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Coupled RNAV approaches

azonic75

Well Known Member
Have a non touch G3X, Trutrax GX pilot and a GTN 650. I haven't had much luck getting the autopilot to fly a coupled approach. It flies in nav mode great but when it gets to the approach it seems like it doesn't couple. I have the approach loaded and activated and the "APPR" button pushed. Any thoughts?

Kai
 
Have a non touch G3X, Trutrax GX pilot and a GTN 650. I haven't had much luck getting the autopilot to fly a coupled approach. It flies in nav mode great but when it gets to the approach it seems like it doesn't couple. I have the approach loaded and activated and the "APPR" button pushed. Any thoughts?

Kai

Hello Kai,

The first thing to establish is whether or not the G3X system is receiving and displaying RNAV glidepath information on the PFD.

Do you see the glidepath indication become visible on the PFD when you pass the waypoint before the FAF?

GP_Vertical_Dev.png

As you probably know, normal operation is for the glidepath vertical deviation information to become visible on the PFD as you cross the FAF-1 waypoint, and the system will capture and fly the glidepath when the vertical deviation diamond reaches the center of this indication. At this point, the flight director vertical mode will transition from ALT hold mode to GP mode and begin the descent.

A common mistake is to intercept the final approach course too close to the FAF. Depending on your altitude, it is possible to over-fly the glidepath and never capture the glidepath.

Steve
 
Thanks

Thanks Steve. I would have assumed the system would fly the entire procedure. NAV to the IAF and then step downs to the FAF
 
Even to LNAV or LNAV/VNAV 400' HAT approaches, you still don't want to dive and drive if you have vertical navigation.

Which systems allow capturing a vertical path outside and above the FAF, then ride the published angle to the 50' above the threshold point while meeting all the altitudes?

It should not need to be a LPV to do this, you just have to go missed at a DA, whether around 400' HAT or higher.

Thanks, apologies if this is too drifty.
 
LPV

That's the question. Will this combination GX pilot, G3X and GTN 650 fly a coupled approach including the vertical component from the IAF on an LPV approach
 
Even to LNAV or LNAV/VNAV 400' HAT approaches, you still don't want to dive and drive if you have vertical navigation.

Which systems allow capturing a vertical path outside and above the FAF, then ride the published angle to the 50' above the threshold point while meeting all the altitudes?

It should not need to be a LPV to do this, you just have to go missed at a DA, whether around 400' HAT or higher.

Thanks, apologies if this is too drifty.

Since you mentioned 400’ HAT I wonder if you meant ‘LNAV or LNAV + V’, the latter being different than LNAV/VNAV? The LNAV+V is really an LNAV approach, with an advisory glide slope. There is no DA, but rather a MAP. And if you treat the point where the advisory GS intersects the MDA as a DA, that’s okay, it’s always before the actual MAP. However, on occasion, you will miss landing, when a ‘dive and drive’ type technique would actually get you home. I’ve seen it happen, where at MDA intercept you see nothing, but, a bit closer to the airport (and still before the actual MAP) the clouds/fog suddenly end, and the runway comes clearly into view.
 
The simple answer is yes. What you have will fly a complete coupled approach including vertical guidance. I had the same setup, now have all Garmin servos.
Garmin/trutrak has a few you tube videos on it. Google it and watch.
 
Have a non touch G3X, Trutrax GX pilot and a GTN 650. I haven't had much luck getting the autopilot to fly a coupled approach. It flies in nav mode great but when it gets to the approach it seems like it doesn't couple. I have the approach loaded and activated and the "APPR" button pushed. Any thoughts?

Kai

I have the identical hardware. It will fly the lateral from an IAF to the IF, but not handle the vertical until the IF. The information is nearly all in the manual. I double check the APPR when it has turned onto the proper LPV course from a procedure turn heading to the IF. There is a vast amount of information being conveyed in the PFD and it does tell you what is happening in each element of arrival and approach. In a post flight analysis, look at the G3X flight data file, it will have everything including AP modes.
 
Which, if any, brands/models will vertically couple outside/before AND above the FAF Altitude?

Both G3x (touch and non-touch) will do that if VNAV is used to get to the IAF but since there is not one specific arrival and altitude to arrive at the IF, then it is not a unique part of the approach for lateral and vertical. I usually use Nav mode for lateral and ATC directions for transition from a series of let downs. Vertical is handled with ALT. The 650 will plan the route from a ways out when the selected IAF is set as the waypoint and the Approach is set. If changing the AP after the approach is active, then APPR will not arm until done so manually. Or it does not for me.

