Identifying DME fixes by GPS

[azure]

I was shooting approaches at KFNT this afternoon with my finish-up CFII. One of the approaches I did was the VOR 27. This approach has an MDA of 1500 unless you can identify the stepdown fix JOBGO, which allows you to come down to 1260 once inside the fix. I think this is the first time I've flown that particular approach in my airplane, and I noticed that although JOBGO is depicted as a fix on my GMX-200 MFD, and must therefore be in the Jepp nav database for the GMX-200, it is NOT depicted at all on my CNX-80 nor is it shown as a waypoint in the procedure. So I was curious whether it was in the databases for other GPS models like the 430, 530, etc.? Does anyone know?

Which brings me to my main question: do I have any legal way in my airplane to identify JOBGO, since it is not in the CNX-80 database? I do not have DME, and normally rely on the GPS database. Of course I could go to the NRST -> VOR page and use the GPS distance to FNT, but I've never been sure whether that's legal.

 

[LDJones]

If the The GPS providing the GMX-200 position info is IFR certified and the database is current I see no reason it couldn't be used as a substitute for DME position.

 

[azure]

If the The GPS providing the GMX-200 position info is IFR certified and the database is current I see no reason it couldn't be used as a substitute for DME position.

The GPS is the CNX-80 and yes the database is current. But it shows the fix as a triangle and the ship's position as a rather large airplane-shaped object. Anyway the GMX-200 is placarded "for situational awareness only". It's not supposed to be used for primary navigation, and there is no way that I know of to use it to accurately determine passage over a fix.

 

[poadeleted20]

Plug FNT VOR in the GPS and look for 2.8 miles to go. If you've loaded the approach, you have a problem if the GPS is showing distance to the runway, not the real MAP (the VOR) and you could step down a touch too soon. Of course, you're supposed to be flying this off the VOR anyway, although it's nice to have the approach in the GPS for situational awareness.

Note than on the 430/530 series, the 2.8 DME fix will be a waypoint in the approach, and you'll see when it sequences so you know when to step down. I thought the CNX80 worked the same way, but your post suggests it doesn't.

 

[azure]

Plug FNT VOR in the GPS and look for 2.8 miles to go. If you've loaded the approach, you have a problem if the GPS is showing distance to the runway, not the real MAP (the VOR) and you could step down a touch too soon. Of course, you're supposed to be flying this off the VOR anyway, although it's nice to have the approach in the GPS for situational awareness.

That's why I said I'd use the nearest VOR page. That way, the distance will be to the VOR not to the runway. I just wasn't sure it was legal to use it because it is horizontal distance whereas DME is slant range. (Yes, I know the difference is slight at <1000 AGL and 2.8 nm out... but I was asking whether it was legal to use.)

Note than on the 430/530 series, the 2.8 DME fix will be a waypoint in the approach, and you'll see when it sequences so you know when to step down. I thought the CNX80 worked the same way, but your post suggests it doesn't.

The CNX-80 does "work" the same way. If a fix is a waypoint in the approach, it sequences when you pass over the fix and you can tell when to step down that way. JOBGO isn't a waypoint in that approach as far as the CNX-80 database is concerned. The reason for my question is that I was under the impression that was the only legal way to identify a stepdown fix using a GPS.

I'm guessing that the problem is with the Jepp database for the CNX-80 and I'm wondering why Jeppesen didn't see fit to include JOBGO, since they obviously have it in the GMX-200 database (not to mention the ones for the 430/530 GPS's).

 

[poadeleted20]

That's why I said I'd use the nearest VOR page. That way, the distance will be to the VOR not to the runway. I just wasn't sure it was legal to use it because it is horizontal distance whereas DME is slant range. (Yes, I know the difference is slight at <1000 AGL and 2.8 nm out... but I was asking whether it was legal to use.)

Yes, it is legal to use for approaches without regard to slant range.

The CNX-80 does "work" the same way. If a fix is a waypoint in the approach, it sequences when you pass over the fix and you can tell when to step down that way. JOBGO isn't a waypoint in that approach as far as the CNX-80 database is concerned.

Check for a fix called D028 or something like that rather than JOBGO showing up in the CNX80 flight plan sequence for the approach. That would be the 2.8 DME stepdown fix.

 

[azure]

Yes, it is legal to use for approaches without regard to slant range.

Thanks.

