G3X/GTN Coupled LNAV+V Approach

[Ezburton]

All,
Did the RNAV 33 into Bridgewater VA (KVBW) this weekend and the Garmin did something I wasn't expecting , which I may have caused. I forgot to bring home the data card, but didn't want to forget about finding out the why. Here's the situation...

The published approach is just LNAV, but when I put it into the GTN, it came up with LNAV +V. So I thought that the GTN would capture the GS. With the approach armed, as evidenced by the white GS, and in altitude hold from MITER to RFLAT, it didn't capture. I noticed the step down in the fly-thru boxes, but it clearly wanted me to do this manually... by that time, it looked like the magenta ball looked to be sitting at the bottom of the vert speed tape... Then I noticed the autopilot was in PIT mode. Easy enough to dial down the pitch so that the velocity vector was sitting at the beginning of the runway and set power accordingly. Steep descend so I never went below the step downs (circled out anyway).

My questions are:

1. Should this have captured and flown the +V just like an LPV and I screwed up somewhere? Or are you on your own to dive and drive the step down on a +V, like in a vanilla LNAV?
2. How did this get into PIT mode... I'm not sure I even know how to get back into pitch after I have been in any other mode; just deselect all other vertical modes? Is it possible that I double pressed a vertical mode and it reverted to PIT?

Without the data cart it's hard to see the chain of events. Any help form the collective would appreciated.
Thanks,

 

[BobTurner]

All,
Did the RNAV 33 into Bridgewater VA (KVBW) this weekend and the Garmin did something I wasn't expecting , which I may have caused. I forgot to bring home the data card, but didn't want to forget about finding out the why. Here's the situation...

The published approach is just LNAV, but when I put it into the GTN, it came up with LNAV +V. So I thought that the GTN would capture the GS. With the approach armed, as evidenced by the white GS, and in altitude hold from MITER to RFLAT, it didn't capture. I noticed the step down in the fly-thru boxes, but it clearly wanted me to do this manually... by that time, it looked like the magenta ball looked to be sitting at the bottom of the vert speed tape... Then I noticed the autopilot was in PIT mode. Easy enough to dial down the pitch so that the velocity vector was sitting at the beginning of the runway and set power accordingly. Steep descend so I never went below the step downs (circled out anyway).

My questions are:

1. Should this have captured and flown the +V just like an LPV and I screwed up somewhere? Or are you on your own to dive and drive the step down on a +V, like in a vanilla LNAV?
2. How did this get into PIT mode... I'm not sure I even know how to get back into pitch after I have been in any other mode; just deselect all other vertical modes? Is it possible that I double pressed a vertical mode and it reverted to PIT?

Without the data cart it's hard to see the chain of events. Any help form the collective would appreciated.
Thanks,

Did you actually see a GS needle displayed at RFLAT? My guess is no. RFLAT is a step down fix. You needed to instruct the AP to step down at RFLAT. Somewhere before the FAF the GS should have come alive, and the AP smoothly intercepted it at the FAF.

 

[Richard Connell]

If you have transition to approach configured in the GTN set up then the procedure is to arm VNAV and APR concurrently (while in ALT hold) the VNAV will take you to the FAF while complying with any vertical constraints - and seamlessly transition to GP.

APR armed alone I don’t believe will be sufficient unless you have somehow gotten yourself in a position to intercept the GS from below in between the IF and the FAF using a different vertical mode.

 

[Ezburton]

Did you actually see a GS needle displayed at RFLAT? My guess is no. RFLAT is a step down fix. You needed to instruct the AP to step down at RFLAT. Somewhere before the FAF the GS should have come alive, and the AP smoothly intercepted it at the FAF.

