CFII Debate - GPS in lieu of DME

Keith Johns

Keith Johns

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Keith Johns
CFIIs & DPEs,

What are you teaching in regards to using GPS in lieu of DME (AIM 1-2-3) on a LOC/DME approach? There is some controversy in my circles about this. Some experienced CFIs and a few examiners I know will not allow you to use the GPS waypoint distance information on the MFD (G1000 or Perspective) to identify the distance to each fix as well as the VDP. This sounds odd to me. As long as I am sourced and tuned to the localizer on the CDI on the approach, I feel I should be allowed to use the distances on the GPS FPL to identify the VDP and MAP.

According to AC-90-108
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.

Why would I not be able to use the most accurate distance information available to identify a fix or the distance to the calculated VDP?

I'd love some opinions on this fellow aviators. Happy flying!
 
The stuff I'm flying/teaching in is all steam. Sorry. :D
 
Not a CFI nor a DPE, but I've never encountered an issue when using GPS data on a certified device in lieu of a DME.
 
jordane93 said:
If it’s a certified GPS and the database is current why wouldn’t you be able to use it in lieu of DME? Sounds like the DPE and CFIs have no idea what they’re talking about.
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That.

That's what I teach as a CFI, and that's what I'm allowed to do as a 135 pilot, both with company check airmen and flight safety international.

You can also fly a NDB via GPS overlay, as long as you're monitoring it elsewhere, same with a VOR. Only thing I do that's not GPS derived is an ILS.


MauleSkinner said:
What is the rationale of the ones who don't allow it?
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Shy of them just not knowing what they are taking about, I could see a student wanting to use a "not for primary navigation" MFD or portable GPS/iPad or something
 
GPS uses lateral distance while DME gives slant distance - consider the case of being overhead of a beacon. However, it is allowed in the rules, so legally can be used unless otherwise stated.
 
sgenie said:
GPS uses lateral distance while DME gives slant distance - consider the case of being overhead of a beacon. However, it is allowed in the rules, so legally can be used unless otherwise stated.
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Talk about picking nits
 
Are you talking about named fixes that are in the database? Or a VDP that is based on LOC/DME which is at the far end of the runway while your GPS is showing distance to the approach end of the runway?
 
sgenie said:
GPS uses lateral distance while DME gives slant distance - consider the case of being overhead of a beacon. However, it is allowed in the rules, so legally can be used unless otherwise stated.
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10 miles out at 6000' AGL your error would be about .05 miles.
12000' AGL it would go all the way up to a whopping .2 miles.
Take it out to 20 miles @ 12k AGL and it's .1 miles.

Less than 6000 feet AGL (which is the only time 1/10ths are going to matter), even at less than a mile away, you're talking < .5 error.
 
sgenie said:
GPS uses lateral distance while DME gives slant distance - consider the case of being overhead of a beacon. However, it is allowed in the rules, so legally can be used unless otherwise stated.
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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.

Edit: I see Salty beat me to it.
 
sgenie said:
Why is it nonsense?
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Because the FAA has made it very clear that:

operators may use a suitable RNAV system in the following ways.
(1) Determine aircraft position relative to or distance from a VOR [snip], 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.
​
The allowances described in this section apply even when a facility is identified as required on a procedure (for example, “Note ADF required”). NOTE: For the purpose of this AC, “VOR” includes VOR, VOR/DME, and VORTAC facilities. NOTE: For the purpose of this AC, compass locator includes locator outer marker and locator middle marker.
​
Nevertheless, folks like the OP is talking about manage to find grey, typically spouting about the small slant rage discrepancy the FAA disregards.
 
Dave Theisen said:
Are you talking about named fixes that are in the database? Or a VDP that is based on LOC/DME which is at the far end of the runway while your GPS is showing distance to the approach end of the runway?
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The requirement for DME replacement is that you use the same facility for distance as the DME would. So if you're using the DME on the I-ABC ILS then you need to have I-ABC in your GPS database and use that for distance. (You could also identify the individual named waypoints or an overlay approach for those.) If the VDP isn't a named waypoint, isn't shown on the overlay approach and you don't have I-ABC then you can't substitute for it.

Edit: I've seen suggestions that if you know that an un-named point on the chart is 2.3 NM inside FUBAR you can use that distance from FUBAR to identify the point. I'd say maybe legal, but a bit more math than I really want to be trying to do in-flight. Of course, I have 2 GPS units, so I can tune #2 to Direct I-ABC if I need DME distances while #1 shows me the named fixes.
 
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While I agree GPS should be good v DME, I do understand the point. It’s the same reason you can’t use GPS for a VOR approach without the VOR raw data displayed somewhere. Some say you need the DME raw data displayed.

I don’t necessarily agree with that, but I believe that is the logic the are using.
 
Kritchlow said:
While I agree GPS should be good v DME, I do understand the point. It’s the same reason you can’t use GPS for a VOR approach without the VOR raw data displayed somewhere. Some say you need the DME raw data displayed.

I don’t necessarily agree with that, but I believe that is the logic the are using.
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But that's exactly what the AIM and AC say: VOR or NDB for lateral guidance on final(and not an 'or GPS' approach), must have raw data. DME can substitute with GPS. If they meant DME raw data was needed they would have said that.
 
This is a bit like the logging vs acting PIC discussion. The number of times I've had to explain that to CFIs is also too damn high.
 
There are only two approaches in the country you can't use the GPS substitution:
KMTN VOR/DME OR TACAN Z RWY 15
KWAL VOR/DME OR TACAN RWY 10
Both use the DME for the final approach course. Both have a bunch of other approaches.
 
flyingron said:
There are only two approaches in the country you can't use the GPS substitution:
KMTN VOR/DME OR TACAN Z RWY 15
KWAL VOR/DME OR TACAN RWY 10
Both use the DME for the final approach course. Both have a bunch of other approaches.
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Are there any “arc in” VOR DME approaches left in the US..??
 