Check for a fix called D028 or something like that rather than JOBGO showing up in the CNX80 flight plan sequence for the approach. That would be the 2.8 DME stepdown fix.

I'll double check next time I'm in the plane. I don't think there's a D028, but I do seem to recall a fix prior to the VOR that I couldn't figure out (might have been FF27 or similar, something that looked like it should have been a FAF except that there isn't a FAF on this approach).

The CNX-80 database has lots of fixes like that with strange names that don't match anything on the plates. I've never seen a key or glossary explaining what the naming convention is. Might be helpful if Jeppesen published one.

 

[poadeleted20]

I'll double check next time I'm in the plane. I don't think there's a D028, but I do seem to recall a fix prior to the VOR that I couldn't figure out (might have been FF27 or similar, something that looked like it should have been a FAF except that there isn't a FAF on this approach).

FF27 (FF for "Final Fix" even though it's not really an FAF and 27 for the runway number) would more likely be the computer navigation fix typically placed 5 miles from the end of the runway used by the GPS along with the end of the runway (RW27 in 430/530-speak) to draw the straight line for the final segment when there's no real FAF, but see if it's 2.8 miles from the VOR.

The CNX-80 database has lots of fixes like that with strange names that don't match anything on the plates. I've never seen a key or glossary explaining what the naming convention is. Might be helpful if Jeppesen published one.

I know the Garmin 430/530 manuals discuss those cryptically-named fixes like FF27, D028, etc, but I can't remember what's in the CNX80/GNS480 manuals on that.

 

[John Collins]

I was shooting approaches at KFNT this afternoon with my finish-up CFII. One of the approaches I did was the VOR 27. This approach has an MDA of 1500 unless you can identify the stepdown fix JOBGO, which allows you to come down to 1260 once inside the fix. I think this is the first time I've flown that particular approach in my airplane, and I noticed that although JOBGO is depicted as a fix on my GMX-200 MFD, and must therefore be in the Jepp nav database for the GMX-200, it is NOT depicted at all on my CNX-80 nor is it shown as a waypoint in the procedure. So I was curious whether it was in the databases for other GPS models like the 430, 530, etc.? Does anyone know?

Which brings me to my main question: do I have any legal way in my airplane to identify JOBGO, since it is not in the CNX-80 database? I do not have DME, and normally rely on the GPS database. Of course I could go to the NRST -> VOR page and use the GPS distance to FNT, but I've never been sure whether that's legal.

JOBGO is in the GNS430W and GTN databases as part of the approach. In the case of the CNX80, many of the ATD fixes are not included in the database, but you can use the FNT 2.8 distance to go on the approach as the step down without a DME.

AC 90-108 Use of Suitable Area Navigation (RNAV) Systems on Conventional Routes and Procedures states (emphasis is mine):

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.

 

[azure]

John, thanks. The parts that have always confused me about this are what is said in 8b and 9b(2) below that paragraph in the same AC (and I'm pretty sure elsewhere as well):

8b. Substitution on a Final Approach Segment. Substitution for the NAVAID (for example, a VOR or NDB) providing lateral guidance for the final approach segment.

9b.(2) Pilots must extract waypoints, NAVAIDs, and fixes by name from the onboard navigation database and comply with the charted procedure or route. Heading-based legs associated with procedures may be flown using manual technique (based on indicated magnetic heading) or, if available, extracted from the aircraft database and flown using RNAV system guidance.

JOBGO is a waypoint on a final approach segment that is determined by DME distance from a navaid. It has never been clear to me whether the FAA would say that something like this was illegally substituting for the navaid, or whether it's okay because you're only using the GPS to get the distance and not for lateral course guidance. So I've read 9b(2) in the most conservative way, and thought of it as a gray area unless the fix was actually in the GPS database for the procedure. Good to know I can use the GPS distance.

 

[poadeleted20]

JOBGO is a waypoint on a final approach segment that is determined by DME distance from a navaid.

Technically, JOBGO is a "DME fix," or "intersection" (of the radial and the DME arc), not a "waypoint," the latter being just a point in space not defined by a ground navaid. You can use a GPS to identify that fix, but if it were only a waypoint, only an RNAV system could identify it.

It has never been clear to me whether the FAA would say that something like this was illegally substituting for the navaid, or whether it's okay because you're only using the GPS to get the distance and not for lateral course guidance.

