Glidepath when going direct to FAF on an RNAV circling approach on the Garmin GPS175/GNC-355/GNX-375

[Narwhal]

I did 5 RNAV GPS approaches last night in my new airplane that has a G3X/GDU-465/GNC-355/G5/GMC-305 setup. They all worked great except for the very last one, which was also the only circling approach.

On that one, I got cleared to the FAF on an RNAV GPS-A and the approach mode on the GMC-305/G3X just did not seem to work at all. I had loaded the approach earlier. When I received the clearance, I tapped the FAF from the flight plan screen on the GNC-355, selected direct, then armed the approach mode on the GMC-305 autopilot. When I got to the FAF, although my lateral mode was "GPS", the CDI and autopilot did not seem to turn to the final approach course until I flew past the fix, and the GP (glidepath) never showed up or captured, despite being at the published altitude at the FAF. Not a big deal, I just disconnected the autopilot, and since it was practice in VMC I just pointed the airplane toward the airport. However, this is the only approach at my home airport so I'd like a thorough understanding of it.

I've been reading that controllers aren't really supposed to issue direct to FAF clearances, and that on the garmin GPS's it's better to select the FAF and choose "activate leg" while on a vector to the course outside of the FAF. Since this was a VFR practice approach, I guess the rule not to clear me to the FAF didn't really apply.

So....is there a way to go direct to the FAF with these garmin GPS's and still have approach logic function correctly (in case it's a visual backed up by an RNAV or a VMC practice approach)?

Is the lack of a glidepath something to expect on circling approaches? Or is there something else that I must've done incorrectly?

Here is the approach if it matters:
https://skyvector.com/files/tpp/2203/pdf/01522RA.PDF

 

[n7421w]

stupid question, did you activate the approach? Also, how did you load the approach? Vectors to final? Start at YOHOE?

 

[Narwhal]

I loaded the approach by selecting "procedure", typed in PAMR, then from the list if procedures selected RNAV GPS-A, and chose YOHOE transition. The approach staring at YOHOE then appeared on the flight plan page, after the missed approach from the previous approach, which I was still flying when I loaded it. Then I hit direct RICKR from the flight plan page when cleared. I never selected activate, maybe that was the problem? I guess I thought going to a fix in the approach would automatically do that.

 

[n7421w]

You have to activate the approach for everything to work properly. Going direct to a fix is just that. Just like going to any other GPS waypoint.

 

[luvflyin]

I don’t think any GPS Navigator allows Direct to an FAF. Paging @midlifeflyer

 

[Hang 4]

I loaded the approach by selecting "procedure", typed in PAMR, then from the list if procedures selected RNAV GPS-A, and chose YOHOE transition. The approach staring at YOHOE then appeared on the flight plan page, after the missed approach from the previous approach, which I was still flying when I loaded it. Then I hit direct RICKR from the flight plan page when cleared. I never selected activate, maybe that was the problem? I guess I thought going to a fix in the approach would automatically do that.

Did you delete the prior approach before loading the new one? Was the prior approach the same one? The GPS might still be thinking you're on the missed. The prior posters that mention activating the approach in the GPS have the missing link I think.

 

[Narwhal]

Did you delete the prior approach before loading the new one? Was the prior approach the same one? The GPS might still be thinking you're on the missed. The prior posters that mention activating the approach in the GPS have the missing link I think.

Previous approach was the RNAV GPS 07L at PANC and the missed approach is straight ahead to 800 then climbing right turn to 3000 direct JUKEP and hold. ATC issued missed was right turn heading 210 maintain 2500, which I flew using HDG and IAS modes passing the MAP.

I ran my scenario through the Garmin emulator and I think what might've happened I was in the "SUSP" mode immdediatley I after I loaded the RNAV GPS A at PAMR and selected direct RICKR, because in the garmin sim, I got the SUSP right then. Maybe I didn't notice it when I was flying the airplane. In the sim, SUSP occurred regardless of whether I "loaded" or "activated" the approach on the garmin simulator. I needed to unsuspend the GNC-355 after going direct RICKR to resume waypoint automatic sequencing. When I did that in the emulator the GNC-355 correctly sequenced RICKR to the MAP for the RNAV GPS A PAMR.