Bob,

I got a "VERTICAL TRACK" aural as I approached RFLAT. I crossed RFLAT at 5000' while turning inbound. I don't recall seeing the magenta ball at RFLAT, just the GS armed indication and the white boxes 1000' below. Since I crossed RFLAT at 5000, I would have thought I was below the GS as in an LPV. I was expecting to intercept an LPV kind of glide slope from somewhere between RFLAT and RIVKE, but instead it showed the step down. Once it didn't occur, I realized I would have to use VS mode to get down to the next step. That is when I noticed had somehow gotten into PIT mode, which allowed me to dial-in my own GS using the velocity vector.

Isn't every fix in this approach a step down fix? Why are you saying an actual GS comes alive by the FAF (between RIVKE and BEEDY) instead of sooner? On an LNAV +V is the GTN generated GS only from the FAF to the MDA?

So far, what I think I learned is in an LNAV +V, don't expect an LPV style GS and be prepared to dive and drive at each fix, as indicated by the white boxes which are a cliff at the upcoming fix. As Richard said, for example, try arming VNAV at the VERTICAL TRACK callout.

All good stuff... thanks for the discussion.

 

[Richard Connell]

For an LNAV/VNAV or LNAV+V you can only expect GP to activate between the IF and the FAF so RIVKE and BEEDY in this case.

If you have an approach activated you’ll hear the vertical track annunciation irrespective of whether you’ve armed the VNAV or not.
In the recent past you couldn’t arm the VNAV any earlier than the annunciation (or 2 mins - I can’t remember exactly)
Now you can not only arm the VNAV as early as you like but also concurrent with the GS/APR. No reason to dive and drive in 2024.
here’s an example from Saturday. VNAV and APR/GS armed an hour out from the IAF.

 

[Ezburton]

For an LNAV/VNAV or LNAV+V you can only expect GP to activate between the IF and the FAF so RIVKE and BEEDY in this case.

If you have an approach activated you’ll hear the vertical track annunciation irrespective of whether you’ve armed the VNAV or not.
In the recent past you couldn’t arm the VNAV any earlier than the annunciation (or 2 mins - I can’t remember exactly)
Now you can not only arm the VNAV as early as you like but also concurrent with the GS/APR. No reason to dive and drive in 2024.
here’s an example from Saturday. VNAV and APR/GS armed an hour out from the IAF.

On my scenario, MITRE is the IAF and RFLAT is an IF. If I understand what you are saying, on LNAV+V or LNAV/VNAV the GS will appear between the FAF and the previous IF (and not just any IF in the approach).

 

[Longez]

All,
Did the RNAV 33 into Bridgewater VA (KVBW) this weekend and the Garmin did something I wasn't expecting , which I may have caused. I forgot to bring home the data card, but didn't want to forget about finding out the why. Here's the situation...

The published approach is just LNAV, but when I put it into the GTN, it came up with LNAV +V. So I thought that the GTN would capture the GS. With the approach armed, as evidenced by the white GS, and in altitude hold from MITER to RFLAT, it didn't capture. I noticed the step down in the fly-thru boxes, but it clearly wanted me to do this manually... by that time, it looked like the magenta ball looked to be sitting at the bottom of the vert speed tape... Then I noticed the autopilot was in PIT mode. Easy enough to dial down the pitch so that the velocity vector was sitting at the beginning of the runway and set power accordingly. Steep descend so I never went below the step downs (circled out anyway).

My questions are:

1. Should this have captured and flown the +V just like an LPV and I screwed up somewhere? Or are you on your own to dive and drive the step down on a +V, like in a vanilla LNAV?
2. How did this get into PIT mode... I'm not sure I even know how to get back into pitch after I have been in any other mode; just deselect all other vertical modes? Is it possible that I double pressed a vertical mode and it reverted to PIT?

Without the data cart it's hard to see the chain of events. Any help form the collective would appreciated.
Thanks,

Don,

To start, there is no such thing as a glideslope (GS) on an RNAV approach. RNAV approaches have a glidepath (GP) and when you arm your G3X Touch autopilot to capture the glidepath by pressing the APR button the white GP indicator will be present on the AP status bar (not GS).