It's the lateral course guidance issue -- you are not permitted to use GPS to sub for a nav system you don't have (either not installed or not working) for lateral guidance on the final segment of the approach. Thus, I can use my GPS to sub for DME on the arc portion of the VOR/DME-A at KWDR because it's only being used for lateral guidance outside the FAF, but not on the VOR/DME or TACAN Z RWY 15 at KMTN because the final approach segment is the DME arc. And on the approach in question, it can be used to identify the 2.8 DME fix for stepdown because even though the fix is on the final segment, the GPS/DME is not being used for lateral guidance at that point -- the VOR is.

Alles klar?

 

[azure]

It's the lateral course guidance issue -- you are not permitted to use GPS to sub for a nav system you don't have (either not installed or not working) for lateral guidance on the final segment of the approach. Thus, I can use my GPS to sub for DME on the arc portion of the VOR/DME-A at KWDR because it's only being used for lateral guidance outside the FAF, but not on the VOR/DME or TACAN Z RWY 15 at KMTN because the final approach segment is the DME arc. And on the approach in question, it can be used to identify the 2.8 DME fix for stepdown because even though the fix is on the final segment, the GPS/DME is not being used for lateral guidance at that point -- the VOR is.

Alles klar?

Ja, danke!

And BTW, I checked today, and FF27 is in fact 2.8 nm from FNT (which is itself, 0.6 nm past RW27). So evidently, FF27 is JOBGO and it is part of the approach in the CNX80, just not by name. What I can't figure out is why Jepp uses CNF naming conventions in the CNX80 database for fixes that actually have real charted names, especially since in this case at least, they use the real charted name in the databases for other models (if I understand correctly what John Collins and others have posted here).

 

[aterpster]

Plug FNT VOR in the GPS and look for 2.8 miles to go. If you've loaded the approach, you have a problem if the GPS is showing distance to the runway, not the real MAP (the VOR) and you could step down a touch too soon. Of course, you're supposed to be flying this off the VOR anyway, although it's nice to have the approach in the GPS for situational awareness.

Note than on the 430/530 series, the 2.8 DME fix will be a waypoint in the approach, and you'll see when it sequences so you know when to step down. I thought the CNX80 worked the same way, but your post suggests it doesn't.

Actually, it is shown as JOBGO in the G-1000. The 400/500 would be the same since they share a common database with the G-1000.

 

[aterpster]

JOBGO is in the GNS430W and GTN databases as part of the approach. In the case of the CNX80, many of the ATD fixes are not included in the database, but you can use the FNT 2.8 distance to go on the approach as the step down without a DME.

JOBGO is not an ATD. It is a published stepdown fix for the VOR IAP.

 

[aterpster]

Ja, danke!

And BTW, I checked today, and FF27 is in fact 2.8 nm from FNT (which is itself, 0.6 nm past RW27). So evidently, FF27 is JOBGO and it is part of the approach in the CNX80, just not by name. What I can't figure out is why Jepp uses CNF naming conventions in the CNX80 database for fixes that actually have real charted names, especially since in this case at least, they use the real charted name in the databases for other models (if I understand correctly what John Collins and others have posted here).

You better check again.

Attached is this IAP from the G-1000.

 

[John Collins]

JOBGO is not an ATD. It is a published stepdown fix for the VOR IAP.

My terminology may not be correct, but it is an along track distance to the VOR and with a DME it 2.8 DME. If the GNS480 defines it in its database as a CNF without a published name, it serves the same purpose as the named fix. The stepdown fix is not a required part of the approach as this approach can be flown with a VOR and no DME or GPS in Lieu of a DME. Being able to identify the stepdown location permits a lower MDA.

 

[John Collins]

You better check again.

Attached is this IAP from the G-1000.

Interesting that the G1000/GNS530W/GTN database defines the MAP at the runway and not at the VOR where the approach charts define it to be.

 

[aterpster]

Interesting that the G1000/GNS530W/GTN database defines the MAP at the runway and not at the VOR where the approach charts define it to be.

I presume Jeppesen has defined database standards for on-airport, no-FAF non-overlay IAPs. On the overlay IAPs they were constrained to have the sensor FAF at 4 miles prior to the threshold. FF27 isn't a sensor FAF, and it's 5.3 miles prior to the threshold. This has to be a Jeppesen standard, or perhaps it is something agreed to at some industry venue. I presume the need is driven by some of the high-end FMSes, which can treat such an IAP as an overlay of sorts.

