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No GS on LPV

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luchetto View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote luchetto Quote  Post ReplyReply Direct Link To This Post Topic: No GS on LPV
    Posted: 13 Feb 2017 at 3:17pm
It has happened no twice to me that the GS did not show up on an LPV approach. I had no GPS integrity warning. Since GPS-Vloc switching happens way too late I put the two boxes on Vloc early enough. The LPV approach into EDTM has a base leg which is perpendicular to the final approach course. The boxes gave no lead in indication when to turn onto the final leg so I slightly overshot. The approaches were of course activated.

The GS has failed me twice on this approach and I don't know if certain conditions lead to this behavior. What could be the reasons for this?

Placido
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bellanca1730a Quote  Post ReplyReply Direct Link To This Post Posted: 13 Feb 2017 at 4:18pm
Shouldn't the IFD remain in GPS mode for an LPV (GPS) approach? VLOC mode utilizes the NAV/LOC/GS receiver.
Sean Andrews
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oskrypuch Quote  Post ReplyReply Direct Link To This Post Posted: 13 Feb 2017 at 4:30pm
Yes, for an LPV approach, the IFD must be in GPS mode, not VLOC.

Additionally, the IFD must have been wired correctly, there are two vertical deviation outputs, only one of them will provide both a GS with an ILS (in VLOC mode) and a GP with APV approaches, in GPS mode.

* Orest

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Post Options Post Options   Thanks (0) Thanks(0)   Quote luchetto Quote  Post ReplyReply Direct Link To This Post Posted: 13 Feb 2017 at 4:53pm
Well, when you activate the approach it shows the usual gps-vloc auto switch mode in the top right corner so I manually switched it. I can't remember if when I got the GS a few weeks ago I let it auto switch. For sure when I flew the approach a few weeks ago with a working gs the box was showing vloc.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote chflyer Quote  Post ReplyReply Direct Link To This Post Posted: 13 Feb 2017 at 5:13pm
Placido, what was the specific approach?
Vince
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MysticCobra Quote  Post ReplyReply Direct Link To This Post Posted: 13 Feb 2017 at 5:37pm
Something's not making sense.  If you were in VLOC mode and you had an active GS, then you were tracking an ILS.

It is completely possible to load an RNAV procedure but then force the box to VLOC mode so that the needles are driven by the NAV receiver instead of the GPS box.  It sounds like that might be what happened.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mfb Quote  Post ReplyReply Direct Link To This Post Posted: 13 Feb 2017 at 5:37pm
Originally posted by luchetto luchetto wrote:

Well, when you activate the approach it shows the usual gps-vloc auto switch mode in the top right corner so I manually switched it. .......... For sure when I flew the approach a few weeks ago with a working gs the box was showing vloc.

That sure sounds like an ILS or a localizer approach to me.  You should never see VLOC on a GPS approach.

The 540 won't automatically switch from GPS to VLOC until the localizer has been tuned and identified and the box senses that it's close to intercepting it. But manually switching to VLOC should have brought up the glideslope, assuming that a valid VHF glideslope signal was present. Was it?

Mike
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Post Options Post Options   Thanks (0) Thanks(0)   Quote luchetto Quote  Post ReplyReply Direct Link To This Post Posted: 13 Feb 2017 at 5:44pm
Vince, it was the RNAv 26 into EDTM.

All, when loading and activating the RNAV LPV approach the top right shows gps-vloc. This airport does not have an ILS approach.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote chflyer Quote  Post ReplyReply Direct Link To This Post Posted: 13 Feb 2017 at 5:52pm
Looking at your first post, Placid, you're referring to EDTM. It looks like this airfield only has one approach, and that is an RNAV approach RNAV (GPS) RWY 26.

As indicated by others, you should never see GPS->VLOC on this or ever see VLOC for that matter because VLOC is only for VHF nav, not GPS.

Since the approach has LNAV, LNAV/VNAV, and LPV, you should only see one of them at the top of the screen. And you'll only get a glideslope with LNAV/VNAV or LPV, but not with LNAV.

If you saw GPS->VLOC with this approach loaded and activated, then there is a serious problem with the software or the Jepp coding of the approach in the navdata..... I honestly can't imagine that happening.



Vince
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Post Options Post Options   Thanks (0) Thanks(0)   Quote chflyer Quote  Post ReplyReply Direct Link To This Post Posted: 13 Feb 2017 at 5:58pm
Here is the approach chart for the benefit of all:

Vince
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Post Options Post Options   Thanks (0) Thanks(0)   Quote chflyer Quote  Post ReplyReply Direct Link To This Post Posted: 13 Feb 2017 at 6:04pm
Check the PG page 5-5 for the possible combinations at the top of the screen. Note that GPS->VLOC should only appear if "the GPS flight plan contains a VHF-based procedure or leg ..."

If this is reproducible, I suggest you fly the approach, take a photo of the screen, and then post it here.




Edited by chflyer - 13 Feb 2017 at 6:05pm
Vince
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luchetto View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote luchetto Quote  Post ReplyReply Direct Link To This Post Posted: 13 Feb 2017 at 6:17pm
Man, maybe my brain saw gps-vloc when instead it was showing gps-lpv. The top right was showing this auto switch mode and it might be that by twisting the knob manually it won't switch to lpv but vloc instead which is wrong.
I tried it on the sim now and it shows gps- l/vnav. If you wait long enough it will auto sequence to l/vnav if instead you twist the knob it shows vloc.

that is not an intuitive behavior, you would expect that the manual switching will give you whatever the top right sequence should be. Well, it looks I learned something new. The auto switching on the Avidyne boxes really happens too late for my gusto, so one is tempted to hand switch. While this works 100 times on the ILS this routine screws your LPV approach. I wonder if turning the knob back while on the final approach course would have cured the problem.

Edited by luchetto - 14 Feb 2017 at 12:54am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote chflyer Quote  Post ReplyReply Direct Link To This Post Posted: 14 Feb 2017 at 1:56am
I agree that the switch from GPS to VLOC on an ILS is very late when being vectored. My instructor wants to see the HSI guidance change from magenta (gps) to green(loc) much sooner and my experience is that I'm on the localiser before that happens.
Vince
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