If you are in altitude hold (ALT) mode and press the ALT button, the flight director (autopilot) will switch to pitch mode. To re-select altitude hold mode, just press the ALT button again.

Yes, the autopilot will capture and fly the glidepath on this LNAV+V approach. One thing to remember is that when the full approach is being flown with a transition (instead of vectors to final (VTF)), the vertical deviation bar showing your altitude relative to the glidepath at the current position is only present and visible next to the altitude tape when you cross the FAF-1 waypoint (RIVKE).


The glidepath on this approach crosses RFLAT at ~5200 ft and RIVKE at ~4200 ft, so when you cross RFLAT in altitude hold at 5000 ft, you will be beneath the glidepath and in a position to intercept it.

Note, however that since the vertical deviation bar will not be present and visible until FAF-1 (RIVKE) and the autopilot will not capture the armed glidepath until this point (when not in VTF), you need to stay below the glidepath until you reach RIVKE by either using a coupled VNAV descent or using some other vertical mode like PIT or VS to descend between RFLAT and RIVKE to stay below the glidepath (but above 4100 ft).

The next time you practice this approach you might try this. Load and activate the approach with the MITER transition and press the APR button to arm for GP capture. Fly in altitude hold at 5000 ft between MITER and RFLAT. At 1 nm prior to RFLAT, select vectors-to-final (VTF) on the navigator.

When you activate VTF, the vertical deviation bar will be immediately active and the glidepath indicator diamond on the vertical deviation bar will be shown above the center indicating that you are below the glidepath. The autopilot will turn the plane the intercept the final course and will automatically capture the glidepath and begin the descent to the runway a short distance past RFLAT.

You will not have to make any step-down to RIVKE because the plane will already be established on the glidepath. It will cross RIVKEY at ~4200 ft.

Have fun with this great system!

Steve

 

[Ezburton]

That's why I'm a member of VAF... such a great knowledge bank!

I've never used VTF... I always load the full approach to get all the fixes in case of a direct-to request. I certainly never thought of changing to VTF in the middle of the procedure. So when you go back to the procedures page in the GTN and switch from MITER to VTF, and you're currently in NAV mode, does it change to HDG, so you have to, and I normally do, slave the heading before changing to VTF)?

I guess you had better really be at 5000 crossing RFLAT... if you are 5200' you'll be above the GP. Coming in from the MITER the plate shows 5000, but the MSA shows 5800', so I held off the descent until the last minute (since that seems to be conflicting information). The lightning bolt shows the GP to cross at RFLAT, so I know I had to be there to intercept.

I think I am a lot smarter about the system now. Thanks to all who contributed.

db

Aside: I assume the plate you were showing is a Jeps plate... the Gov plates don't show the GP above RFLAT (see attached). I thought Jeps would have showed the dive and drive in the profile view because an LNAV doesn't have a GP?

========================================
Are you going to RR this weekend Steve?

 

[mburch]

One question I have about the OP's setup: What unit is providing VNAV guidance? This may have been mentioned above and I just missed it, but it's worth thinking about.

If "VNAV" is configured on the GTN, then all enroute vertical navigation related to the GTN's GPS flight plan will come from the GTN. On the GTN's flight plan page you will enter any altitude constraints manually, and/or check that they are loaded automatically as a consequence of having an approach in the flight plan. If the GTN gives you the ability to enter VNAV altitudes on the flight plan page, then you can assume this is enabled. Richard is correct above that you will want to also have "Transition To Approach" enabled to take full advantage of the flight director capabilities.

If "VCALC" is configured on the GTN, then the GTN's VNAV feature is disabled, and the G3X Touch system will attempt to provide its own VFR-oriented enroute VNAV guidance when using the GTN's flight plan data. This will typically be a default flight path angle / descent rate aimed at getting you to pattern altitude. During the latter stages of an approach, this guidance will disappear so as not to conflict with the approach vertical guidance.

