-brief-use-of-gps-in-lieu-of-dme-adf

 

[Bob - S50]

1 hour ago, Steve W said:

Ok, where's the regulatory guidance on this. I see nothing that requires that you have to have any entries in a database besides the actual VOR/DME or VORTAC to fly a DME arc based on a GPS. As a matter of fact AOPAs guidance says exactly the opposite of what you said:

"DME arcs associated with instrument approaches may be flown using GPS distance provided the DME transmitter, on which the arc is based, is identified in the GPS database"

https://www.aopa.org/advocacy/advocacy-briefs/air-traffic-services-brief-use-of-gps-in-lieu-of-dme-adf

 

AIM 1-1-17.b.2.(b).(3) "(3) All approach procedures to be flown must be retrievable from the current airborne navigation database supplied by the equipment manufacturer or other FAA−approved source. The system must be able to retrieve the procedure by name from the aircraft navigation database, not just as a manually entered series of waypoints. Manual entry
of waypoints using latitude/longitude or place/bearing is not permitted for approach procedures."

And it's probably in your GPS documents as well.

[Steve W]

24 minutes ago, Bob - S50 said:

AIM 1-1-17.b.2.(b).(3) "(3) All approach procedures to be flown must be retrievable from the current airborne navigation database supplied by the equipment manufacturer or other FAA−approved source. The system must be able to retrieve the procedure by name from the aircraft navigation database, not just as a manually entered series of waypoints. Manual entry
of waypoints using latitude/longitude or place/bearing is not permitted for approach procedures."

And it's probably in your GPS documents as well.

Exactly, the DME fix must be in the database, you can't enter it. Substituting GPS for DME is not "an approach procedure"

Here's how the FAA says to fly an Arc with a GPS, note it does NOT say you need to load the procedure, because unless it's an overlay approach it's probably not in many IFR databases any more due to size constraints:

(Instrument Flying Handbook, Pg 9-27 https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/FAA-H-8083-15B.pdf )

"To Fly a DME Arc:

1. Verify aircraft GPS system integrity monitoring is functioning properly and indicates satisfactory integrity.

2. Select from the airborne database the facility providing the DME arc as the active GPS WP. 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.

3. Maintain position on the arc by reference to the GPS distance instead of a DME readout."

And as far as what my GPS flight manual says:

"GPS based instrument approaches must be flown in accordance with an approved instrument approach procedure that is loaded from the navigation database." So, not applicable to an approach requiring a DME arc that is not a GPS based approach(VOR, ILS, NDB)

[Piloto]

7 minutes ago, Bob - S50 said:

AIM 1-1-17.b.2.(b).(3) "(3) All approach procedures to be flown must be retrievable from the current airborne navigation database supplied by the equipment manufacturer or other FAA−approved source. The system must be able to retrieve the procedure by name from the aircraft navigation database, not just as a manually entered series of waypoints. Manual entry
of waypoints using latitude/longitude or place/bearing is not permitted for approach procedures."

And it's probably in your GPS documents as well.

There is no Lat/Long indicated on approach plates. But in the old days of Delco Carousel everything was entered in Lat/Long, including your initial position.

[Piloto]

Back in the 80's the B757 and B767 were equipped with scanning DMEs feeding the FMS for position determination. It had a position accuracy of better than 0.1nm. The DMEs updated the FMS position solution until the oceanic environment were the FMS would use the IRS/INS. A very expensive system ($100k for each FMS/IRS+10K for DME) and there would be three of these on each plane. And you complain about your GPS price.

José

[jaylw314]

On ‎1‎/‎21‎/‎2018 at 12:10 PM, Steve W said:

And for the regulatory guidance, check out AC 90-108 https://www.faa.gov/documentLibrary/media/Advisory_Circular/90-108.pdf  Basically it can substitute for the DME or ADF in any phase of flight, unless the DME is the primary lateral guidance on the final approach segment(I think the only one that would qualify is the Martin State VOR/DME approach all the others only use DME to identify fixes for step-downs and procedure turns.)

Holy cow, I hadn't heard of DME for lateral guidance before...

[kortopates]

On 1/22/2018 at 10:50 AM, Bob - S50 said:

AIM 1-1-17.b.2.(b).(3) "(3) All approach procedures to be flown must be retrievable from the current airborne navigation database supplied by the equipment manufacturer or other FAA−approved source. The system must be able to retrieve the procedure by name from the aircraft navigation database, not just as a manually entered series of waypoints. Manual entry
of waypoints using latitude/longitude or place/bearing is not permitted for approach procedures."

And it's probably in your GPS documents as well.

What your quoting applies to GPS approaches only. We're not using GPS for lateral or vertical guidance in doing ILS, LOC or VOR approaches, therefore we don't need a GPS database approach procedure to fly them just like before GPS existed in the cockpit and we flew these without GPS. But we are able to use GPS to substitute for DME as long as the DME waypoint is in the database. As @Steve W says above, it does not need to come from a loaded procedure, but when its available its certainly legit and can be used for added situational awareness. Its certainly still legal since we are navigating by raw radio signal (e.g. VOR, ILS, LOC/BC, LDA, or SDF) and we may use a IFR certified GPS in the panel to substitute for the DME distance providing we have DME facility in the DB, which will work for any VOR based DME. An interestingly you will find many ILS, LOC, LDA DME facilities in the Garmin databases also including what you need for your VDU LDA-A approch.