However, it still did not draw a glidepath in the emulator. Does the GNC-355 not draw a glidepath for cirlcing approaches? Even flying the full approach from YOHOE would not draw an advisory glidepath from the to the missed approach point. That would be nice, but maybe it's not possible aside from using the Vcalc utility to compute vertical speed (which does give me the white boxes to fly through on the G3X PFD, just no vertical path indicator next to the altitude tape).

 

[asicer]

Previous approach was the RNAV GPS 07L at PANC and the missed approach is straight ahead to 800 then climbing right turn to 3000 direct JUKEP and hold. ATC issued missed was right turn heading 210 maintain 2500, which I flew using HDG and IAS modes passing the MAP.

I ran my scenario through the Garmin emulator and I think what might've happened I was in the "SUSP" mode immdediatley I after I loaded the RNAV GPS A at PAMR and selected direct RICKR, because in the garmin sim, I got the SUSP right then. Maybe I didn't notice it when I was flying the airplane. In the sim, SUSP occurred regardless of whether I "loaded" or "activated" the approach on the garmin simulator. I needed to unsuspend the GNC-355 after going direct RICKR to resume waypoint automatic sequencing. When I did that in the emulator the GNC-355 correctly sequenced RICKR to the MAP for the RNAV GPS A PAMR.

The box should have put you in SUSP mode as soon as you hit the MAP on the RNAV GPS 07L at PANC approach (or any other approach for that matter). You have to un-SUSP to fly the missed approach procedure. Perhaps you didn't notice because ATC gave you a different missed than what was published?

 

[Narwhal]

The box should have put you in SUSP mode as soon as you hit the MAP on the RNAV GPS 07L at PANC approach (or any other approach for that matter). You have to un-SUSP to fly the missed approach procedure. Perhaps you didn't notice because ATC gave you a different missed than what was published?

The GNC-355 gives a pop-up prompt to "Activate Missed Approach" or "remain suspended" when reaching the MAP. I selected "activate missed approach" passing the MAP as I was flying the ATC assigned missed approach. As I understand it, this selection termintaes the suspension. The subsequent suspension didn't occur until I went direct RICKR (2nd approach FAF) in the emulator. I'm not sure why it suspsends sequencing when loading or activating the next approach and going direct to the FAF, but it does in the simulator/emulator.

 

[midlifeflyer]

I don’t think any GPS Navigator allows Direct to an FAF. Paging @midlifeflyer

It will. It's just a series of waypoints. Tap direct to the FAF and it will go direct there and then sequence to the runway. If it did not sequence laterally, there was an error.

Vertical guidance is a different issue.

 

[midlifeflyer]

However, it still did not draw a glidepath in the emulator. Does the GNC-355 not draw a glidepath for cirlcing approaches?

Garmin has a series of criteria for generating advisory vertical guidance (+V) for an approach. It has probably changed periodically but if I recall correctly, one of them is the availability of straight in minimums.

 

[Narwhal]

Garmin has a series of criteria for generating advisory vertical guidance (+V) for an approach. It has probably changed periodically but if I recall correctly, one of them is the availability of straight in minimums.

Thanks! Is there anyplace they publish this information for the GNC-355? I could not find it in the pilot's guide or installation manual.

I have no idea why the devices present the magent "SUSP" annunciation when I leave the missed approach of the prior approach and go direct to FAF on the subsequent approach that has been loaded on the flight plane page, with the IAF for the next approach appearing after the holding waypoint of the previous missed approach.

Here's a series of pictures from the sim/emulator that depicts what happens as I fly the RNAV GPS 07L at ANC then fly the ATC missed approach in HDG/IAS (RT H210, Maintain 2500), then try to load the RNAV GPS-A at PAMR and go direct to the FAF (RICKR). It doesn't matter if I select "load approach " or "activate approach" when I load the RNAV GPS-A, it goes to SUSP either way. If I hit unsuspend, the approach works, I just want to know why it goes into SUSP in this scenario.