In either case, the "Vertical Track" audio alert is associated with VNAV, not approach guidance - though with the right setup, in many cases the VNAV transition to the FAF will neatly match the approach glidepath.

 

[Longez]

I've never used VTF... I always load the full approach to get all the fixes in case of a direct-to request. I certainly never thought of changing to VTF in the middle of the procedure. So when you go back to the procedures page in the GTN and switch from MITER to VTF, and you're currently in NAV mode, does it change to HDG, so you have to, and I normally do, slave the heading before changing to VTF)?
Are you going to RR this weekend Steve?

Don,

When you go to the GTN (or GPS 175, GNC 355, or GNX 375) and activate Vectors-To-Final (VTF), you are not "switching from MITER to VTF", just activating vectors to the final approach course at the current position on the procedure.

The autopilot will NOT change to HDG mode and it is not necessary to change to HDG mode before activating VTF. When you activate VTF at 1 nm outside RFLAT as described above, you do not have to touch the autopilot or change anything. It will still be slaved to the approach course laterally and will remain in altitude hold at 5000 ft as it automatically turns onto the final approach course and eventually automatically captures the glidepath and begins the descent.

Personally, I like to stay high as long as possible and only descend after coupled to the glidepath. While this is not possible on all LPV and LNAV+V approaches, it is for the majority of them.

For example, here is one of the 2 LPV approaches into my home airport. Since the glidepath crosses the SOKYO IAF at 4800 ft, I typically arrive at the SOKYO waypoint at 4000 ft (well below the glidepath) and remain at that altitude until the glidepath is intercepted 9.5 nm prior to the runway threshold which is 4.4 nm prior to the ROKSE FAF. In my opinion, there is no need to fly a stepdown to 2600 ft between SOKYO and ROKSE and wait for a glidepath intercept at ROKSE.

Since this approach does not have a FAF-2 waypoint, there is not really a benefit to activating VTF since the vertical deviation bar becomes active at the SOKYO IAF. Many LPV approaches at small airports are of this simple design with a single waypoint in front of the FAF.



Some of the approach plates designed outside the U.S. will be depicted in the above manner showing the relative position of the glide path above the minimum approach entry altitude. I like this method and think it adds significant value and clarity and used it for these examples to support the discussion.

Steve

 

[Bavafa]

Don,

To start, there is no such thing as a glideslope (GS) on an RNAV approach. RNAV approaches have a glidepath (GP) and when you arm your G3X Touch autopilot to capture the glidepath by pressing the APR button the white GP indicator will be present on the AP status bar (not GS).

If you are in altitude hold (ALT) mode and press the ALT button, the flight director (autopilot) will switch to pitch mode. To re-select altitude hold mode, just press the ALT button again.

Yes, the autopilot will capture and fly the glidepath on this LNAV+V approach. One thing to remember is that when the full approach is being flown with a transition (instead of vectors to final (VTF)), the vertical deviation bar showing your altitude relative to the glidepath at the current position is only present and visible next to the altitude tape when you cross the FAF-1 waypoint (RIVKE).

View attachment 70893


Note, however that since the vertical deviation bar will not be present and visible until FAF-1 (RIVKE) and the autopilot will not capture the armed glidepath until this point (when not in VTF), you need to stay below the glidepath until you reach RIVKE by either using a coupled VNAV descent or using some other vertical mode like PIT or VS to descend between RFLAT and RIVKE to stay below the glidepath (but above 4100 ft).


Steve

Steve, would you please remind/teach me what in this plate indicates that the GP will be visible to the system at FAF-1 (RIVKE)? Why not at RFLAT or BEEDY?

 

[Longez]

Steve, would you please remind/teach me what in this plate indicates that the GP will be visible to the system at FAF-1 (RIVKE)? Why not at RFLAT or BEEDY?

There is nothing in the approach plate which identifies when the glidepath vertical guidance will become visible on the PFD. This is determined when the FAF waypoint becomes the active waypoint.

