Late GPS -> LPV to LPV GS needle occur at FAF
Printed From: Avidyne
Category: Avidyne General
Forum Name: IFD 5 Series & IFD 4 Series Touch Screen GPS/NAV/COM
Forum Description: Topics on Avidyne's IFD 5 Series and IFD 4 Series Touch Screen GPS/NAV/COM
URL: http://forums.avidyne.com/forum_posts.asp?TID=1594
Printed Date: 29 Apr 2025 at 8:41am
Software Version: Web Wiz Forums 12.01 - http://www.webwizforums.com
Topic: Late GPS -> LPV to LPV GS needle occur at FAF
Posted By: Cllarsonmd
Subject: Late GPS -> LPV to LPV GS needle occur at FAF
Date Posted: 07 Sep 2018 at 9:45am
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This for my 1 1/2 year old IFD 540 I'm noticing that when I fly an LPV from an outer Initial Approach Fix...I get GPS--LPV....switching late....to LPV at the FAF. ..well after I'm established inbound. This occurs even when activating the approach early. With this, the Glide Slope Needle is absent until the FAF..The flight director is the same. As a result I have to catch up..dive down..to.the Glide Slope after the FAF. Contrast this with Vectors to Final...where I get the LPV indication sooner and the GS Needle comes alive 10 or more miles out. I appreciate your thoughts. Thank you Chris
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Replies:
Posted By: rolfe_tessem
Date Posted: 07 Sep 2018 at 10:08am
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My memory is a bit hazy on this, but I think the GPS doesn't switch to approach mode until 2 miles from the FAF. Could that be part if the issue here? Rolfe |
Posted By: Cllarsonmd
Date Posted: 07 Sep 2018 at 10:39am
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Thanks Rolfe Maybe... Vectors to final gives me an active GS Needle more than 10 miles out...and the LPV Mode Alert is active much earlier. The full approach...or flying waypoint to waypoint with the IAF and FAF in the string, Activating the Approach early on the LSK side of the 540...LPV and the GS needle are absent until the FAF. That means that I'm high and have to dive to intercept the GS This makes me want to use VTF only |
Posted By: rolfe_tessem
Date Posted: 07 Sep 2018 at 4:05pm
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Well, I can point to any number of RNAV LPV approaches where the IAF is a long, long way from the FAF. I don't think you can count on the GS coming alive just because you're past the IAF, especially if there are multiple step down fixes before the FAF. You may have to hit at least the first one yourself, before you get the switch to LPV. Again, I think this depends on the distance from the FAF. Rolfe
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Posted By: Cllarsonmd
Date Posted: 07 Sep 2018 at 4:16pm
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Thank you Rolfe The same approach with VTF comes alive way outside 10 mil es...much sooner Chris
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Posted By: chflyer
Date Posted: 07 Sep 2018 at 5:51pm
I'm still struggling to understand the logic here, and not just for GPS->LPV. I have the same issue with GPS->VLOC for an ILS approach. I was on an ILS yesterday with VTF and GPS->VLOC showing. I was being vectored to a point just a mile or so outside the FAF at a 25° intercept with automatic sequencing configured and the sequence from GPS guidance to VLOC(LOC/GS) didn't happen until I passed the FAF. Besides your issue about being high, the FAF is very late to discover any eventual issue with a missing glideslope or simply getting a view of the glideslope position. I am told that with Garmin sequencing to VLOC occurs much earlier so there is lots of time to do a proper LOC & GS intercept and confirm presence of both and proper operation. I have now decided to manually sequence to VLOC as soon as I get a reasonable intercept, to force LOC/GS display.
