Vertical Navigation of Approach Stepdowns for ILS with VNAV - GFC500/GTN750Xi

[Marc_B]

3 minutes ago, midlifeflyer said:

The distance between WELDS and BUFFS is almost 7 nm. At 3 degrees, you need less than 1.5 miles to descend those 400'. Staying at 8,000 you'd still intercept the GS from bellow.

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Exactly!  But my comment was that this is why I like autoswitch CDI (rather than manual).  Because the scenario that would put me above the GS would be that I fly though the GS.  i.e. I didn't switch CDI and AP kept me at ALT, or I didn't have APR armed.  But the gotcha is specifically with an ILS it's much more dangerous to try to capture an ILS from above due to the possibility of false repeated GS.

[midlifeflyer]

3 minutes ago, Marc_B said:

Exactly!  But my comment was that this is why I like autoswitch CDI (rather than manual).  Because the scenario that would put me above the GS would be that I fly though the GS.  i.e. I didn't switch CDI and AP kept me at ALT, or I didn't have APR armed.  But the gotcha is specifically with an ILS it's much more dangerous to try to capture an ILS from above due to the possibility of false repeated GS.

I don't rely on autoswitch on an ILS. It's just a technique difference, but for me, autoswitch is a backup to my failure to switch at an appropriate time. Plus, I can see the glideslope right away to know I'm below it

The concerns about false glideslopes are a bit overblown. Basically, between design criteria and the controller rules you mentioned, it should not happen. But if it did, you'd have to be at 8500' crossing BUFFS and, I think (my math is probably off) over 10,000' crossing WELDS to get the 1st out-of-phase false glideslope. If ATC was keeping me that high,  (a) they would be violating their own rules, as you pointed out, and (b) my situational awareness alarm bells would be ringing because of the  4,000' dive to the FAF.

[Ibra]

2 hours ago, Pinecone said:

Hmm, I wonder if there is an approach that the altitude at the last fix before the FAF is above the glide path?

For ILS & LPV, the geometric glidepath is designed to clear any stepdown SDF altitude after FAF from LOC & LNAV under ISA, hence, this only works when temperature is ISA or bellow ISA 

In extreme hot days, the official glidepath of ILS & LPV may go into stepdown fix altitudes at or after FAF, however, there is no high temperature restriction: you can always fly 3D official glide even if penetrate SFD MDA from 2D NPA procedure (ILS GP and LPV GP won't penetrate anything solid like ground or obstacle, it retain the same geometric shape and won't move at all all year around, it's stepdown fix altitudes that move up on hot day, as such, a regid glidepath may go into raising hot air mass)

On vectoring, ATC take care of altitude assignement, they may even have a separate chart for cold vs hot days and will vector you to FAP on higher altitudes than the one published for FAF 

On procedures, you are expected to adjust altitude of intermediate segment and mda due to temperature, however, only for cold days

L/VNAV with BARO glide is completely different, it even does the opposite: the glide go into terraiinand obstacles (origin and angle) with wrong QNH or in low temperatures, hence, they are restricted

L/VNAV on SBAS glide behave like SBAS for the glide (lateral guidance or design may differ) 

+V advisory glide from Garmin SBAS will also clear every SDF altitude after FAF at ISA and remain static over time in an identical fashion to LPV GP (however, this is a moot point as +V is only advisory while SDF altitudes are official when flying an LNAV procedure on CDFA)

There are advisory glides and vnav that rely on BARO input, however, they are irrelevant for single piston GA pilots? most of us use SBAS input in vertical navigation...

If you lose GP on ILS during a hot day, can you fly LOC minima?

It's a compliance question: I don't think one will hit anything being dead on ILS glide while being bellow LOC fix altitude at FAF  

Edited January 17 by Ibra

[Pinecone]

2 hours ago, Marc_B said:

@Pinecone What I'm referring to is if you're held high and are far enough out you are still under the GS.  You start below the GS, but once you cross the GS prior to the FAF you're above it.  i.e. if you are at 7000 feet and using GTN and GFC500, the AP will keep you at 7000 feet and intercept the GS several NM prior to the FAF then descend.  If you don't capture at that point (APR not armed, or CDI not switched) the AP will fly you though the GS and you're now above it.  This is accentuated if (for traffic separation) ATC gives you a "maintained 8000 till established, cleared ILS approach" where they intentionally keep you higher for traffic separation.