Passing the FAF inbound on the first approach:

Reaching the MAP on the first approach:



After I select "Activate Missed Approach", but actually flying the ATC issued missed in HDG::



After I hit the "procedures" key, previous approach still active on the missed segment:



Retrieving the next approach, full procedure, from the procedures menu while still flying the previous missed:


Now I'm off the previous missed approach and on an ATC vector of 010. What the flight plan page looks like as I'm getting vectored towrd the FAF. JUKEP MAHP is still the active waypoint, followed by JUKEP (Hold 4.0 NM) and the IAF for the next approach, YOHOE.



Getting cleared direct to the FAF for the RNAV GPS-A at PAMR:


Suspended once I hit Direct Activate to the FAF. This occurs regardless of whether or not the approach was "activated" or "loaded". The only difference is activating the approach makes YOHOE the active/magenta waypoint before hitting direct RICKR, whereas loading retains JUKEP MAHP as the active/magenta waypoint. Either way, selecting RICKR and going direct results in a SUSP annunciation which must be unsuspended to resume sequencing onto the final approach segment.


Flight plan page during suspension. No turn toward RW07 after RICKR and a messsage prompt saying aproach not active.


Selecting "unsuspend" and now the waypoint sequences and I turn on the final approach segment and descend to MDA.


Question is, why is the suspension happening after loading OR activating the second approach while on the missed for the first, making the unsuspend necessary in this scneario?

 

[Narwhal]

Ok, I think I figured it out.

The problem was I was going direct to the FAF, RICKR ("inside") with too steep of an intercept angle to the final approach course? Apparently it doesn't want to allow sequencing automatically unless you wait until the turn results in less than a 45 degree intercept go direct to the FAF. In this case with a 089 final aproach course from RICKR to RWY 07R, I think you need to be past the 224 radial/044 bearing from RICKR (FAF) before going direct to RICKR, otherwise you get the SUSP.

 

[midlifeflyer]

Thanks! Is there anyplace they publish this information for the GNC-355? I could not find it in the pilot's guide or installation manual.

I don't know. I know that the manuals say that +V is not available for all approaches and that there are tolerances for when it is available, but I'm not aware of a specific document which talks about them. @John Collins?

 

[wayneda40]

Just a general recommendation wrt "going Direct to a FAF"... ATC should never instruct this, and if they do you should say "unable, request vector to intercept outside the FAF" (or request direct to a waypoint outside the FAF).

As Mark notes, the Direct to FAF will work laterally as it would any sequence of waypoints, but any vertical guidance (formal or advisory) will be out the window... for the simple reason that the Flight Director/Autopilot needs (for terrain/obstacle clearance reasons) to always capture the lateral (GPS or LOC) before capturing the vertical (GP or GS). Going Direct to the FAF, and the inherent "turn anticipation", pretty much rules out that lateral-then-vertical capture from happening.

 

[Narwhal]

I tried that RNAV GPA-A again last night, except this time on an IFR flight plan and this time I got a vector to the intermediate approach segment between the IF and the FAF. I just activated the leg to the FAF, and the GNC-355 intercepted the leg using the approach mode. Again no Vertical guidance, except for the Vcalc generated white box "pathways" or "highway in the sky" on the attitude indicator because I programmed VCALC to be at the MAP at the MDA. Also, I'm learning that the VNAV switch on my GMC-305 apparently does nothing, even though I will get a "VNAV" armed annucnication on the flight mode annunciations on the top of the PFD when I press it. Nothing ever goes active, I think due to the omission of VNAV from the GNC-355. Best I can do is flying the Vcalc generated V/S using the V/S autopilot/FD mode.

One question, I found it interesting that there was a 3 degree discrepancy between all the approaches I flew on the "DTK" fields of the flight page page (and the course selection of the HSI on my PFD) while I was established on the approaches, vs the depicted course on the approach plates. I've read that this is some kind of computational discrepancy because of the way the magnetic variation is accounted for in approach plates vs the garmin databases? It is a 16E magvar in the area; still doesn't seem like that'd result in a 3 degree discrepency over ~ 5 mile approach segments. The emulator/sim has does this, the same as the real airplane, noticeable in the above screenshots.

