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
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
Agreed Panos, I've never seen that on our NG's.
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."
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
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
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
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
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
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
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.
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.
The final approach T/D display logic has been modified in PSX 10.100:
http://aerowinx.com/board/index.php?topic=4191.0
|-|ardy
Hardy,
I can verify that there is no more T/D mark
on ILS rwy 07R at VHHH when G/S is captured!
Panos
Hardy,
On the RNAV app at VHHH 07R on the FAF VH721 appears
a T/D mark at 1600 while executing a LNAV/VNAV app
Panos
This is because my FMC assumes the descent path to VH721 is shallower than the path thereafter. There's no level-off, but there's a knee at VH721.
The algorithm of the real NG FMC on this subject is unknown. We can only speculate.
|-|ardy
I just learned from a real-world EGLL ILS 27R situation: The ND shows the T/D about 0.1 nm after the FAF (math roundings like in PSX), and the T/D remains displayed even when the G/S pitch mode is engaged. So the AFDS mode cannot be the condition for showing and hiding the T/D. The T/D only disappears when it is passed.
|-|ardy