As you cross RFLAT, RIVKE becomes the active waypoint, which is not the FAF, so the vertical guidance is not yet available. As you cross RIVKE, the FAF waypoint (BEEDY) becomes active and the vertical guidance becomes available.

When you activate a vectors-to-final, the FAF waypoint becomes the active waypoint which explains why the vertical deviation information becomes available regardless of your position on the final approach course.

Steve

 

[Bavafa]

There is nothing in the approach plate which identifies when the glidepath vertical guidance will become visible on the PFD. This is determined when the FAF waypoint becomes the active waypoint.

As you cross RFLAT, RIVKE becomes the active waypoint, which is not the FAF, so the vertical guidance is not yet available. As you cross RIVKE, the FAF waypoint (BEEDY) becomes active and the vertical guidance becomes available.

When you activate a vectors-to-final, the FAF waypoint becomes the active waypoint which explains why the vertical deviation information becomes available regardless of your position on the final approach course.

Steve

Thank you Steve for explaining. I had wondered and so that I can fully understand, we would know this ONLY when we are on that approach and paying attention to when the GP becomes available and not by studying the plate, correct?
Most of my approaches have been either ILS or LPV so this situation has not come up for me.

 

[Longez]

Thank you Steve for explaining. I had wondered and so that I can fully understand, we would know this ONLY when we are on that approach and paying attention to when the GP becomes available and not by studying the plate, correct?

Not exactly. You can look at any approach plate and know when the FAF waypoint will become active (when you cross the FAF-1 waypoint) at which point the vertical deviation guidance becomes available.

The only exception to this rule is when you select vectors-to-final which forces the FAF to be the active waypoint ignoring the waypoints prior to the FAF.

Steve

 

[BobTurner]

Thank you Steve for explaining. I had wondered and so that I can fully understand, we would know this ONLY when we are on that approach and paying attention to when the GP becomes available and not by studying the plate, correct?
Most of my approaches have been either ILS or LPV so this situation has not come up for me.

Take a look at RNAV(GPS) rwy 30 KTCY. Regardless of whether you intend to shoot the LNAV or LPV, the IF/IAF at Nampy is a step down fix. But looking at the government plate it sure looks like there is a glide path starting there. Now look at the Jepp plate for this approach. The ‘dive and drive’ symbology is unmistakably a step down. Thru the years the faa has slowly adapted the features of the Jepp plates, I don’t know why they haven’t copied this, too. (40 years ago the government plates were god-awful.) As described above, ‘vectors to final’ will show an extended glide path. There was a time when VTF also removed many ‘unneeded’ fixes - unneeded, that is, until ATC suddenly threw you a hold at a fix that was no longer displayed! I *think* that VTF with recent software updated has restored those fixes.(?).

 

[Ezburton]

Take a look at RNAV(GPS) rwy 30 KTCY. Regardless of whether you intend to shoot the LNAV or LPV, the IF/IAF at Nampy is a step down fix. But looking at the government plate it sure looks like there is a glide path starting there. Now look at the Jepp plate for this approach. The ‘dive and drive’ symbology is unmistakably a step down. Thru the years the faa has slowly adapted the features of the Jepp plates, I don’t know why they haven’t copied this, too. (40 years ago the government plates were god-awful.) As described above, ‘vectors to final’ will show an extended glide path. There was a time when VTF also removed many ‘unneeded’ fixes - unneeded, that is, until ATC suddenly threw you a hold at a fix that was no longer displayed! I *think* that VTF with recent software updated has restored those fixes.(?).

On the GTN simulator, you have to choose either a transition or VTF. If you choose VTF, you get all fixes on the approach, but none of the transitions. I guess if you wanted VTF and a transition, you would have to insert the transition into the flight plan ahead of time (or so it seems).

