"Visual segment - obstacles" and "LP/LNAV+V" advisory glidepath

RussR

RussR

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This is going to be a really techie question about RNAV (GPS) approaches and the GPS coding behind the scenes, regarding the advisory glideslope of an LP or LNAV approach when the "Visual Segment - obstacles" note is displayed on the chart.

@John Collins @aterpster , please help if you can.

I flew the 1F0 RNAV (GPS) RWY 17 approach the other day using a Garmin 530W. https://skyvector.com/files/tpp/2009/pdf/05399R17.PDF

It has the note "Visual segment - obstacles" in the profile view (and boy does it ever have such obstacles, trees and big poles just off your wingtip, placed there apparently just to hold red lights for the trees).

upload_2020-9-8_13-31-27.png

It was my understanding that when this note is on the chart, that the glidepath angle would be coded as "0.00" in the ARINC record, and this would cause the advisory glidepath to not be displayed. But when I flew it, I did in fact get an advisory glidepath.

Here is the FAS data record from the IFP Gateway:

upload_2020-9-8_13-48-26.png

Now, this is a sample size of 1, so even if my understanding is correct it could be an error on just this procedure.

John, Wally, anybody else knowledgeable about this type of issue?
 
RussR said:
This is going to be a really techie question about RNAV (GPS) approaches and the GPS coding behind the scenes, regarding the advisory glideslope of an LP or LNAV approach when the "Visual Segment - obstacles" note is displayed on the chart.

@John Collins @aterpster , please help if you can.

I flew the 1F0 RNAV (GPS) RWY 17 approach the other day using a Garmin 530W. https://skyvector.com/files/tpp/2009/pdf/05399R17.PDF

It has the note "Visual segment - obstacles" in the profile view (and boy does it ever have such obstacles, trees and big poles just off your wingtip, placed there apparently just to hold red lights for the trees).

View attachment 89647

It was my understanding that when this note is on the chart, that the glidepath angle would be coded as "0.00" in the ARINC record, and this would cause the advisory glidepath to not be displayed. But when I flew it, I did in fact get an advisory glidepath.

Here is the FAS data record from the IFP Gateway:

View attachment 89648

Now, this is a sample size of 1, so even if my understanding is correct it could be an error on just this procedure.

John, Wally, anybody else knowledgeable about this type of issue?
Click to expand...

Don't know the answer but I'm wondering something. Did you get an LP+V Annunciation? Or did you get the 'needles' without it?
 
RussR said:
This is going to be a really techie question about RNAV (GPS) approaches and the GPS coding behind the scenes, regarding the advisory glideslope of an LP or LNAV approach when the "Visual Segment - obstacles" note is displayed on the chart.

@John Collins @aterpster , please help if you can.

I flew the 1F0 RNAV (GPS) RWY 17 approach the other day using a Garmin 530W. https://skyvector.com/files/tpp/2009/pdf/05399R17.PDF

It has the note "Visual segment - obstacles" in the profile view (and boy does it ever have such obstacles, trees and big poles just off your wingtip, placed there apparently just to hold red lights for the trees).

View attachment 89647

It was my understanding that when this note is on the chart, that the glidepath angle would be coded as "0.00" in the ARINC record, and this would cause the advisory glidepath to not be displayed. But when I flew it, I did in fact get an advisory glidepath.

Here is the FAS data record from the IFP Gateway:

View attachment 89648

Now, this is a sample size of 1, so even if my understanding is correct it could be an error on just this procedure.

John, Wally, anybody else knowledgeable about this type of issue?
Click to expand...

That 8260/10 on the IFP Gateway is dated 10-15-2015. The Amendment date on the Chart is 01FEB18. There might have been an Amendment that made the Glidepath Angle(GPA) something other than zero, like 3.00, and the VDA/TCH thingy didn’t get put on the Chart. Why the older 8260/10 is still in the IFP Gateway five years later and a newer one isn’t, who knows
 
Last edited:
luvflyin said:
That 8260/10 on the IFP Gateway is dated 10-15-2015. The Amendment date on the Chart is 01FEB18. There might have been an Amendment that made the Glidepath Angle(GPA) something other than zero, like 3.00, and the VDA/TCH thingy didn’t get put on the Chart. Why the older 8260/10 is still in the IFP Gateway over five years later and a newer one isn’t, who knows
Click to expand...

You're looking at the wrong date. That's the date of the last superseded amendment. The date of the form is 1 Feb 18.
 
If you have access to the approach procedure ARINC 424 data, see if it has an angle coded for the LNAV. The data packer may have packed that angle.
 
When the old note was used "Descent Angle NA", it was true that the VDA was set to zero. This prevented any advisory vertical guidance. When the updated note was available that reads as "Visual Segment - Obstacles", the VDA was coded in the database record and Jeppesen used it to provide +V on LP or LNAV procedures. The two notes were intended for the same reason, obstacles along the VDA path below the MDA. The theory on the setting the database VDA to zero was to block anyone from using +V, but it had a side consequence of crashing many of the Garmin WAAS navigators. The Garmin bug was eventually fixed. Concurrent with the new chart note and the inclusion of the VDA in the database, Jeppesen took the position they were going to chart it and also provide +V. AIS decided they would not chart the VDA or TCH when the new note was added. So +V with LP/LNAV with the note "Visual Segment - Obstacles" on the chart is to remove those idiots from the gene pool that follow the +V below the MDA. If you can't figure out the note is telling you you might hit a rock, a tree, or a tower below the MDA, you should not be flying these procedures.
 
Well said John.

A far too common error is to treat the advisory vertical guidance like it’s a glide slope, meaning it represents an obstacle-free path to the runway. It’s far better to consider any path below the MDA as flight in an obstacle-rich environment.
 
Rebel said:
Well said John.

A far too common error is to treat the advisory vertical guidance like it’s a glide slope, meaning it represents an obstacle-free path to the runway. It’s far better to consider any path below the MDA as flight in an obstacle-rich environment.
Click to expand...
Another way to look at it, if it isn't ILS, LPV, or LNAV/VNAV you may want to have familiarity with the airport before flying in there at night or during IMC.
 
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