I understand.  I was just wondering if there was an approach that remaining at the altitude restriction of the last fix would cause an issue.

And there is also the issue of ATC holding you too high, but I try to be proactive and start asking if I can have lower.

It would be nice if they can get this fixed for the GFC-500 soon.  And it all becomes moot.

 

[Marc_B]

14 minutes ago, Pinecone said:

It would be nice if they can get this fixed for the GFC-500 soon.

100%!!

[Pinecone]

Maybe in Feb and release new software by early March. 

Just wishing, installs started this week.
 

[Marc_B]

A few VNAV limitations from Garmin I learned at training class…

VNAV is unavailable if:

-meeting the constraint requires a climb;

-meeting constraint requires aircraft to exceed max FPA (6deg down) or max VS (4000 FPM);

-it results in TOD behind the current position;

-it is within a leg that does not support altitude constraints (holds, heading legs, altitude terminating legs);

-it is for a waypoint past the FAF. 

Of course you first need to have all the basics met that are listed in the pilot guide…

Enhanced VNAV will not function if any of the following conditions are true:

    - Navigation source is not set to GPS

    - VNAV not enabled on VNAV profile page

    - No waypoints exist in the flight plan with appropriate altitude constraints

    - Lateral mode is not actively engaged to GPS

    - OBS mode is active

    - Dead reckoning mode is active

    - Parallel Track is active

    - Aircraft is on the ground

    - Aircraft is between the FAF and MAP of an approach.

 

I find it strange that VNAV won’t work on GPS parallel track. Seems like it should be able to.  The others seem pretty straightforward and obvious after some experience with it. 
Still no update regarding Transition to Approach. Of course the training team and development team are different so hard to say if this is in the works or not. 

[Marc_B]

The training crew did mention that part of the issue with Transition to Approach is with extreme temp that the fixed glide path intercept may not be the same as the baro FAF so there may be a discrepancy that causes an issue. 
 

Made it sound like issue is matching up VNAV path to the GP to intercept smoothly is the issue. 

[donkaye]

13 minutes ago, Marc_B said:

The training crew did mention that part of the issue with Transition to Approach is with extreme temp that the fixed glide path intercept may not be the same as the baro FAF so there may be a discrepancy that causes an issue. 
 

Made it sound like issue is matching up VNAV path to the GP to intercept smoothly is the issue. 

If that is the case, then why does it work with the GFC 600 and why can't they do the same with the GFC 500?

[Marc_B]

@donkaye it could all be just a matter of certification…not sure. But it sounds like Garmin teams are fairly siloed so hard to say what a different team is seeing or working on. 

[donkaye]

33 minutes ago, Marc_B said:

A few VNAV limitations from Garmin I learned at training class…

VNAV is unavailable if:

-it is within a leg that does not support altitude constraints (holds, heading legs, altitude terminating legs);

 

With regards to certain holds, I'm not sure this "holds" true.  In particular HILPT.  For example the ILS into KSCK.  If I am at 5,000 feet and receive the following clearance, "Cross IPDEW at 3,000, cleared for the ILS 29R Approach", and have set up VNAV, the plane will meet the crossing restriction at IPDEW and descend in the hold to meet the 1,800 foot crossing back at IPDEW.  At least in my experience.

[PT20J]

2 hours ago, donkaye said:

With regards to certain holds, I'm not sure this "holds" true.  In particular HILPT.  For example the ILS into KSCK.  If I am at 5,000 feet and receive the following clearance, "Cross IPDEW at 3,000, cleared for the ILS 29R Approach", and have set up VNAV, the plane will meet the crossing restriction at IPDEW and descend in the hold to meet the 1,800 foot crossing back at IPDEW.  At least in my experience.

You are correct, Don. I have done other HILPT approaches and VNAV works. Just for fun, I entered a flight plan on the simulator with one leg between two VORs about 20 nm apart and set the aircraft position to the first VOR at 3000. I created a holding pattern at the distant VOR and entered an altitude constraint of 2000. The simulator set a TOD and VNAV arrived at the second VOR at 2000 and entered the hold. I believe the requirement as listed in the Pilot's Guide is that the leg type has to support an altitude constraint. So, if you cannot enter an altitude constraint in the flight plan, you cannot use VNAV.

