Approach Types in Garmin GNC 355

[Llewtrah381]

I currently have a Garmin 300XL as my only certified GPS and, needless to say, it's getting long in the tooth although it's still working well. Looking at upgrade options, I'm wondering about swapping it out for a GNC 355, since those are very similar in profile and that's several AMUs cheaper than a GTN 650, especially with installation labor. I have a Nav/Comm as well (MAC 1700, which I like a lot, actually), so I receive VORs and ILSs.

The 300XL database only includes GPS approaches, so I can't use the 300XL to "follow along" on an ILS or VOR approach unless I hand-load the waypoints (and that's only a partial solution: the "final" point is the airport and not the runway threshold). My question, for those with a GNC355, is if that unit includes VOR and ILS approaches in the database or if it, too, only has the ones "permitted" to be used by the unit. Because a GNC 650, etc. is GPS plus Nav, what's in the databases for those units won't answer my question.

 

[David Lloyd]

GNX375, but pretty much the same; has GPS, VOR and ILS approaches in the database. Have never loaded a VOR approach but loading an ILS approach a warning pops up that it is for practice only and not to used as an actual instrument approach. Fancier wording but that’s the gist.

 

[Llewtrah381]

GNX375, but pretty much the same; has GPS, VOR and ILS approaches in the database. Have never loaded a VOR approach but loading an ILS approach a warning pops up that it is for practice only and not to used as an actual instrument approach. Fancier wording but that’s the gist.

Perfect. Thanks. I tried searching the manual for this, with no success, but was searching for VOR approaches. Searching for ILS gave the language you described.

My GPS is connected to a DAC roll steering and it would be great to have those approaches in the database, even just for "monitoring", to feed the autopilot. Sounds like that would be a big improvement for me, even if it's only for ILS (and I haven't seen if VORs are or are not included, but those are less important).

Question answered. Thanks again!

 

[DGlaeser]

The 355 is GPS/COM only, just like the 300XL. The good news is that all non LPV approaches have an advisory glide slope
The 375 adds an ADSB Transponder.
The 175 is just the GPS navigator, no COM or Transponder.
The navigator is the same in all of them.
Any non-gps approaches in the database are GPS overlay procedures- advisory only as noted.
If you google Garmin 355, you can find the manual.

 

[RussR]

The 355 is GPS/COM only, just like the 300XL. The good news is that all non LPV approaches have an advisory glide slope

I don't think that's quite true. Certainly the vast majority of them do. But I don't think it's "all".

Any non-gps approaches in the database are GPS overlay procedures- advisory only as noted.

Nitpicky pet peeve here I know, but "GPS Overlay" is a specific term starting back in the 90's for procedures where the name was changed from "VOR RWY xx" to "VOR or GPS RWY xx" as a quick method to make GPS useful as actual GPS procedures were developed. There are still some out there. But loading a VOR or ILS (or NDB) procedure from a GPS database does not constitute an "overlay" procedure.

 

[DGlaeser]

I guess I can’t say all, since I haven’t tried all, but haven’t found an approach yet without an advisory GS.

Since those navigators don’t have VOR or ILS receivers, the best they can do is replicate the approach with GPS. I don’t know what, or if, there is an official term for that, so I just used the term overlay. Sorry for any confusion.

 

[David Lloyd]

KCQW GPS runway 8 is an LNAV with vertical guidance. Seems like when I selected that approach there was a message that said the glide path was advisory only. Glide slope displayed, autopilot armed, captured and followed just like an LPV. Guess you could either use the advisory or do a dive and drive. Oh, so many new tricks for an old dog.

 

[midlifeflyer]

KCQW GPS runway 8 is an LNAV with vertical guidance. Seems like when I selected that approach there was a message that said the glide path was advisory only. Glide slope displayed, autopilot armed, captured and followed just like an LPV. Guess you could either use the advisory or do a dive and drive. Oh, so many new tricks for an old dog.

Here's the profile view for that approach. The "3.04" is telling you that a stabilized 3.04 degree descent begun at the FAF will bring you to to a point 45' above the runway threshold. The FAA refers to it as the "Continuous Descent Final Approach," an more stabilized alternative to "dive and drive." You can do it manually and that's basically what your Garmin box is emulating. With the newest boxes, you even get them on VOR approaches.

The point to remember is that the approach was not designed for vertical guidance. That's the reason for the disclaimer that it is advisory only. You do not have a DA. It's still just an MDA.

 

[RussR]

I guess I can’t say all, since I haven’t tried all, but haven’t found an approach yet without an advisory GS.

I seem to remember that approaches that have the profile note "Visual Segment - Obstacles" and therefore do not have a VDA published, will not have advisory glidepaths. But I'm not 100% sure on that.

Example - the 4G4 RNAV procedures have the note and no VDA. I "think" this means they will not have an advisory glidepath.

 

[peet]

KCQW GPS runway 8 is an LNAV with vertical guidance. Seems like when I selected that approach there was a message that said the glide path was advisory only. Glide slope displayed, autopilot armed, captured and followed just like an LPV. Guess you could either use the advisory or do a dive and drive. Oh, so many new tricks for an old dog.