------------- Vince |
Cllarsonmd wrote:Posted By: rolfe_tessem
Date Posted: 08 Sep 2018 at 10:27am
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[/QUOTE] I'm still struggling to understand the logic here, and not just for GPS->LPV. I have the same issue with GPS->VLOC for an ILS approach. I was on an ILS yesterday with VTF and GPS->VLOC showing. I was being vectored to a point just a mile or so outside the FAF at a 25° intercept with automatic sequencing configured and the sequence from GPS guidance to VLOC(LOC/GS) didn't happen until I passed the FAF. Besides your issue about being high, the FAF is very late to discover any eventual issue with a missing glideslope or simply getting a view of the glideslope position. I am told that with Garmin sequencing to VLOC occurs much earlier so there is lots of time to do a proper LOC & GS intercept and confirm presence of both and proper operation. I have now decided to manually sequence to VLOC as soon as I get a reasonable intercept, to force LOC/GS display.
[/QUOTE]
Well, the criteria Avidyne uses for the switch to VLOC are pretty strict -- I think there are 5, including morse identified, within x degrees of the LOC heading, etc. Personally, I always switch manually, no later than the final vector if VTF. It is just one less thing to have to check in the scan at a busy time. Rolfe |
Posted By: rolfe_tessem
Date Posted: 08 Sep 2018 at 10:37am
Chris, I can't explain that, but I offer this approach which I routinely fly. http://155.178.201.160/d-tpp/1809/06204R3.PDF The glideslope doesn't come alive until HAROY -- I have to descend from KROWL to HAROY on my own, which is when I get the switch to LPV. As an aside, that's when I put my autopilot into APP mode as well. Rolfe
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Posted By: Cllarsonmd
Date Posted: 09 Sep 2018 at 12:25am
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I flew 2 other full approaches starting from the outer most IAF...everything sequenced appropriately...LPV activated...and the GS needle came alive early enough to be ready for decent. I think the Approach I wrote about earlier has an issue.. Chris
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Posted By: Gring
Date Posted: 09 Sep 2018 at 7:37am
| Rolfe, the RNAV 03 into 1B1 is exactly as you say, if you don’t begin your descent manually at KROWL you’ll end up above the glide slope when it comes in after HAROY. It is an odd one. |
Posted By: rolfe_tessem
Date Posted: 09 Sep 2018 at 4:21pm
Geoff, Glad to know I'm not crazy! It took some head scratching to figure out to delay the autopilot APP mode on this approach :-). Rolfe |
Posted By: Cllarsonmd
Date Posted: 09 Sep 2018 at 10:04pm
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Curious. If you fly VTF, will the GS needle come alive sooner allowing intercept without manually descending before HAROY? Chris
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Posted By: rolfe_tessem
Date Posted: 10 Sep 2018 at 1:08pm
I've never done it because Albany will always send you to an IAF for this approach, but will check next time I'm out there... Rolfe |
Posted By: Cllarsonmd
Date Posted: 10 Sep 2018 at 1:50pm
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Thanks Rolfe.. Or fly a practice Approach to see what happens
Chris |
Posted By: Gring
Date Posted: 10 Sep 2018 at 2:33pm
| It will depend on what altitude ATC has you at and where they have you intercept the "localizer". |
Posted By: Cllarsonmd
Date Posted: 13 Sep 2018 at 7:38am
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Good morning Rolfe I’m attending our Duke Fly In in OWB next weekend. Gary Reeves will be teaching his Avidyne 540 Mastery Course on Friday. I’ll be in attendance. I’ll ask him about our problem. My problem, like yours, occurs on an LPV RNav where the Approach is started at a remote IAF and the Glide Slope needle does not come alive until the FAF. Manual flying of the first leg of the descent is necessary to intercept GS when the needle comes alive. For me, this doesn’t happen with VTF. VTF does everything perfectly. Please send me details of your plane and panel. I have a Duke The HSI is the Bendix King 525a with glide slope The Auto Pilot is the King KFC 250 I have the Avidyne IFD 540 Thank you. Chris |
Posted By: PA20Pacer
Date Posted: 13 Sep 2018 at 8:28am
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Note that approaches are not designed for descent on the glideslope outside the FAF. It is possible to have altitude deviations while following the glideslope outside the FAF. See: https://www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/info/all_infos/media/2011/InFO11009.pdf Regards, Bob ------------- Bob Siegfried, II Brookeridge Airpark (LL22) Downers Grove, IL |
Posted By: Cllarsonmd
Date Posted: 13 Sep 2018 at 8:46am
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The problrm is no Glide slope needle until the FAF...where, unless manual descent was started outside this approach gate, the path is one dot high at the FAF..I agree..descent begins at Glide Slope intercept as depicted by the Glide Slope Needle...There's no needle however until the FAF..so, manual descent needs to be flown until the needle comes alive, hand flying the descent to the FAF MDA..it's using the poor man's glide slope...flying to the MDA of each fix until the GS needle comes alive.. Chris