So from my original post... my expectations of capturing the GP at RFLAT (with APR, NAV, ALT Modes active) would have been correct if the additional fix (RIVKE) between BEEDY and RFLAT not been there. Hence the need to use VTF to extend the GP out for the entire approach, because a controller could insert you anywhere onto extended centerline.

Sure glad I asked... had this approach not had the extra step down, I would have just gone along fat, dumb and happy.

Thanks all!

 

[Longez]

Take a look at RNAV(GPS) rwy 30 KTCY. Regardless of whether you intend to shoot the LNAV or LPV, the IF/IAF at Nampy is a step down fix.

Not seeing a NAMPY waypoint on this approach, but if you activate vectors-to-final before crossing NAPYI at 3400 ft you can continue to fly level with no step down and intercept the glidepath at 9.9 nm from the runway threshold (1.3 nm ahead of HELVA). This is particularly easy if you fly the procedure turn because you can activate vectors-to-final after you complete the inbound turn towards NAPYI.

I am no IFR expert, but not sure why you would perform the complexity of a step-down when you can just fly straight ahead into the glidepath at 3400 ft.

 

[Longez]

On the GTN simulator, you have to choose either a transition or VTF. If you choose VTF, you get all fixes on the approach, but none of the transitions. I guess if you wanted VTF and a transition, you would have to insert the transition into the flight plan ahead of time (or so it seems).

I always load and activate the entire approach and only activate vectors-to-final close to the point when needing to turn inbound if I am actually going to fly a VTF. It just takes 2 button presses to do that.

Steve

 

[BobTurner]

I am no IFR expert, but not sure why you would perform the complexity of a step-down when you can just fly straight ahead into the glidepath at 3400 ft.

No argument from me - except that it somehow seems morally wrong to push a button (VTF) which is different from the clearance I received(!). Why can’t it look like an ILS, where the GS is received no matter where you are on the approach, and regardless of vectors to final or the full approach?
Q: I like those charts, with GP altitudes printed. Do you have a source?

 

[RV8Squaz]

For an LNAV/VNAV or LNAV+V you can only expect GP to activate between the IF and the FAF so RIVKE and BEEDY in this case.

If you have an approach activated you’ll hear the vertical track annunciation irrespective of whether you’ve armed the VNAV or not.
In the recent past you couldn’t arm the VNAV any earlier than the annunciation (or 2 mins - I can’t remember exactly)
Now you can not only arm the VNAV as early as you like but also concurrent with the GS/APR. No reason to dive and drive in 2024.
here’s an example from Saturday. VNAV and APR/GS armed an hour out from the IAF.

I’m not saying one way or another is better or the right way to do it. But the way Richard has stated in both of his posts, VNAV to intercept a glide path, is the way we do it in the airlines. It provides a seamless transition as he stated.

Sometimes it’s necessary to make the FAF-1 altitude constraint a hard altitude. In this case I would change RIVKE from an “At or above” to a hard altitude of “At” 4100’.

Jerry

 

[Longez]

No argument from me - except that it somehow seems morally wrong to push a button (VTF) which is different from the clearance I received(!). Why can’t it look like an ILS, where the GS is received no matter where you are on the approach, and regardless of vectors to final or the full approach?
Q: I like those charts, with GP altitudes printed. Do you have a source?

I agree with Richard and others that if you have a GTN that can fly VNAV coupled step downs, then by all means use that technology.

My GNX 375 (and the GPS 175 and GNC 355) don't have that same capability, but for me this is pretty much a don't care because I don't often see a need to fly step downs anyway.

It has been years since I first saw that non-U.S. approach plate using the nice glidepath indications I am showing above in my examples and I no longer remember the source.

I just calculated the glidepath crossing altitudes and labeled these for the purposes of this discussion to illustrate that the glidepath seems to be always higher than the minimum approach entry altitude at the IAF (often much higher), so it is possible to enable glidepath guidance/coupling at that point thus making it unnecessary to descend further before intercepting the glidepath and getting on with the stabilized phase of the approach if that is what you want to do.

Steve