From: https://www.ifr-magazine.com/technique/a-bit-over-ninety/

You’ve probably noticed that GPS course guidance is often a degree or three different from the courses shown on the chart. The reason is the magnetic variation used by your GPS, the chart designer, and the VOR defining an airway may all be different. If you program the course BTV to XIMKY in a GPS with a database and date as of this writing, the course appears as 141, not 142, so the restriction of 051-231 is correct.

Hate to be picky, but the FMC's in the big jets don't have this issue for some reason, except on very long airway segments that are a couple of hundred miles long because of the point on the route at which the magvar is selected for calculation. On approach segments I don't ever recall a discrepancy between an approach plate and the FMC depicted final approach course.

 

[midlifeflyer]

A difference between a VOR radial that has a preset direction that has not been reset in years and is no longer aligned with the current magnetic course, and the direct course computed between two points in space by an RNAV box, is very common.

i know of a pilot who busted an instrument checkride because they didn't understand this natural discrepancy.

 

[John Collins]

Radials rarely use current magnetic variation. Neither do airport runways or approaches to them. In the case of VOR's, the magnetic variation is the one that existed when the VOR was either installed or last recalibrated. In most cases this never happens for a VOR and they still use the magnetic variation in effect when they were installed. Magnetic variation has a different name, the declination, which is the magnetic variation at the time of install. Many go back to the 1960's. For runways, a magnetic variation is established based on a specific magnetic year epoch. Example, my airport KUZA uses the magnetic variation of 5W based on the 1990 epoch which is 32 years ago, the current magnetic variation is 8W. You can check this out on ForeFlight, by going to Airport>KUZA>Info>Features>Magnetic Variation>5W. But if you tap on a nearby waypoint such as AZAKA, you will see the current magnetic variation of 8W. Do this at a VOR such as FLO, and you will see the declination is 3W (the magnetic variation in 1965), but a nearby fix JONAP, the current magnetic variation is 9W. If you want to keep the VOR CDI centered on the airway, you have to set the OBS to the radial or you will be way off course. FLO088016 will only line up with the airway fix CARLI on V139 if the declination of 3W is used.

 

[Larry in TN]

FLO088016 will only line up with the airway fix CARLI on V139 if the declination of 3W is used.

Which the RNAV unit should take care of for you as long as you build the waypoint by reference to FLO.

 

[455 Bravo Uniform]

Forget all that crap. You said “in my new airplane”. Do you have a link to that thread in POA forum with pictures? If not, we need a pic, here and now. Thanks for your compliance.

 

[Narwhal]

Forget all that crap. You said “in my new airplane”. Do you have a link to that thread in POA forum with pictures? If not, we need a pic, here and now. Thanks for your compliance.

I've been working on an overview of the airplane for a few days, just got it finished (and learning new video software). First 7:30 is just a cursory walkaround, last 4:00 is some flying. Keep in mind I'm not that good at flying it and there is about a 10 mph quartering headwind. I think the wind was about 100 @ 10 landing runway 16, but variable between 090 and 130 with gusts up to 15-20.

 

[Martin Pauly]

Just a general recommendation wrt "going Direct to a FAF"... ATC should never instruct this, and if they do you should say "unable, request vector to intercept outside the FAF" (or request direct to a waypoint outside the FAF).

I agree - ATC may tell an IFR aircraft to proceed direct to the FAF is in preparation for a visual approach, but not when planning to fly the actual instrument approach. That needs to start at an IAF or IF, or through vectors to somewhere before the FAF.

Now I have noticed that ATC sometimes applies some shortcuts when dealing with practice approaches in VMC, such as vectoring us to somewhere past the approach gate. In my opinion, they should not do that - it can confuse the heck out of instrument students, who are trying to learn how to do this the right way.