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Posted By: Cllarsonmd
Date Posted: 13 Sep 2018 at 11:19am
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Note: On VTF GS is alive 16 miles out, and GPS>LPV has already transitioned to LPV This doesn't happen on this approach if waypoints are flown instead of VTF See indication for approach 9.4 NM froom FAF   |
Posted By: Catani
Date Posted: 13 Sep 2018 at 11:22am
| There should be glideslope indications on ILS and LPV approaches well before glidepath intercept. The indication should appear pegged at the top of the case or vertical deviation indicator or whatever terminology you have for the glideslope scale. You should look for when it comes "alive" and starts moving down, but you can't do that if it's not there. Not sure what the problem is, but if you have no glideslope needle until reaching the intercept, there is definitely a problem. Either the box(es) is (are) not working correctly, or any required initial conditions are not being met in time. |
Posted By: rolfe_tessem
Date Posted: 13 Sep 2018 at 12:52pm
On an LPV, the GS does not generally appear pegged at the top of the display -- it just appears wherever it is when the box switches to approach mode. Sometimes this is midscale, depending on the approach. Rolfe |
Posted By: Cllarsonmd
Date Posted: 13 Sep 2018 at 4:39pm
| it Appears that we now have 2 LPV approaches, one at SBM and One at 1B1, where the GS needle is delayed.. when flying waypoint to waypoint, flying the full approach..with the IFD 5$0. . For me at SBM, Vectors to final fixes this.. |
Posted By: Cllarsonmd
Date Posted: 14 Sep 2018 at 8:45am
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...and that's what we're both findind...different aircraft ...different airports....both using the IFD 540... flying an LPV approach from an IAF. ..The LPV activates late...which means no GS needle until the FAF...so GS intercept is not possible unless first leg of descent is hand flown. ..otherwise...at the FAF...when the needle comes alive, altitude is 1 dot too high. ...unless. ATC gives VTF...then LPV...and GS Needle active 16 miles out... Chris
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Posted By: Catani
Date Posted: 14 Sep 2018 at 5:52pm
I switch to approach mode manually, and always far enough away from the FAF so that the glideslope always appears at the top of the VDI for me. So I'm never so close to the descent point that the glideslope is already on its way down when I first see it. I'd suggest anyone having issues with the glideslope to try to do the same, and see if that helps resolve anything.
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Posted By: Cllarsonmd
Date Posted: 14 Sep 2018 at 11:00pm
| I've manually switched to VLOC...I didn't know we could switch from GPS>LPV to LPV |
Posted By: Ed Trautman
Date Posted: 21 Sep 2018 at 6:22pm
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there is also an autopilot config dependency for auto vloc switching. I have a 225. Ed
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Posted By: Ed Trautman
Date Posted: 21 Sep 2018 at 6:29pm
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from the user guide. Ed  |
Posted By: nhflydoc
Date Posted: 29 Mar 2022 at 6:13am
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I have the same issue flying the lpv RNAV 32 at KIZG when the gs appears I'm above it and my AP will not capture it Alan
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Posted By: chflyer
Date Posted: 29 Mar 2022 at 6:52am
I do the same. This is one of the major criticisms my instrument instructor has of my Avidyne (I'm the only pilot he knows with Avidyne... all the rest are G* users). When I activate an approach, he wants to see the VLOC indications on the HSI NOW. On the Avidyne, I don't usually get the glideslope indication and transition to VLOC until very/too close to the FAF. This is correct behaviour in line with the Avidyne PG (see below), but I tend to agree with him that I should be seeing the VLOC indication as soon as the approach is activated.  ------------- Vince |
Posted By: Gring
Date Posted: 29 Mar 2022 at 7:50am
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I don't think that is actually the case with Garmin. Garmin, like Avidyne has a number of steps in a checklist that must be met for the box to auto switch to VLOC. These include properly tuned and identified frequency, withing a certain number of degrees of the final approach course, certain CDI deflection amounts, etc. I think the Avidyne box is actually better because it annunciates its intention - GPS -> VLOC, whereas the Garmin box does not. The Garmin box does not automatically display VLOC on the HSI when the approach is activated. It only displays that message when the box is put into VLOC mode either manually or Automatically as per the TSO.
