DME question.

[Bob - S50]

19 hours ago, kortopates said:

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.

 

Yes, we are talking about an approach.  The OP asked about substituting GPS for DME to fly an arc for an approach.

[Bob - S50]

22 hours ago, Steve W said:

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)

Steve.  That manual is from 2012.  The most current version I have no longer contains that section.

I also looked at the STC for my GPS (GTN650) and it contains the following:

"Pilots are prohibited from flying any approach path that contains manually entered waypoints."

[Bob - S50]

20 hours ago, kortopates said:

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:

Not a problem for me, I just used the KNS80.

[kortopates]

2 hours ago, Bob - S50 said:

Not a problem for me, I just used the KNS80.

okay, so on a practice LDA-A approach, while your navigating by the PDX ILS and using the KNS80 for DME distance, have you ever tried loading ILS-DME facility, i.e. I-VDG waypoint - not the airport, as the active waypoint and comparing the DME read out distance to the GPS distance from I-VDG? (This goes to your original post earlier when you correctly stated that you can not use the airport waypoint for a DME distance - you need need to use the ILS-DME facility waypoint which is I-VDG). Since you have both, KNS80 for DME and the correct GPS waypoint for the DME facility I-VDG, you could enlighten us on what the difference is?

 

2 hours ago, Bob - S50 said:

Steve.  That manual is from 2012.  The most current version I have no longer contains that section.

I also looked at the STC for my GPS (GTN650) and it contains the following:

"Pilots are prohibited from flying any approach path that contains manually entered waypoints."

Absolutely agreed ""Pilots are prohibited from flying any approach path that contains manually entered waypoints." But no one has suggested loading one or more waypoints into the GPS flight plan and then using this flight plan to fly the "approach path" which means to use the flight plan for lateral guidance. All that has been suggested is 1) setting the GPS to have the active waypoint the DME facility GPS waypoint to display the GPS distance and 2) while flying the approach using your CDI/HSI ILS signal for approach path guidance. How is that flying an approach path with manually entered waypoints?  

[Bob - S50]

18 hours ago, kortopates said:

okay, so on a practice LDA-A approach, while your navigating by the PDX ILS and using the KNS80 for DME distance, have you ever tried loading ILS-DME facility, i.e. I-VDG waypoint - not the airport, as the active waypoint and comparing the DME read out distance to the GPS distance from I-VDG? (This goes to your original post earlier when you correctly stated that you can not use the airport waypoint for a DME distance - you need need to use the ILS-DME facility waypoint which is I-VDG). Since you have both, KNS80 for DME and the correct GPS waypoint for the DME facility I-VDG, you could enlighten us on what the difference is?

 

Absolutely agreed ""Pilots are prohibited from flying any approach path that contains manually entered waypoints." But no one has suggested loading one or more waypoints into the GPS flight plan and then using this flight plan to fly the "approach path" which means to use the flight plan for lateral guidance. All that has been suggested is 1) setting the GPS to have the active waypoint the DME facility GPS waypoint to display the GPS distance and 2) while flying the approach using your CDI/HSI ILS signal for approach path guidance. How is that flying an approach path with manually entered waypoints?  

I have not tried comparing the DME and GPS distance.  Next time I head down that way I'll try to remember to do that, but I sometimes forget why I came into... what was I going to say?

Agreed.  You can do just about anything you want for situational awareness as I suggested earlier when I suggested manually entering the waypoints into the GPS but using the ILS for guidance.

[kortopates]

2 hours ago, Bob - S50 said:

I have not tried comparing the DME and GPS distance.  Next time I head down that way I'll try to remember to do that, but I sometimes forget why I came into... what was I going to say?

Agreed.  You can do just about anything you want for situational awareness as I suggested earlier when I suggested manually entering the waypoints into the GPS but using the ILS for guidance.

