GPS in lieu of DME - this months IFR Killer Quiz

TMetzinger

TMetzinger

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Tim
in this month's IFR quiz, they dissect the KMTN VOR DME 15 approach, which is a fun arc approach.

One of the questions (#11) indicates that an IFR approach GPS can't be used to substitute for DME on the approach, but the current AIM says that flying a DME arc is one of the things that CAN be done with IFR GPS. I asked the editor to get the authors to elaborate.

I'm aware there's an email from AFS-400 out there somewhere that says basically "we didn't really mean what we said in the AIM when it comes to approaches", because the AIM states that it can't be used as the principal nav source i.e lateral guidance.

The problem I have with that is that any arc will use distance for lateral guidance, whether it's part of the final approach course, a lead-in to an approach, a missed, something enroute, whatever. You look at your distance and turn left or right based on that.

Has there been any "final" word from FAA chief counsel on this yet? One email does not make policy.

This is almost as bad as the "interpretations" we got from John Lynch (no ire here - he was flat out doing his best) on the part 61 rewrite, several of which were flat out contradicted by the chief counsel when a formal opinion was sought.

One would think if there was a policy change in summer 2005 it would have made it into the AIM by 2006, or as an SFAR or something.
 
i think rev. ron has all the answers on this one. basic gist is that right now, GPS can be used to fly a DME arc, but not on an approach? right? huh? what?
 
I think on the red board someone else got a letter from AFS-420 saying you CAN use it on an arc. Typical goverment operation.
 
Ron is STILL waiting for a dead certain final answer from someone out there. See the thread (it's long) on the red board. The approach in question in Ron's correspondence though is an approach where the arc IS the final approach course, not a transition to the final approach course, but the actual course itself IS an arc. I see that as a whole different ball of wax than the arc as transition, making the interpretation of very limited scope. But that's just my non-aviation lawyer-pilot opinion of the letters that Ron has received.

Maybe Reverend Ron will give us a short recap of the approach in question, and the two emails/letters he has received?

Jim G
 
I think the arc in question is the KMTN approach, the arc is a curved path to the MAP.

I wouldn't have a problem with the FAA coming out and saying that you cant substitute GPS for DME when the arc is part of the final approach segment, but they haven't said that, and what I understand they HAVE said so far (don't use it for lateral guidance) applies to all arcs.
 
TMetzinger said:
I think the arc in question is the KMTN approach, the arc is a curved path to the MAP.

I wouldn't have a problem with the FAA coming out and saying that you cant substitute GPS for DME when the arc is part of the final approach segment, but they haven't said that, and what I understand they HAVE said so far (don't use it for lateral guidance) applies to all arcs.
Click to expand...


Unfortunately, the stuff on the red board (for those who don't know it, that's the aviation forum at AOPA) is lengthy. The language from the latest tome from Ron's FAA correspondence is there, quoted by Ron Levy. If I get time when I get back from court, I'll try to find it and cut and paste over here.

The letter reads to me as pretty limiting, i.e. to the KMTN approach which has the arc as final approach track. But it is still not clear. My interpretation is that you can't use GPS in lieu of DME on THAT particular approach, which sort of makes sense.

But that's my non binding, non legal personal 2 pennies on the subject. YMMV.

Jim G
 
There is are internal differences inside the FAA on this, and I'm trying to get them to give the same answer. AFS-400 in DC said one thing, and AFS-420 in OKC (which works for AFS-400 but didn't get a copy of AFS-400's guidance) said another. The bottom line is that AFS-400 says you may not use a GPS to substitute for a DME to track a DME arc inside the IAF. You can use it for identifying DME fixes, but not for the "primary lateral guidance" on a DME arc on an approach. It doesn't make sense to me, but that's what they said. More when AFS-400 gets its act together.
 
Thanks, Ron, I checked out the thread on AOPA, and will look for further info there.

Can't wait to see what IFR says assuming this gets cleared up sometime.
 
Here is the language that Ron posted over on AOPA on this issue.

"Answer just came in. For the record, the query I sent was:


I received a note from someone who spoke to Harry Hodges in AFS-420 in OKC regarding the item below. This person writes that Mr. Hodges "concluded, without question, you can use an IFR GPS in lieu of DME for a DME arc. Further, there is no restriction if the arc is enroute, terminal or (rare) inside the FAF. He also mentioned that the AIM specifies how the procedure must be done, in paragraph 1-1-19(f)(c)(2)." That seems to conflict with the first paragraph in your email below as it would apply to an approach such as the VOR/DME or TACAN Z Rwy 15 approach to Martin State Airport (KMTN) on which the final approach course itself is a DME arc. Your message indicates this approach may NOT be flown using an IFR GPS as a substitute for DME to fly the arc inside the IAF, i.e., as "the 'principal instrument approach navigation source' used for lateral guidance."

I would appreciate clarification of what appears to be conflicting guidance from two FAA sources.


The reply was:

The restriction still applies in this case. The requirement for DME can not be met using GPS as the principal instrument approach navigation source. The VOR/DME or TACAN RWY 15 approach to Martin State Airport requires VOR/DME or TACAN in that the DME arc is the final approach course..