I do think your KVUO LDA-A approach that uses the PDX ILS is a most interesting issue since the procedure is not in your Database. I've given it a bit more thought and assuming we're looking for options for GPS equipped aircraft without DME to fly an approach IFR, I came up with these two:

1) firstly, one option you've probably considered. Just get clearance to fly the PDX ILS 10L, with intent to cancel and land at VDU.  If you ceiling are above 1100-1200' this should work since you'll go right over VDU at right around JALAG at 1100'. Although you'll be  able to load this approach, JALAG isn't in the database procedure so you'll be going more by distance to the missed for 10L to realize your passing VDU.

2) You can still fly the VDU LDA-A approach. Granted there is no procedure in your database. But you do have the I-VDG facility in the database, so simply dial in the ILS for PDX, identify it, and substitute your GPS distance for DME to identify each and every DME fix to the missed. Both GNS and GTN databases have these facility fixes. If you have dual GPS's like I do, its even easier since one GPS  could have the KVDU airport while the  other has the I-VDG facility for your DME distance. That method would get you the 860' minimums and be still legal although I have not had a need to do this, but you have an interesting issue.

Anyway, I am confident this is valid (using the I-VDG waypoint). Back in 1998 when the FAA first granted us the ability to substitute GPS for DME & ADF, they had 3 limitations at that time, one concerned ADF (had to be an overlay like today), one for requiring a non-GPS alternate if an alternate was required (since this was before WAAS) and thirdly "DME transmitters associated with a localizer may not be retrievable from your GPS until the manufacturer incorporates them in the database. Pilots are not authorized to manually enter coordinates." That quote is right out of the old AOPA document courtesy of Steve above. Well that problem was solved some time ago and these facilities are now in the database. 

Hope that helps you out.

Edited January 23, 2018 by kortopates

[Browncbr1]

4 hours ago, jaylw314 said:

Holy cow, I hadn't heard of DME for lateral guidance before...

That’s what a DME arc is.    ...  lateral guidance. 

[Browncbr1]

On the gtn, you can create a waypoint by entering VOR radial and DME.  Then, you can set the gtn to hold at that waypoint.   

I don’t think you can make the magenta line curve along a user defined DME arc.    I know it will do it if it is a published procedure though.  

[xcrmckenna]

This seems to be getting as clear as mud:) can I use the IFD’s measurement for this dme arc to final in this approach?


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[Steve W]

6 minutes ago, xcrmckenna said:

This seems to be getting as clear as mud:) can I use the IFD’s measurement for this dme arc to final in this approach?
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Yes.

Direct, DSD, make sure it says 'Deschutes VOR' , make the distance stay '22', done (you can also switch to OBS mode and use the GPS Waypoint as your VOR for the turn 10 twist 10 part)

Alternatively, load the approach, and select one of the arc IAFs, most will have ILS approaches in the database.

As usual be sure to switch to the LOC for the localizer part. Also, if you're substituting for DME on the localizer and you don't have the approach in the database then make sure you can follow the note and you do have I-RDM in the database.

 

[xcrmckenna]

Yes.
Direct, DSD, make sure it says 'Deschutes VOR' , make the distance stay '22', done (you can also switch to OBS mode and use the GPS Waypoint as your VOR for the turn 10 twist 10 part)
Alternatively, load the approach, and select one of the arc IAFs, most will have ILS approaches in the database.
As usual be sure to switch to the LOC for the localizer part. Also, if you're substituting for DME on the localizer and you don't have the approach in the database then make sure you can follow the note and you do have I-RDM in the database.
 

It does have the approach in the database, just wanted to make sure legally I could use it for the turn ten twist 10 since that’s how I’m working on my arc’s. And a little plug for the wonderful Avidyne IFD systems it will switch the obs for you.
Thank you for your time and responses!!!! Always enjoy learning from Mooney Space members.


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[kortopates]

3 hours ago, xcrmckenna said:

It does have the approach in the database, just wanted to make sure legally I could use it for the turn ten twist 10 since that’s how I’m working on my arc’s. And a little plug for the wonderful Avidyne IFD systems it will switch the obs for you.
Thank you for your time and responses!!!! Always enjoy learning from Mooney Space members.


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Of course if you load the full procedure to include an IAF on the Arc, you will no longer need to turn 10 and twist 10, the GPS will give you lateral guidance along the arc. The ability for the unit to update OBS is actually a plug for the autoslewing capability of your CDI or HSI. All modern GPS's are capable of that, its the CDI or HSI which may not be.

But if learning DME arc for your IFR checkride, of course you'll want to learn how to do them manually as well without following the GPS lateral guidance to fly them.

Also to clarify an apparent misunderstanding - A DME Arc does not constitute lateral guidance. Lateral guidance refers to following a course as depicted on course center line or lateral deviation indicator i.e. your CDI or HSI. I think the original remark was meant to say that GPS can not be used to substitute for lateral guidance provided by a navaid on final approach segment; which is applicable only to VORs, LOC's, SDF, & LDA's only - again since DME is only distance.  Further, the MTN VOR/DME  final approach segment consisting of an DME arc isn't lateral guidance either - this approach has no lateral guidance.