I would be really interested to hear how it works out. I get to fly with all kinds of avionics but can't remember last being in one with both DME and GPS, so its always one or the other except for the sim. And its only on the sim I ever get to fly what amounts to handful or conventional approaches in the US requiring DME but using the GPS without a database procedure to load. I don't have any nearby approaches that need DME GPS substitution without a database procedure to load like your VDU LDA. I am glad you brought it up, I am thinking of adding it to my class section on LDA approaches using GPS for its uniqueness. 

[carusoam]

Paul,

Get a ride in an O1 with the original BK nav suite... Many got the KLN89 or 90B that were new technology for the time...  they had the full standard nav/com/DME/adf from the prior technical era.

Fly over a VOR at 5,700’, The GPS will say you are at 0.0 miles to the station.  The DME should be telling you have 1.0 miles to go before it starts counting up again...

If your DME reads 0.0 miles to the station, you may be closer than you think!    

Want to go for a ride?

Best regards,

-a-

[kortopates]

Paul,
Get a ride in an O1 with the original BK nav suite... Many got the KLN89 or 90B that were new technology for the time...  they had the full standard nav/com/DME/adf from the prior technical era.
Fly over a VOR at 5,700’, The GPS will say you are at 0.0 miles to the station.  The DME should be telling you have 1.0 miles to go before it starts counting up again...
If your DME reads 0.0 miles to the station, you may be closer than you think!    
Want to go for a ride?
Best regards,
-a-

Thanks Anthony, I'd go for ride with you anytime
But I am very familiar with the difference between DME slant range distance and GPS distance. In the example above, several miles out and down pretty low, its going to calculate out to within a tenth or two of a mile - but I am too lazy to do the math. My point is there is no need for DME with a modern GPS. But I would be surprised if the older KLN89 would have all the DME facility waypoints.


Sent from my iPhone using Tapatalk

[Steve W]

Because this topic was apparently asked on every pilot forum I monitor I did do the math on another site:

Just for fun I did the math on a 20DME arc(as mentioned in another post on this same stupid subject) at 6000' above the station:
When your GPS reads: 20.0NM your DME will read: 20.0NM, the difference is 149 feet.

3 miles out on the localizer at 1000': GPS: 3.0NM, DME: 3.0NM, the difference being 27 feet.

You're never going to be using DME to identify station passing so I don't think the 0 DME case is very relevant.

[Andy95W]

Here's a more extreme example of slant range:

 

Edited May 2 by Andy95W

[carusoam]

Andy,

30k’ up?

You are the best for delivering detailed, easy to absorb, facts. 

Thanks for posting the annotated graphic.

Best regards,

-a-

[Andy95W]

Thanks, Anthony.  I figured @Steve W would be able to figure out the altitude like he did with the DME arc.  But yes, above 30k.

[wishboneash]

33k if I am not mistaken.

Oops, 38k (corrected, used sm instead of nm!)

[Andy95W]

Oh crap.  Where's a high school geometry teacher when you need one? 

IIRC, the Pythagorean Theorem says for a right triangle, the sum of the square of the two sides equals the square of the hypotenuse.

0.7nm X 0.7nm + b X b = 6.3nm X 6.3nm

b X b = 39.69 - .49

b = 6.261 nm X 6,076nm/feet = 38,041 feet 

That's closer than I was expecting!  FL380.  It's a good thing CAE is pretty close to sea level or I would've been WAY off!

[donkaye]

On 1/24/2018 at 9:54 AM, kortopates said:

I get to fly with all kinds of avionics but can't remember last being in one with both DME and GPS, so its always one or the other except for the sim. 

I knew there was a reason I kept my KN 63 when I upgraded my panel.  Reads beautifully on the G500. 

[kortopates]

11 hours ago, donkaye said:

I knew there was a reason I kept my KN 63 when I upgraded my panel.  Reads beautifully on the G500. 