The altitude constraints on missed approach holds are reference only so VNAV will not use them (although you would not normally need VNAV on a missed approach anyway, so it's academic); perhaps there are other published holds that VNAV won't use.

[donkaye]

13 minutes ago, PT20J said:

Just for fun, I entered a flight plan on the simulator with one leg between two VORs about 20 nm apart and set the aircraft position to the first VOR at 3000. I created a holding pattern at the distant VOR and entered an altitude constraint of 2000. The simulator set a TOD and VNAV arrived at the second VOR at 2000 and entered the hold.

I would expect that to work because the constraint is the VOR.  In my case above the constraint was the waypoint, but then there was the descent in the hold back to the waypoint.  I ran the simulator and it showed a constraint in the hold.

[Marc_B]

I think the clarification is that an altitude constraint to enter a hold is okay. But I’m not aware of the GTN/GFC500 being able to make VNAV descent in a hold. If it’s a HILPT into an approach that’s not quite the same thing. 
 

VNAV will allow program of altitude constraint into the hold entry fix. But say you’re at 16,000 and you get “descend at pilot’s discretion, cross Waypoint A at 10,000 and descend in hold to 2000, then proceed to Waypoint B” I don’t think VNAV will do it. It will cross at 10,000 and then you’ll have to manage VS and bugged descent yourself inside the hold.

I think it all depends on what hold “leg” is active. A hold entry leg has a constraint and you can use VNAV. Inside the hold (hold leg) does not from my understanding (i.e. when hold suspended and you're circling).  The Garmin Trainer allows entry of an altitude constraint for the entry waypoint and for the exit waypoint.  But it only descends when unsuspended.

@PT20J & @donkaye Are you saying you are able to program a start hold constraint as well as a finish hold constraint to descend using VNAV inside the hold (while suspended)?
 

6 hours ago, PT20J said:

The altitude constraints on missed approach holds are reference only

VNAV is only for descent and the missed approach wouldn’t be VNAV but rather climbing. Sure, you can still plug in altitude constraints in your flight plan page, but that’s not VNAV. 
 

I set up the GTN to show active and selected constraints on the map in settings and show all constraints on the map on the G500TXi. Then you can easily see constraints coming up as well as using the Baro Minimums bug at times for an altitude bug for climbing (i.e. say on an ODP when you don’t wanna turn before “x” feet)

Edited March 15 by Marc_B
clarity

[PT20J]

29 minutes ago, Marc_B said:

@PT20J & @donkaye Are you saying you are able to program a start hold constraint as well as a finish hold constraint to descend using VNAV inside the hold?

I played around with this a bit on the sim. It will VNAV descend to an altitude constraint entered in the hold entry. Once in the hold it will not descend further even if the hold altitude constraint is lower. Unsuspending while in the hold will cause it to calculate a TOD and descend to the hold altitude constraint as it finishes the holding pattern and returns to the holding fix.

[donkaye]

5 minutes ago, PT20J said:

I played around with this a bit on the sim. It will VNAV descend to an altitude constraint entered in the hold entry. Once in the hold it will not descend further even if the hold altitude constraint is lower. Unsuspending while in the hold will cause it to calculate a TOD and descend to the hold altitude constraint as it finishes the holding pattern and returns to the holding fix.

In actual practice I don't find that to be the case.  OTOH I've only done it with the Stockton ILS.   To add a little complexity to the situation, the VNAV descent is occurring within the loaded approach, on the first turn where the hold is unsuspended, but before the FAF.

[Marc_B]

So Hold entry leg allows a constraint and hold exit leg allows a constraint.  I think a HILPT just has a hold entry leg and a hold exit leg so the constraints/VNAV sequences...my perception of the slide shown at Garmin (comments above) was that VNAV unavailable with hold leg/suspended hold.

"Heading legs" and "altitude terminating legs" I've seen programed with missed approaches and with departure procedures.  I've never seen a way to program these myself.  But both of these would be typically seen with climbing, and not with descents; Garmin definitely pointed out that the VNAV is an enhanced descent mode only.

[PT20J]

42 minutes ago, donkaye said:

In actual practice I don't find that to be the case.  OTOH I've only done it with the Stockton ILS.   To add a little complexity to the situation, the VNAV descent is occurring within the loaded approach, on the first turn where the hold is unsuspended, but before the FAF.