Kritchlow said:
Are there any “arc in” VOR DME approaches left in the US..??
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I'm not sure what you're asking. Those two I quoted use the DME arc as the final approach course, down to the runway. The MTN one even uses the arc as the missed appraoch (just turn around and follow it back the way you came in).

MTN is a mixed use reliever east of Baltimore.

KWAL is a NASA facility, so you're not likely landing there.
 
flyingron said:
There are only two approaches in the country you can't use the GPS substitution:
KMTN VOR/DME OR TACAN Z RWY 15
KWAL VOR/DME OR TACAN RWY 10
Both use the DME for the final approach course. Both have a bunch of other approaches.
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I thought there were two, but I could only find KMTN...thanks!
 
flyingron said:
I'm not sure what you're asking. Those two I quoted use the DME arc as the final approach course, down to the runway. The MTN one even uses the arc as the missed appraoch (just turn around and follow it back the way you came in).

MTN is a mixed use reliever east of Baltimore.

KWAL is a NASA facility, so you're not likely landing there.
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That sounds like what I’m asking. The DME arc is the lateral nav and the radials are the step down fixes. A regular VOR DME sort of in reverse...
 
Dave Theisen said:
Are you talking about named fixes that are in the database? Or a VDP that is based on LOC/DME which is at the far end of the runway while your GPS is showing distance to the approach end of the runway?
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This is the nut of the issue in my debate with my colleagues. Using GPS waypoints to derive the VDP (on a LOC/DME where it not published) and using the GPS distances to identify the VDP on a LOC/DME approach. I know this sounds like nitpicky crap, and it is!
 
Keith Johns said:
This is the nut of the issue in my debate with my colleagues. Using GPS waypoints to derive the VDP (on a LOC/DME where it not published) and using the GPS distances to identify the VDP on a LOC/DME approach. I know this sounds like nitpicky crap, and it is!
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I'm confused...are you trying to figure out if it can be used to identify a descent point where a VDP isn't published, a published VDP, or both?
 
MauleSkinner said:
I'm confused...are you trying to figure out if it can be used to identify a descent point where a VDP isn't published, a published VDP, or both?
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Both.
Let me chime in since I know under what circumstances the original question arose (Keith happens to be my CFII).
We fly G1000 equipped SR20. I never saw any VDPs displayed in G1000, regardless if published or not. I have no idea why G1000 doesn't display published VDPs but this happens to be the case. So the way I happen to identify published VDPs is to watch the GPS distance to RWY XX map because this is the distance you always get from G1000 - and this is the distance you always get on approach chart (for example 2.2 nm at airport O69, RNAV approach). So for example in case of this LNAV approach the only way for me to identify VDP is to wait until RWY29 map becomes active waypoint and wait for the distance to count down to 2.2.

But at KLVK, LOC RWY 25R since the published VDP is identified explicitly through the localizer distance 2.9 nm and our SR20 is not DME equipped and doesn't give you this distance - I again resorted to using the published VDP to threshold (gps) distance (2 nm) that some very senior CFII in the club considered unacceptable.
 
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olasek said:
But at KLVK, LOC RWY 25L since the published VDP is identified explicitly through the localizer distance 2.9 nm and our SR20 is not DME equipped and doesn't give you this distance - I again resorted to using the published VDP to threshold (gps) distance (2 nm) that some very senior CFII in the club considered unacceptable.
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I'll answer with "I don't know". If you set "Direct I-LVK" and use that, and when it says 2.9 for the VDP then I'd say all is fine. The problem is do you 100% know that the RW25R(I assume you mean 25R) waypoint in your database is the same place as I-LVK(1) and that the VDP is 2 miles outside that because the chart doesn't explicitly name that waypoint or like the RNAV chart give the distance(now based on the AIM this is almost certainly 100% fine but I can understand people whining about it). If you told me that at ZABGO you saw the distance to RW25R is 4.9 and you went until it said 2.0(thus the 2.9 miles depicted to the VDP) I'd say that was fine.

And this is why I'm not a CFI/CFII.

Edit: And out here in the real world we'd just fly the bloody RNAV or ILS.
 
If a VDP is advisory, does it matter how you identify it?

Is there a source stating use of the VDP is mandatory?
 
Keith Johns said:
CFIIs & DPEs,
What are you teaching in regards to using GPS in lieu of DME
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First.. DPE's don't teach.

Second... the GPS as DME is commonplace. The question is what you tune the GPS to. Are you putting the airport or the navaid the DME distance is measured from? [the second answer is correct]

If you're flying, per chance, the LOC/DME RWY 24 http://155.178.201.160/d-tpp/1801/05136LD24.PDF

You'd have the GPS tuned to IFUL for distance information only and the nav set to 108.9 with approach course 243 on the CDI.

Fly the needles and reference the GPS only for distance information. Kinda like having a DME for distance information.

Hmm.. why does this question keep coming up?
 
Is it just me or are there missing posts / links ?
 
Keith Johns said:
This is the nut of the issue in my debate with my colleagues. Using GPS waypoints to derive the VDP (on a LOC/DME where it not published) and using the GPS distances to identify the VDP on a LOC/DME approach. I know this sounds like nitpicky crap, and it is!
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What is the regulatory function of a VDP on an approach?
 
midlifeflyer said:
What is the regulatory function of a VDP on an approach?
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We have discussed this before.
It has always been my belief that if it’s published, it’s mandatory.... But one should always calculate one as good practice.
 
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