GFC500 & VNAV

[Bob - S50]

A question for those of you that are now flying a GFC500 and using VNAV.  I have questions about how the GFC would fly an approach with multiple step down fixes.  I've included a link to one such approach in my local area.  You probably have not flown this approach but may have flown something similar in your local area.  Here's the link:

RNAV 29 at BVS

If you look at this approach, let's assume I'm approaching PAE from the SE at 6000' and I'm told to cross PAE at 6000', cleared for the RNAV 29 at BVS.  Let's assume I've also ensured that the FAF - OPIXE is set as a constraint at or above 3000' as shown.  I plan to set our default descent angle at 1.89 degrees which is 200'/NM.  Also, assume that after I get this clearance I set my altitude bug to 3000' and push the VNAV button.  I'll then also press the APR button for the approach.

First question.  With my default descent angle it looks like the GFC will need to cross KIREC at 4300' because the descent after that will need to be steeper than my default angle.  So I'm guessing that TOD will be set 8.5 NM prior to KIREC and the GFC will stay at 6000' until it reaches TOD.  The path will then be set to cross KIREC 'at' 4300'.  Is that correct?

Second question.  With my default descent angle, it looks to me like the descent from KIREC to OPIXE will need to be steeper than my default, but not as steep as a 3 degree glideslope.  Since I pressed APR, VNAV guidance will stop at ENNGL and the GFC will level off and wait to capture the GS.  So the question is, will the GFC strive to cross ENNGL at 3500' or will it cross ENNGL a bit higher along a constant rate flightpath between KIREC and OPIXE that would cross OPIXE at 3000'?

Third question.  If I leave a FAF crossing restriction as an 'at or above', does the GFC VNAV path strive to hit the FAF 'at' that altitude?  Or do I need to change the restriction to an 'at' restriction for that to happen?

Thanks

[donkaye]

 Also, assume that after I get this clearance I set my altitude bug to 3000' and push the VNAV button.  I'll then also press the APR button for the approach.

According to Steve at Garmin, for the GFC 500 you do not have to set the altitude bug below the lowest altitude.  This conflicts with the Supplement to the POH, which is written incorrectly.  Steve is correct.  I have tried it.  Also, if TOD is more than 5 minutes away, you will have to push it  again within the 5 minute range.  I have TOD as one of the data boxes on the GTN 750.

First question.  With my default descent angle it looks like the GFC will need to cross KIREC at 4300' because the descent after that will need to be steeper than my default angle.  So I'm guessing that TOD will be set 8.5 NM prior to KIREC and the GFC will stay at 6000' until it reaches TOD.  The path will then be set to cross KIREC 'at' 4300'.  Is that correct?

Yes, that is correct.

Second question.  With my default descent angle, it looks to me like the descent from KIREC to OPIXE will need to be steeper than my default, but not as steep as a 3 degree glideslope.  Since I pressed APR, VNAV guidance will stop at ENNGL and the GFC will level off and wait to capture the GS.  So the question is, will the GFC strive to cross ENNGL at 3500' or will it cross ENNGL a bit higher along a constant rate flightpath between KIREC and OPIXE that would cross OPIXE at 3000'?

It will cross ENNGL at 3500'.  Remember VNAV ends at ENNGL, one waypoint before the FAF.  GP should be armed and enabled within 2 miles of the FAF.  If the GP shows above you on the G5 or G500TXi you are good to go and will intercept it and the GFC will fly you down.  If you are above the GP then you will need to push V/S and go down to meet it before the GFC 500 will capture and fly it.

Third question.  If I leave a FAF crossing restriction as an 'at or above', does the GFC VNAV path strive to hit the FAF 'at' that altitude?  Or do I need to change the restriction to an 'at' restriction for that to happen?

VNAV isn't applicable after one waypoint before the FAF.