- Martin

 

[Flying Keys]

Chiming in late with the relevant excerpt from JO 7110.65Z:



No HILPT, no IAF even (YOHOE is only an IF). An approach without an IAF is fairly rare - must be in a radar environment and usually has other conflicting airspace or procedures nearby. But the tight VTFaf expectation likely exceeded the GPS unit’s limitations, which are:

 

[Flying Keys]

Ok, I think I figured it out.

The problem was I was going direct to the FAF, RICKR ("inside") with too steep of an intercept angle to the final approach course? Apparently it doesn't want to allow sequencing automatically unless you wait until the turn results in less than a 45 degree intercept go direct to the FAF. In this case with a 089 final aproach course from RICKR to RWY 07R, I think you need to be past the 224 radial/044 bearing from RICKR (FAF) before going direct to RICKR, otherwise you get the SUSP.

45° sounds much more reasonable, but the manual note I posted has me curious. I’m not near the GPS sim right now, but I might try loading it up later and seeing whether an intercept just on the TO side of the fix (so an 89°ish degree intercept) will resume sequencing as stated in the manual.

 

[Narwhal]

Chiming in late with the relevant excerpt from JO 7110.65Z:

View attachment 109768

No HILPT, no IAF even (YOHOE is only an IF). An approach without an IAF is fairly rare - must be in a radar environment and usually has other conflicting airspace or procedures nearby. But the tight VTFaf expectation likely exceeded the GPS unit’s limitations, which are:

View attachment 109769

Just so we're on the same page, I never selected VTaF on the GNC-355 in the original scenario. I "loaded" the approach, then went to the "flight plan" page and selected the FAF, and hit Direct, direct activate, resulting in the SUSP. In the simulator this will result in a suspension unless the difference between the DTK to the FAF and the final approach course is 45 degrees or less. If you unsuspend it will still intercept and sequence, although it'll probably overshoot final. I was too ignorant to know what suspensions were when I first did this.

I get that ATC isn't supposed to issue clearances that way, but I was just practicing in VMC/VFR and I think the way it played out was: ATC: "Can you take RICKR or do you need YOHOE"? ME: "I can take RICKR". ATC: "resume own navigation RICKR, maintain VFR". I'm sure they know not to do that in IMC/IFR, and in this case it was actually good because I learned a lot about the avionics, which was the point of the flight.

On an unrelated note, I guess I am slightly disappointed that I can't shoot RNP AR approaches with this instrument package, or can I (maybe there is a setting to allow it?) There are a lot of those approaches nearby. I'm also annoyed that Connext for flight plan transfer doesn't seem to work with android phones without some kind of additional hardware. Minor quibbles.

 

[Flying Keys]

Ya, RNP AR is a whole different ball of wax. Those databases are highly protected and there’s additional levels of compliance involved.

To clarify, I was referring to ATC giving you a vector direct to the FAF, not you selecting VTF in the GPS.

However, I loaded up the approach and got the same results you did - it sequenced (unsuspended) when I was within a 45°offset from the final approach course.

(edit: fat fingered the send button too early)

 

[RussR]

I get that ATC isn't supposed to issue clearances that way, but I was just practicing in VMC/VFR and I think the way it played out was: ATC: "Can you take RICKR or do you need YOHOE"? ME: "I can take RICKR". ATC: "resume own navigation RICKR, maintain VFR". I'm sure they know not to do that in IMC/IFR, and in this case it was actually good because I learned a lot about the avionics, which was the point of the flight.

Okay, that's an important distinction, and different than being cleared direct to RICKR. "Own navigation" to RICKR implies that you're going to act as your own ATC, self-vectoring to final prior to the FAF, much like is done all the time in instrument training with the CFII providing the vectors.

On an unrelated note, I guess I am slightly disappointed that I can't shoot RNP AR approaches with this instrument package, or can I (maybe there is a setting to allow it?) There are a lot of those approaches nearby.

Flying RNP AR approaches (the "AR" means "Authorization Required") involves a much higher level of equipment than is usually found in the light GA fleet. Details are in AC 90-101A:



In addition, you must have specific RNP training policies contained in your operations manual and some other things.