Obviously, there is no-harm, no-foul, in missing prior to the on airport VOR. Nonetheless, the chart is the official reproduction of source, which indeeds shows the MAP at the VOR, which is what criteria require.

 

[poadeleted20]

Obviously, there is no-harm, no-foul, in missing prior to the on airport VOR.

Not necessarily. It is entirely possible for there to be a big rock to one side short of the MAP such that starting a missed with a turn in that direction far enough prior to the MAP could compromise obstruction clearance. That is why, although we can start the climb, we are directed not to make any turns prior to the MAP even if we decide to go missed early. I suppose that when the MAP is on the airport, starting the missed at the approach end of the runway almost certainly won't have that result, but I sure wouldn't turn before reaching that threshold.

 

[aterpster]

Not necessarily. It is entirely possible for there to be a big rock to one side short of the MAP such that starting a missed with a turn in that direction far enough prior to the MAP could compromise obstruction clearance. That is why, although we can start the climb, we are directed not to make any turns prior to the MAP even if we decide to go missed early. I suppose that when the MAP is on the airport, starting the missed at the approach end of the runway almost certainly won't have that result, but I sure wouldn't turn before reaching that threshold.

Gee, I didn't know that.:wink2:

Flying the database missed approach will preclude that.

 

[azure]

You better check again.

Attached is this IAP from the G-1000.

I will check it again, but I am fairly (say, 98%) sure that the distances from FF27 in the CNX80 database were the same as from JOBGO in your attachment. I was on the ground warming up the engine, not flying, so I had plenty of time to look carefully and make sure that I was reading the distances correctly. Still, I'll triple check it to be 100% sure.

Since FF27 is a CNF, does it necessarily have to be the same fix across platforms?

 

[aterpster]

I will check it again, but I am fairly (say, 98%) sure that the distances from FF27 in the CNX80 database were the same as from JOBGO in your attachment. I was on the ground warming up the engine, not flying, so I had plenty of time to look carefully and make sure that I was reading the distances correctly. Still, I'll triple check it to be 100% sure.

Since FF27 is a CNF, does it necessarily have to be the same fix across platforms?

FF27 is not provided by the FAA but I would think it would remain constant across platforms because I believe it was created by Jeppesen, not the avionics vendors. I can't give you an authoritative answer, though.

Added: FF27 serves no purpose except for a few high-end systems in any case. Sensor FAFs are useful only on overlay IAPs that do not have procedural FAFs. This is not an overlay IAP for most of us.

 

[aterpster]

Not necessarily. It is entirely possible for there to be a big rock to one side short of the MAP such that starting a missed with a turn in that direction far enough prior to the MAP could compromise obstruction clearance. That is why, although we can start the climb, we are directed not to make any turns prior to the MAP even if we decide to go missed early. I suppose that when the MAP is on the airport, starting the missed at the approach end of the runway almost certainly won't have that result, but I sure wouldn't turn before reaching that threshold.

I stand corrected. If in autoflight, the G-1000 will start the missed approach turn .6 miles prior to the VOR. That is a major screw-op, probably on the part of Jeppesen.

 

[poadeleted20]

Flying the database missed approach will preclude that.

Agreed. I was just concerned that someone might be misled by the unqualified statement "there is no-harm, no-foul, in missing prior to the on airport VOR," as there can be harm if you don't go to at least the database MAP before turning.

 

[poadeleted20]

I stand corrected. If in autoflight, the G-1000 will start the missed approach turn .6 miles prior to the VOR. That is a major screw-op, probably on the part of Jeppesen.

It will happen on every approach with straight-in mins where the "real" MAP is at an on-field navaid, as the final segment in the database is always anchored on the runway threshold, not the navaild, and that's where the system suspends.

 

[aterpster]

It will happen on every approach with straight-in mins where the "real" MAP is at an on-field navaid, as the final segment in the database is always anchored on the runway threshold, not the navaild, and that's where the system suspends.

I certainly would believe that to be the case. Nonethess, it is not coding that is faithful to the procedure.

 

[aterpster]

Agreed. I was just concerned that someone might be misled by the unqualified statement "there is no-harm, no-foul, in missing prior to the on airport VOR," as there can be harm if you don't go to at least the database MAP before turning.

I would add going to at least, or just to, the database MAP may not be safe.