I am not sure why you would try and use VNAV when ATC is directing, but could be used if COMs are out.

If there is a system that really does like you ask please explain how it is going to work in in the real/active system environment.

Edit: I admit to "touch" envy though, the GMC507 will enable TOGA (even with a non-touch) and it is a work saver at a high work load time. 5-10 seconds saved then is very helpful for me. There is more, but lack of TOGA bugs me the most.
 
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Thanks

If being vectored to the FAF I wouldn’t worry about coupling. But when you have to fly the complete approach including a course reversal the having the autopilot fly the whole approach coupled vertically and laterally would seem pretty useful
 
If being vectored to the FAF I wouldn’t worry about coupling. But when you have to fly the complete approach including a course reversal the having the autopilot fly the whole approach coupled vertically and laterally would seem pretty useful

Fortunately, all G3X/G3X Touch systems will fly a complete coupled approach including a course reversal with little or no pilot action after activating the approach.

Since you are from Salem, let's look at the RNAV 31 approach at McNary Field (KSLE) as an example.

RNAV_RWY31_Profile.png

Since you want to fly a procedure turn in this example, we will approach the field from the NW and use the ARTTY transition. We will enter the approach at ARTTY at 3,000' MSL and not need to manually make any additional altitude or other changes before reaching the runway.

At this point in the approach we are in altitude hold at 3,000' inbound before ARTTY. The approach is loaded and activated, the APR button has been pushed on the GMC to arm for approach (GP) capture, and the baro minimum for the LPV approach has been selected. Even though the full procedure turn and approach/descent are ahead of us, we are done making ANY changes until we hear the "Minimums, Minimums" aural annunciation at the bottom of the glidepath and either land or go missed (by pressing a single TO/GA button).

LPV31_1.png

In this example we have passed the ARTTY transition, and are 17 nm outside of the NECIP IAF where we start the procedure turn. Again, nothing to do but monitor the flight.

LPV31_2.png

Here we have passed NECIP outbound and are completing the procedure turn.

PT.png

As we approach the NECIP IAF inbound, you can see that the vertical deviation bar is not yet present on the PFD.

LPV31_3.png

After crossing the IAF, the vertical deviation bar is present on the PFD and we are well below the glidepath as the glidepath crosses NECIP at 4,000' MSL.

LPV31_4.png

Finally, we capture the glidepath several nm ahead of the FAF at our present altitude and begin the descent. No descent is ever needed at the waypoint before the FAF (AFAIK). You just maintain the published altitude for the FAF-1 waypoint, and fly straight into the glidepath.

LPV31_6.png

At the bottom of the glidepath, you can see the baro min indicator rising in the altitude tape.

LPV31_5.png

A tip for flying the missed approach. At ANY time during the above procedure, once you are established in ALT hold at 3,000', you can change the selected altitude to 3,700' in preparation for flying the missed approach. If you go missed, just add power and press the TO/GA button which transitions both the G3X system and the Garmin navigator to fly the missed to the ARTTY hold at 3,700'. The autopilot will automatically couple to the missed approach procedure, climb to 3,700', and enter the hold at ARTTY. It will stay in this hold until you give it directions to do otherwise.

While it seems that many, if not most, RNAV approaches do not have intermediate fixes that require step down(s) between the IAF and the waypoint before the FAF, there are some that do. In those cases these step downs can be handled by either using GTN VNAV, or simply using the altitude select and VS mode when instructed by ATC. As Bill mentions above, in the real world of flying IFR with ATC instructions, it is often the case that it is simpler to just manually manage the altitude changes when cleared to do so.

BTW, if it were up to me, I would show the profile for this approach in this manner to make it clear that when arriving at NECIP at 3,000', you can fly level from this point to intercept the glidepath.

RNAV_RWY31_Profile_Modified.png

Steve
 
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Have you set up the GTN650 for VNAV or VCALC? You will find this in the configuration mode of the GTN. VCALC will not give you transition to approach, as it must be selected after selecting VNAV. The GTN will get air data through the installed ARINC connection and you should be able to set minimums and watch it do its magic.
 
BTW, if it were up to me, I would show the profile for this approach in this manner to make it clear that when arriving at NECIP at 3,000', you can fly level from this point to intercept the glidepath.
Steve

+1. This is one area where the Jepp charts remain clearly superior (Jepp uses a symbology for step down fixes that cannot be confused with a glide slope.).
 
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