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Author Topic: TD mark on ILS app  (Read 436 times)

Panos Bilios

  • Join date: Jun 2014
  • Location: LGKR
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TD mark on ILS app
« on: Sat, 15 Feb 2020 11:37 »
Hi all
I have noticed on ils approaches there is on NG FMC
A TD mark as if the approach was under Vnav but no vnav pointer is
Present am I doing something wrong here?

Thanks
Panos

Hardy Heinlin

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Re: TD mark on ILS app
« Reply #1 on: Sat, 15 Feb 2020 21:12 »
Hi Panos,

there is an explanation in the Aerowinx NG manual, page 68.

As far as I understand this new feature, the T/D marker is independent of the approach type. It just refers to the end of the level-off constraint, -- and that end exists on any approach type that contains a level-off segment. The E/D is also displayed on any approach type.


Regards,

|-|ardy

andmiz

  • Join date: Jan 2019
  • Location: Hong Kong
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Re: TD mark on ILS app
« Reply #2 on: Sun, 16 Feb 2020 02:07 »
Agreed Panos, I’ve never seen that on our NG’s.
« Last edit: Mon, 17 Feb 2020 05:16 by andmiz »

Hardy Heinlin

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Re: TD mark on ILS app
« Reply #3 on: Sun, 16 Feb 2020 02:31 »
Quote from Boeing:

"6.11. Additional Top of Descent Profile Points
(NG FMC) Additional T/D profile points will be displayed on the ND at the end of a computed level segment in the descent path, such as closer to the approach indicating capture of the ILS Glide Slope from the ILS capture altitude."

Panos Bilios

  • Join date: Jun 2014
  • Location: LGKR
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Re: TD mark on ILS app
« Reply #4 on: Sun, 16 Feb 2020 08:39 »
Hi,

Thanks Hardy for your reply, I think that I understood why there is a T/D mark
on an ILS app, for example, on VHHH 07R ILS there is a level segment between CI07R and FI07R
of 1700 feet, a T/D mark appears on FI07R and a VNAV pointer as well,
as you near the glidescope capture at FI07R(at 1700 feet) and the aircraft descents both
the T/D marker and VNAV pointer dissapears. It appears  that some ILS app
have a level segment and some don't. Correct me if I am wrong

Panos

Hardy Heinlin

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Re: TD mark on ILS app
« Reply #5 on: Sun, 16 Feb 2020 08:55 »
If the path is a continuous descent, there's no T/D.

Whether a level-off occurs also depends on your descent rate; e.g. if you fly from a 4000 ft constraint waypoint to a 2000 ft constraint waypoint, and you have reached 2000 before passing that waypoint, you have to level off. This causes the T/D to appear. If you fly at a descent rate that brings you to the 2000 ft and the waypoint at the same time, the T/D will not appear.

Whether the vertical deviation indicator on the ND appears or not is a different story: It disappears when the G/S pitch mode is engaged -- so that the crew won't confuse it with the G/S scale (the G/S pointer may differ from the VNAV pointer; the G/S is independent of baro and OAT, and may include antenna related deviations etc.).


Regards,

|-|ardy

Panos Bilios

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Re: TD mark on ILS app
« Reply #6 on: Sun, 16 Feb 2020 09:32 »
On the Vnav app I understand the logic but my
Confusion is why a T/D mark appears on an Ils app
While the plane has captured the G/s

Panos

Hardy Heinlin

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Re: TD mark on ILS app
« Reply #7 on: Sun, 16 Feb 2020 09:50 »
It shows you on the map where the descent starts. So you see the distance to go to the capture point. When you are descending along the G/S, the T/D should not be shown. It can only appear while in level flight.

Did you capture the G/S before reaching the T/D?


|-|ardy

Panos Bilios

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Re: TD mark on ILS app
« Reply #8 on: Sun, 16 Feb 2020 10:00 »
Yes I was descending in the G/S but the T/D was present
In front showing 1700 on FI07r on the example in VHHH

Panos   

Hardy Heinlin

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Re: TD mark on ILS app
« Reply #9 on: Sun, 16 Feb 2020 10:19 »
OK, I see what you mean. The reason is this:

If you zoom the T/D in to the max, you will see that it may sit a few meters past the waypoint. The lat/lon values in the database are rounded. My FMC model calculates the T/D using the E/D location and the G/S angle from the database. The calculated beam goes backwards from the E/D up to the FAF altitude constraint. If this reversal beam intersects the constraint shortly before the FAF, my FMC considers this gap a level-off segment -- even if the gap is just a few meters long. I thought about implementing a tolerance zone to inhibit this effect.


|-|ardy

andmiz

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Re: TD mark on ILS app
« Reply #10 on: Mon, 17 Feb 2020 08:28 »
I had a look through some approaches on the NG and while its very rare to see one of these T/D points in our operation, I did find an example of one into MMMX.  I don’t think it occurs into VHHH however so there may be some interesting coding going on in the nav database.

Hardy Heinlin

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Re: TD mark on ILS app
« Reply #11 on: Fri, 21 Feb 2020 01:37 »
Another T/D example has been observed on the real 744: At KSFO on the BDEGA 3 RNAV arrival for Rwy 28L at the level segment from CORKK to BRIXX at 11000 ft.

Hardy Heinlin

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Re: TD mark on ILS app
« Reply #12 on: Thu, 27 Feb 2020 00:59 »
The final approach T/D display logic has been modified in PSX 10.100:

http://aerowinx.com/board/index.php?topic=4191.0


|-|ardy

Panos Bilios

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Re: TD mark on ILS app
« Reply #13 on: Thu, 27 Feb 2020 19:41 »
Hardy,

I can verify that there is no more T/D mark
on ILS rwy 07R at VHHH when G/S is captured!

Panos