The "someone" who spoke to Harry Hodges may have gotten a TERPS interpretation of how using GPS in lieu of DME for an arc certainly meets the obstacle clearance requirements. A DME arc has a primary 4 NM either side of the course and a 2 NM secondary on either side of the course. (12 NM wide obstacle clearance) DME is considered to be accurate within 1/2 NM or 3% of the distance, whichever is greater, while GPS on final is considered to be 0.3 NM. Although DME is slant range and GPS is not, the results from using GPS in lieu of DME, if the procedure was in the data base, (or GPS), would probably, from a TERPS standpoint, provide an equivalent level of safety"



There you are. As one used to looking at stuff as an attorney, I would look at the following statements as limiting this restriction. "The restriction still applies IN THIS CASE. The requirement for DME can not be met using GPS as the principal instrument approach navigation source. The VOR/DME or TACAN RWY 15 approach to Martin State Airport requires VOR/DME or TACAN in that the DME arc is the final approach course.." I've added the emphasis here.

Both these statements seem to me to make it clear that they are limiting this opinion to: A. This particular approach and B. Circumstances where the DME arc IS the final approach course. To my mind, this seems like a very limited circumstance. Just so everyone can be aware of the particular approach in question, here, let me see if I can put that link in.

http://download.aopa.org/iap/20060216/NE-3/mtn_vor_dme_or_tacan_z_rwy_15.pdf

Hopefully this works. If so, you can see this isn't your garden variety arc. Rather, this is the arc itself as the final approach course. And I think this makes a great deal of difference, from, say, an arc being used inside the IAF to line up with a straight in final approach course.

Again, this is just my personal opinion of what we are seeing here. Mind you, Ron's conservative advice that you not use the GPS for the DME arc inside IAF is a good conservative recommendation that will keep you for sure from a violation. Unfortunately, of all the aircraft that I rent, only one old archer has DME, while most have a KLN94, Garmin 430 or G1000 system. This interpretation is limiting for those aircraft.

YMMV and none of this is a legal opinion on my part. Just a personal analysis of the specific language quoted.

Jim G
 
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All very nice, but I'd like to see the ORIGINAL message that imposed the limitation, not the thirdhand version. And the response quoted above completely ignores the fact that the GPS or DME is ALWAYS the primary nav source for lateral guidance when flying an arc, no matter what the purpose of the arc is.

Again, if they want to limit what the AIM says, they need to:
Say it in english (as opposed to taking the absurd position that the AIM already limits this)
Publish it in a procedurally correct fashion.

So while I'm not arguing with the limitation they are trying to impose, I am arguing with the way they are trying to impose it.
 
reading the AIM closely, I now halfheartedly agree that the exclusion of 1-1-19 F b 6 .."except for use as the principal instrument approach navigation source" could mean that one couldn't fly:

Any approach where DME is required to establish your position on the approach, i.e. an arc. But that would apply to an arc ANYWHERE in the approach, not just in the final segment, because distance is ALWAYS your lateral guidance on an arc.

But nothing in that language implies "lateral" guidance only. That logic would apply to straight in approaches where DME was used to determine stepdown fixes, or DME is used to determine the MAP. Hence the memphis incident. As les says- what the AIM says is apparently not what was intended, since the "lateral guidance" part was left out. Since it's been that way for years, wonder why they don't fix it.

I am certain that the exclusion was put in primarily to cover ADF (which makes sense), so you couldn't just punch in an ADF ID and fly an NDB approach - that requires an overlay with named fixes etc stored in the database
This seems like a stretch to make it fit DME as well, and in light of conflicting opinions in the FAA, I hope they clarify it soon. Especially since I don't know of DME arcs being used outside of approaches except maybe for DPs/STARS (which are normally in the database anyway).

I still think it's arguable that Les is wrong, but I can see how his position is logical (even if it's dumb in my opinion). What's not logical is putting any sort of interpretation to "allow" GPS arcs on approaches up to the FAF but not past it - if you accept Les' interpretation of the AIM as written, GPS arcs are verboten in approaches period. They should just fix the AIM so it says what they want it to say, so that pilots might actually read and understand it

Best Wishes (to Les, too!)
 
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TMetzinger said:
All very nice, but I'd like to see the ORIGINAL message that imposed the limitation,
Click to expand...
A bit of background: The AIM language, "Charted requirements for ADF and/or DME can be met using the GPS system, except for the use as the principal instrument approach navigation source.", was intended to mean that GPS could not be used as a substitute for the "principal instrument approach navigation source" used for lateral guidance. Thus, the intent was to allow the use of GPS as a substitute for DME on a VOR/DME approach. However, the AIM language is what it is and can properly be interpreted as requiring both VOR & DME for this approach with no GPS substitution allowed.
In the case at hand, the refusal to clear an aircraft to conduct a VOR/DME approach when the required DME was out of service appears to me to be correct.
The GPS Approach Overlay program, which I think is not being done anymore, does allow use of GPS under certain conditions. If this approach had the label "or GPS" it could have been completed with GPS.
NACO charts are identified by their procedure name, based on the NAVAIDS required for the final approach. The VOR/DME RWY 18 approach at Tupelo requires both VOR and DME to conduct the procedure.
The AIM, table 1-1-6, indicates GPS may be used for certain equipment In lieu of ADF and/or DME under certain conditions. The AIM, paragraph 1-1-19 f. outlines the use of GPS in lieu of ADF and DME. This AIM paragraph lists operations allowed and restrictions.
AIM, paragraph 1-1-19 f. (b) (6) states, "Charted requirements for ADF and/or DME can be met using the GPS system, except for the use as the principal instrument approach navigation source."
The VOR and the DME are the princpal instrument approach navigation
source(s) for the VOR/DME RWY 18 approach at Tupelo. With the DME out of service , in this case, the charted requirement for DME can not be met using the GPS system.
It is also a concern that the correct policy is understandable by the pilot community. The GPS guidance section has been expanded over the years. In this case, it requires reading the whole section and determining how it applies. We expect that in the future the use of GPS will be further expanded just as we have added more information due to the commissioning of WAAS.
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