Any instances stand out that you were glad you had it? Or has it been more a convenience thing? With 2 GPS's loaded with all DME facility waypoints I've never missed it but I also don't have anything odd to deal with like Bob's LDA aprpoach example into Vancouver WA which is rather interesting but still doable with GPS. 

[donkaye]

1 hour ago, kortopates said:

Any instances stand out that you were glad you had it? Or has it been more a convenience thing? With 2 GPS's loaded with all DME facility waypoints I've never missed it but I also don't have anything odd to deal with like Bob's LDA aprpoach example into Vancouver WA which is rather interesting but still doable with GPS. 

I kept it because I never knew when it might be useful, and it displayed on the G500 through the GAD 43e.  I got the added benefit of DME hold with the GAD 43e that I didn't have before.  Before KSJC got their ILS DME, with my old EFIS 40, I had to spend a great deal of money to put a switch in so that I could get DME displayed on the EFIS 40 while in ILS mode.  Coming in VFR to KSJC from the East where they want you to pass over VPEMB, I use it to quickly tell the Tower my position.  Enroute, if GPS were ever lost (I actually had it happen once on the way back from Salt Lake City), when on an airway it would prove to be useful.  And then there is the interesting LDA approach described above, that could be done thoughtlessly.  I also, use it enroute for quick relative position, like when to start descending into San Diego when VFR.  All in all I still find having DME useful, and it really didn't cost me anything to keep it during the upgrade.

[Bob - S50]

One of the nice things I like about the KNS80 is the Hold function.  When flying an ILS 17 to KOLM (Olympia, WA), DME comes from the VOR, not the ILS.  So on the way to OLM I tune the VOR and get the DME.  I then press the Hold button which keeps the DME on the current channel.  I then tune the VOR to the ILS frequency.  Perfect!  ILS guidance with DME to the OLM VOR.

[Bob - S50]

On 1/23/2018 at 12:37 PM, kortopates said:

okay, so on a practice LDA-A approach, while your navigating by the PDX ILS and using the KNS80 for DME distance, have you ever tried loading ILS-DME facility, i.e. I-VDG waypoint - not the airport, as the active waypoint and comparing the DME read out distance to the GPS distance from I-VDG? (This goes to your original post earlier when you correctly stated that you can not use the airport waypoint for a DME distance - you need need to use the ILS-DME facility waypoint which is I-VDG). Since you have both, KNS80 for DME and the correct GPS waypoint for the DME facility I-VDG, you could enlighten us on what the difference is?

Just an update for you:

1.  It's now moot since the approach is now available in the database.

2.  I had a chance to fly by VUO yesterday so I tuned up IVDG at Portland.  I entered IVDG as a direct to (yes, it will let me do that) and looked at the range.  They were withing 0.2 miles of each other but considering I was at 6500' and about 20 miles away, I'm guessing slant range would account for the difference.  So yes, you could theoretically make IVDG a waypoint and use distance to that point to substitute for DME.

[kortopates]

2 hours ago, Bob - S50 said:

Just an update for you:

1.  It's now moot since the approach is now available in the database.

2.  I had a chance to fly by VUO yesterday so I tuned up IVDG at Portland.  I entered IVDG as a direct to (yes, it will let me do that) and looked at the range.  They were withing 0.2 miles of each other but considering I was at 6500' and about 20 miles away, I'm guessing slant range would account for the difference.  So yes, you could theoretically make IVDG a waypoint and use distance to that point to substitute for DME.

Thanks for confirming Bob. Yep, the only difference is in slant range and a lower approach altitude should make the difference negligible. Guess that's good news that Garmin or Jeppesen is able to over  come the issue that prevented them for putting the procedure in the DB. But this was such a great example on how to use GPS to substitute for DME that I am going to have to try to find another similar one for the college IFR class I teach since the DB procedure makes it too easy  

Edited April 20, 2018 by kortopates

[gacoon]

2 hours ago, kortopates said:

Thanks for confirming Bob. Yep, the only difference is in slant range and a lower approach altitude should make the difference negligible. Guess that's good news that Garmin or Jeppesen is able to over  come the issue that prevented them for putting the procedure in the DB. But this was such a great example on how to use GPS to substitute for DME that I am going to have to try to find another similar one for the college IFR class I teach since the DB procedure makes it too easy  

KSUN NDB/DME-A might be a good replacement.