Don, during the HILPT when you are just using it as a course reversal and not going around the holding pattern a time or two the GTN does not suspend sequencing when first crossing the holding fix. As long as it is not suspended, it will calculate a VNAV TOD and descend on to cross the holding fix inbound observing the altitude constraint designated in the flight plan. It is only when remaining in a holding pattern with the sequencing suspended that VNAV will not descend to a lower constraint designated for a holding pattern.

[donkaye]

12 hours ago, PT20J said:

Don, during the HILPT when you are just using it as a course reversal and not going around the holding pattern a time or two the GTN does not suspend sequencing when first crossing the holding fix. As long as it is not suspended, it will calculate a VNAV TOD and descend on to cross the holding fix inbound observing the altitude constraint designated in the flight plan. It is only when remaining in a holding pattern with the sequencing suspended that VNAV will not descend to a lower constraint designated for a holding pattern.

I think that is what I said above.  My "complexity" comment referred to the fact that VNAV is not supported to occur within the "Approach" segment, but it does in this case.

[PT20J]

25 minutes ago, donkaye said:

I think that is what I said above.  My "complexity" comment referred to the fact that VNAV is not supported to occur within the "Approach" segment, but it does in this case.

I think we are saying the same thing. I love to watch the automation work, and would never go back to life without it, but I do think things were easier to understand when I learned with two VORs and an ADF.  I bragged to my DPE friend once that I used to be able to do intersection holds with a single VOR and he bested me by saying he did the same thing with a VOR receiver that you had to tune manually. Thank God I never had to orient on a four-course range.

[flyboy0681]

25 minutes ago, PT20J said:

 I love to watch the automation work, and would never go back to life without it

Just be current and on your toes for when that automation fails.

[Marc_B]

I think the easiest way to think about this is figure out Garmin logic and definitions.

On 3/14/2024 at 9:12 AM, Marc_B said:

VNAV is unavailable if:

-it is within a leg that does not support altitude constraints (holds, heading legs, altitude terminating legs);

Basically a Hold Entry Leg and a Hold Exit Leg can have an altitude constraint added to the flight plan and VNAV can use these for vertical sequencing.  So for a simple HILPT it's works.  However with a Suspended Hold, that adds a "Hold Leg"  (shown in the pilot guide graphic a few posts up) that will circle indefinitely in the hold until unsuspended, and VNAV will no longer be available for vertical navigation.  HOWEVER, when you press UNSUSP, it then sequences a variable length "Hold Exit Leg" that can then resume VNAV.  But depending on when you unsuspend the hold, the total length of the Exit leg could be anything from essentially an entire loop in the hold to a tiny segment if you unsuspended right before the fix. 

So you can't always count on the Exit Leg altitude constraint unless you plan appropriately.  My thought is that if you're given a "Hold at X and descend and maintain 2000 feet in the hold" then you'd be better off using VS and your altitude bug.

 

[donkaye]

44 minutes ago, Marc_B said:

I think the easiest way to think about this is figure out Garmin logic and definitions.

Basically a Hold Entry Leg and a Hold Exit Leg can have an altitude constraint added to the flight plan and VNAV can use these for vertical sequencing.  So for a simple HILPT it's works.  However with a Suspended Hold, that adds a "Hold Leg"  (shown in the pilot guide graphic a few posts up) that will circle indefinitely in the hold until unsuspended, and VNAV will no longer be available for vertical navigation.  HOWEVER, when you press UNSUSP, it then sequences a variable length "Hold Exit Leg" that can then resume VNAV.  But depending on when you unsuspend the hold, the total length of the Exit leg could be anything from essentially an entire loop in the hold to a tiny segment if you unsuspended right before the fix. 

So you can't always count on the Exit Leg altitude constraint unless you plan appropriately.  My thought is that if you're given a "Hold at X and descend and maintain 2000 feet in the hold" then you'd be better off using VS and your altitude bug.

 

More than once different Garmin manuals have conflicting statements.  This is one of them.  In one VNAV only in the Enroute Phase, then another where it works with a HILPT within the Approach.  Then differences between the G1000 functionality and the GTN functionality doing the same things.  As an instructor, it can make your heard spin trying to remember which is which and when.