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Posted By: chflyer
Date Posted: 29 Mar 2022 at 8:07am
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I'm not familiar with the newer Garmin units myself. But I suspect that I am the only Avidyne user in Switzerland, and my instructor flies Garmin all the time. He expects/wants to see HSI guidance switch from GPS to VLOC as soon as an ILS approach is activated. So I'm not sure why you're seeing something different with Garmin. Perhaps the Garmin TSO allows automatic switching much earlier than that Avidyne. My observation with the Avidyne is that the key reason for late automatic switching to VLOC are points 3-5 in the list above. As mentioned, my work-around is to manually switch from GPS to VLOC when I activate the approach.
------------- Vince |
Posted By: Gring
Date Posted: 29 Mar 2022 at 10:25am
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I think you are missing my point. The HSI will only annunciation the mode of the navigator - either GPS or VLOC. It only changes from GPS to VLOC via a manual push of a Button, or automatically when the right criteria is met. The Garmin and the Avidyne criteria are nearly the same functionally (although described slightly differently). Here in the US, I’ve not found a practical difference flying either box. Not sure about your instructor’s ability to think out of the box and wonder how many tools he has in his toolbox. I think if he is expecting things to work just one way, then he needs to rethink his approach. I never rely on the auto switch, and if you do, “verify proper navigation mode” should be part of your approach checklist.
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Posted By: chflyer
Date Posted: 30 Mar 2022 at 10:35am
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I understand what you're saying, but I don't have any experience with the Garmin behaviour. My point is that with the Avidyne the auto switch is very late so I always switch from GPS to VLOC manually after I activate the approach, so that I have VLOC info on the HSI in good time. My instructor has just said that when activating a vectored ILS approach on the Garmin, it switches to VLOC immediately and doesn't require that manual step..... but perhaps I've misunderstood him. ------------- Vince |
Posted By: Gring
Date Posted: 30 Mar 2022 at 11:27am
| My experience is that in practice they work similarly. Once vectored to the FAF, I generally manually switch to VLOC rather than wait for auto switch. |
Posted By: nhflydoc
Date Posted: 01 Apr 2022 at 6:10am
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The vizion/aerocruz100 autopilot has to see the glide slope above you for 6 seconds before it will arm and couple. With a late GS signal on a LPV this doesn't work. The IFD440 uses the codes from Jeppensen.If only it would arm the GS 6 seconds early all would work. Alan
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Posted By: chflyer
Date Posted: 01 Apr 2022 at 8:39am
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LPV being a GPS approach, the GS is being calculated by the IFD? In that case, by a "late GS signal" I assume you mean that the IFD starts sending the GS information very late ? I don't recall anything in the Avidyne manuals about what criteria the IFD has for generating LPV GS output to an autopilot. The "armed vs engaged/active" criteria posted above are in the context of GPS->VLOC, but perhaps the same criteria apply in the case of an RNP approach, only GPS->VLOC is replaced by GPS->LPV, etc. Or is this described elsewhere in the manual?
------------- Vince |
Posted By: nhflydoc
Date Posted: 01 Apr 2022 at 2:40pm
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From what I know the GS signal is coded for the approach from the FAA and Jeppensen supplies the data to Avidyne. With my autopilot Trutrak Vizion it cannot fly the LPV at KIZG as I'm too high on the gs to arm. other autopilots namely Dynon can capture a glide slope that is below. Alan
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