So the bar is pretty high, and not likely to be achievable by typical owners of light aircraft.

 

[Narwhal]

Okay, that's an important distinction, and different than being cleared direct to RICKR. "Own navigation" to RICKR implies that you're going to act as your own ATC, self-vectoring to final prior to the FAF, much like is done all the time in instrument training with the CFII providing the vectors.



Flying RNP AR approaches (the "AR" means "Authorization Required") involves a much higher level of equipment than is usually found in the light GA fleet. Details are in AC 90-101A:

View attachment 109778

In addition, you must have specific RNP training policies contained in your operations manual and some other things.

So the bar is pretty high, and not likely to be achievable by typical owners of light aircraft.

That's too bad. There are several runways at the main airport here that only have RNP AR approaches. I never really looked into the requirements much until now, because my previous airplane had nothing but a single VOR reciever, but even in the new plane, since they were not in my GNC-355 database, I figured they were off limits. I did read the GTN series navigators can potentially do them though.

Knowledge gaps like this are the downsides of having earned an instrument rating in 2002 then spending 20 years flying clapped out planes with the latest 60's tech.

 

[John Collins]

That's too bad. There are several runways at the main airport here that only have RNP AR approaches. I never really looked into the requirements much until now, because my previous airplane had nothing but a single VOR reciever, but even in the new plane, since they were not in my GNC-355 database, I figured they were off limits. I did read the GTN series navigators can potentially do them though.

Knowledge gaps like this are the downsides of having earned an instrument rating in 2002 then spending 20 years flying clapped out planes with the latest 60's tech.

Where did you read that GTN series of navigators can be used for RNP AR procedures. Note also the vertical guidance for an RNP AR procedure uses BaroVNAV which is not found on piston level GA aircraft.

 

[Ryan F.]

That's too bad. There are several runways at the main airport here that only have RNP AR approaches. I never really looked into the requirements much until now, because my previous airplane had nothing but a single VOR reciever, but even in the new plane, since they were not in my GNC-355 database, I figured they were off limits. I did read the GTN series navigators can potentially do them though.

Knowledge gaps like this are the downsides of having earned an instrument rating in 2002 then spending 20 years flying clapped out planes with the latest 60's tech.

RNP AR is not coming to piston GA aircraft anytime soon. They are left purely to the realm of highly automated flight decks with levels of redundancy we can't afford (in money or weight) for our little airplanes.

At my last flight department, I maintained RNP AR currency for years and flew one actual RNP AR approach (in VMC, at that.) They are quite something to watch in the sim; the proximity of the aircraft to the terrain is really eye-opening. There are some "exotic" missed approach methodologies too, one of which is to simply press the ALT button on the FGP.

We have the equipment necessary at my current flight department (same Gulfstream and Bombardier products I flew for the last company) but we don't maintain RNP AR. We just can't justify the time and expense (mostly time) for hardly ever flying an RNP AR.

 

[Martin Pauly]

I did read the GTN series navigators can potentially do them though.

Is it possible you are confusing RNP and RNP AR?
They are very different.

- Martin

 

[Narwhal]

I read it briefly in this post: https://www.pilotsofamerica.com/community/threads/rnp-in-ga.110742/#post-2515073 , turns out the poster was mistaken and was mistaking RNP AR for RNP as you say.

 

[ShaneWhite]

In the United States, we have RNAV, RNAV (GPS) VOR or GPS and RNAV RNP in our approach titles. ICAO has switched to calling all of the Aforementioned approaches that have a GPS authorization RNP approaches, and the USA equivalent of an RNAV RNP approaches is called RNP AR.

Stated another way, RNAV RNP (FAA) = RNP AR (ICAO).

Both RNAV RNP and RNP AR require special aircrew training, dual GPS, ANP vs. RNP performance warning (UNABLE RNP), multiple autopilots, V-Track and Cross-Track error display, and a slew of other considerations. The closest GA gets is WAAS / LPV approaches. Many airliners do not have WAAS and cannot fly LPV approaches.