Garmin does a horrible job of documentation on VNAV.  They should give examples.  Beechtalk had the best thread where Steve answered many questions.  It can be found here:

https://www.beechtalk.com/forums/viewtopic.php?f=21&t=171218

The best thing is to go out and practice on the approach to see what it does versus what you expect it to do.

[Bob - S50]

40 minutes ago, donkaye said:

 Also, assume that after I get this clearance I set my altitude bug to 3000' and push the VNAV button.  I'll then also press the APR button for the approach.

According to Steve at Garmin, for the GFC 500 you do not have to set the altitude bug below the lowest altitude.  This conflicts with the Supplement to the POH, which is written incorrectly.  Steve is correct.  I have tried it.  Also, if TOD is more than 5 minutes away, you will have to push it  again within the 5 minute range.  I have TOD as one of the data boxes on the GTN 750.

First question.  With my default descent angle it looks like the GFC will need to cross KIREC at 4300' because the descent after that will need to be steeper than my default angle.  So I'm guessing that TOD will be set 8.5 NM prior to KIREC and the GFC will stay at 6000' until it reaches TOD.  The path will then be set to cross KIREC 'at' 4300'.  Is that correct?

Yes, that is correct.

Second question.  With my default descent angle, it looks to me like the descent from KIREC to OPIXE will need to be steeper than my default, but not as steep as a 3 degree glideslope.  Since I pressed APR, VNAV guidance will stop at ENNGL and the GFC will level off and wait to capture the GS.  So the question is, will the GFC strive to cross ENNGL at 3500' or will it cross ENNGL a bit higher along a constant rate flightpath between KIREC and OPIXE that would cross OPIXE at 3000'?

It will cross ENNGL at 3500'.  Remember VNAV ends at ENNGL, one waypoint before the FAF.  GP should be armed and enabled within 2 miles of the FAF.  If the GP shows above you on the G5 or G500TXi you are good to go and will intercept it and the GFC will fly you down.  If you are above the GP then you will need to push V/S and go down to meet it before the GFC 500 will capture and fly it.

Third question.  If I leave a FAF crossing restriction as an 'at or above', does the GFC VNAV path strive to hit the FAF 'at' that altitude?  Or do I need to change the restriction to an 'at' restriction for that to happen?

VNAV isn't applicable after one waypoint before the FAF.

Garmin does a horrible job of documentation on VNAV.  They should give examples.  Beechtalk had the best thread where Steve answered many questions.  It can be found here:

https://www.beechtalk.com/forums/viewtopic.php?f=21&t=171218

The best thing is to go out and practice on the approach to see what it does versus what you expect it to do.

Thanks Don.

I know I don't need to set the altitude bug below my current altitude for VNAV to start the descent, but I think that is a bad technique.  I don't EVER want to hit VNAV until ATC (or me when VFR) gives me a lower altitude.  Set the altitude and hit VNAV.  Just like in the big airplanes.

As for question #2.  Let's say I plan to fly the LNAV instead of the LPV.  So instead of selecting APR, I just leave it in NAV so it doesn't intercept the glidepath.  In that case, VNAV works all the way to the FAF.  What will it do as far as altitude at ENNGL?

[donkaye]

4 hours ago, Bob - S50 said:

Thanks Don.

I know I don't need to set the altitude bug below my current altitude for VNAV to start the descent, but I think that is a bad technique.  I don't EVER want to hit VNAV until ATC (or me when VFR) gives me a lower altitude.  Set the altitude and hit VNAV.  Just like in the big airplanes.

As for question #2.  Let's say I plan to fly the LNAV instead of the LPV.  So instead of selecting APR, I just leave it in NAV so it doesn't intercept the glidepath.  In that case, VNAV works all the way to the FAF.  What will it do as far as altitude at ENNGL?

I'd need to go out and try it, but having flown a VOR approach, the VNAV still stopped at one before the FAF in NAV mode.  I expect that would be the same, but I'm going to fly an LPV approach in NAV mode tomorrow and see what happens from one waypoint before the FAF to the FAF.