Well thats a way to wind up six feet under..
Just because it gives you a ball to fly doesnt mean its going to clear the towers stacks trees or whatever is there that stopped the LPV approach from being approved.

 

[David Lloyd]

Yes, I understand the difference in an LPV and LNAV with a glide path advisory and the danger of blind faith in an advisory. Does everyone?

 

[John Collins]

I seem to remember that approaches that have the profile note "Visual Segment - Obstacles" and therefore do not have a VDA published, will not have advisory glidepaths. But I'm not 100% sure on that.

Example - the 4G4 RNAV procedures have the note and no VDA. I "think" this means they will not have an advisory glidepath.

View attachment 105013

That is no longer the case. It is true that the VDA will not be charted on the FAA charts when the note "Visual Segment -Obstacles" is shown on the chart, however with current GPS software, the advisory LNAV+V or LP+V will be provided. At one point, the VDA was coded as a zero and the chart note was "Descent Angle NA". The Garmin navigators at the time would fail with a zero value in the VDA, so any such approaches were removed from the database. All of those procedures have been amended to include the new note and the VDA (non zero)/TCH is included in the source data. The current GPS software will generate the +V if the VDA and TCH is provided. It the note is present, it means that flight test determined that following the VDA below the MDA has obstacles and that following the VDA below the MDA (which is prohibited anyway) could be hazardous. In one case, flying the VDA below the MDA takes you through a ridge.

 

[John Collins]

Here is 4G4 RNAV RWY 10 showing +V.

 

[John Collins]

Yes, I understand the difference in an LPV and LNAV with a glide path advisory and the danger of blind faith in an advisory. Does everyone?

It is not a blind faith issue, it is an understanding of the purpose of an advisory glidepath or LP+V or LNAV+V. The purpose of the +V is to aid the pilot in flying a stabilized and constant angle descent on a Non Precision straight in approach between the FAF and the MDA. It was never intended to be used below the MDA and does not provide guidance between the MDA and the runway that is clear of obstacles. A non precision approach does not provide a clear path below the MDA to the runway, it only assures that obstacles below the MDA must be avoided visually. A vertically guided procedure such as an LPV or LNAV/VNAV has a decision altitude (DA) and meets requirements for obstacle clearance from the DA to the runway.

 

[David Lloyd]

Exactly. However, for some it is a blind faith issue because they don’t understand there is a difference.

 

[Randomskylane]

It is not a blind faith issue, it is an understanding of the purpose of an advisory glidepath or LP+V or LNAV+V. The purpose of the +V is to aid the pilot in flying a stabilized and constant angle descent on a Non Precision straight in approach between the FAF and the MDA. It was never intended to be used below the MDA and does not provide guidance between the MDA and the runway that is clear of obstacles. A non precision approach does not provide a clear path below the MDA to the runway, it only assures that obstacles below the MDA must be avoided visually. A vertically guided procedure such as an LPV or LNAV/VNAV has a decision altitude (DA) and meets requirements for obstacle clearance from the DA to the runway.

Anyone have an example of a runway with problematic obstacles after mda? I’m just starting the IFR journey and would never have thought of this. Yes I knew lpv, lnav +v etc are different; just no experience

TIA

 

[John Collins]

For an LNAV, the obstacle clearance requirement is just a level plane. The MDA is 250 feet above the controlling obstacle along the final approach course inside the FAF. So once you have descended 250 feet from the MDA, you can hit an obstacle if you don't see it visually. An LNAV MDA can easily be 500 or 600 feet above the runway. So take the height above the runway into consideration when flying an LNAV procedure. Some clues you should consider when flying an RNAV straight in approach that will indicate the obstacle environment in the visual segment below the MDA (MDH).

1) High MDH means there are high obstacles
2) The cleanest obstacle environment is if you see the stipple shown between the VDP and the runway. The stipple is the gray triangular arrow head symbol that extends from the VDP to the runway on the chart and indicates the visual segment is clear on a 34 to 1 slope. You will see the stipple on an ILS and on some RNAV.
3) If the slope from the VDP to the runway is clear on a 20 to 1 slope, a VDP will be charted. No VDP, then you can't count on 20 to 1. BTW 19.08 to 1 is a 3 degree slope. Also if the 20 to 1 is not clear in the visual segment, then you will most likely see Procedure NA at night, particularly when there are unlit obstacles and there isn't a VGSI available with a higher descent angle to mitigate it.
4) No VDP, no Stipple, NA at night and you see the chart note "Visual Segment -Obstacles" in the profile view, then not only is the 34 to 1 not clear, nor is the 20 to 1 clear, but you might even hit an obstacle if you follow the +V. This note appears when the flight test requires the flight test aircraft to maneuver on final approach to avoid coming dangerously close to hitting an obstacle.

So to answer your question there are tons of such approaches. Examples KBML RNAV 18 (Not clear on 20 to 1), high MDH. KEOE RNAV 22 (clear on 20 to 1, VDP). KOGB RNAV 23 (LNAV is clear on 34 to 1, stipple and VDP), 4G4 RNAV 10 (no stipple not clear on 34 to 1, no VDP not clear on 20 to 1, Has profile note "Visual Segment - Obstacles" and if you fly it using +V below the MDA, you may get removed from the gene pool).