[Jerry 5TJ]

/* Warning:  This post contains techno-geek material.

Slant range is one contributor to DME accuracy.  But slant range error is generally small compared to intrinsic system performance limits. 

The DME system achieves a 95% accuracy expectation of around 180 meters, about the same as the +0.1 nmi resolution on standard DME displays.  

The 95% horizontal accuracy of WAAS enabled GPS is under 10 meters, so DME is about 20 times less accurate.  

In addition GPS accuracy is independent of the number of users, while the ground-based DME transponder is reply-rate limited.  This means in dense traffic areas, or far from the ground station,  individual users may receive fewer updates.  During such intervals the airborne DME interrogator’s display “coasts” for a few seconds with no updates before blanking out until further replies come in.  

The reply-rate limiting is done in part by reducing the ground-based transponder’s receiver sensitivity.  This has the effect of favoring near-by users more than distant airplanes.   It also favors higher power such as airliners over our lower power GA units.  

Typical DME interrogators in airliners transmit 300 watt pulses compared to 50 watts from a KN-64, giving them nearly 8 dB advantage. 

Note that what we refer to as DME is also known as DME-N where the N designated “non-precision” or “narrowband.”  There is  also a standard for DME-P meaning  “precision” associated with the Microwave Landing System or MLS.  All common DME in our planes are DME-N only.  

A reference:  Tran, Michael, "DME/DME Accuracy," Proceedings of the 2008 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2008, pp. 443-451.

End of Geek warning. */

[Marauder]

27 minutes ago, Jerry 5TJ said:

/* Warning:  This post contains techno-geek material.

Slant range is one contributor to DME accuracy.  But slant range error is generally small compared to intrinsic system performance limits. 

The DME system achieves a 95% accuracy expectation of around 180 meters, about the same as the +0.1 nmi resolution on standard DME displays.  

The 95% horizontal accuracy of WAAS enabled GPS is under 10 meters, so DME is about 20 times less accurate.  

In addition GPS accuracy is independent of the number of users, while the ground-based DME transponder is reply-rate limited.  This means in dense traffic areas, or far from the ground station,  individual users may receive fewer updates.  During such intervals the airborne DME interrogator’s display “coasts” for a few seconds with no updates before blanking out until further replies come in.  

The reply-rate limiting is done in part by reducing the ground-based transponder’s receiver sensitivity.  This has the effect of favoring near-by users more than distant airplanes.   It also favors higher power such as airliners over our lower power GA units.  

Typical DME interrogators in airliners transmit 300 watt pulses compared to 50 watts from a KN-64, giving them nearly 8 dB advantage. 

Note that what we refer to as DME is also known as DME-N where the N designated “non-precision” or “narrowband.”  There is  also a standard for DME-P meaning  “precision” associated with the Microwave Landing System or MLS.  All common DME in our planes are DME-N only.  

A reference:  Tran, Michael, "DME/DME Accuracy," Proceedings of the 2008 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2008, pp. 443-451.

End of Geek warning. */

Man, do I love watching a former Bendix King engineer geek out. So, if I translate your post into layman english, you are saying "DME ain't so accurate and especially for us GA types".

[kris_adams]

On 1/23/2018 at 3:37 PM, kortopates said:

 1) setting the GPS to have the active waypoint the DME facility GPS waypoint to display the GPS distance and 2) while flying the approach using your CDI/HSI ILS signal for approach path guidance. How is that flying an approach path with manually entered waypoints?  

That's exactly how I was taught...of course haven't actually flown one ever nor do I think I'll ever have to.