Heading ILS Fixes not correct

[TDPlane]

I noticed that the heading of the fixes pertaining to the ILS approach into KMCO17L are not correct. The IXEG FMS shows a heading of 172 on the ILS; they should read between 184-186 according to real world navdata. This also occurred with fixes on the ILS of KDTW22L. I'm on the stock navdata of the IXEG, i.e. 1509.

The problem corrects itself when I get close by the first approach fix. All the 172's turn into the right heading.

I really cannot tell if this is part of the issues with VNAV in general.

[cmbaviator]

19 hours ago, tdplane said:

I noticed that the heading of the fixes pertaining to the ILS approach into KMCO17L are not correct. The IXEG FMS shows a heading of 172 on the ILS; they should read between 184-186 according to real world navdata. This also occurred with fixes on the ILS of KDTW22L. I'm on the stock navdata of the IXEG, i.e. 1509.

The problem corrects itself when I get close by the first approach fix. All the 172's turn into the right heading.

I really cannot tell if this is part of the issues with VNAV in general.

I don't understand why are you showing this view, the 172 at SEEDO is refering to the calculated Vnav Speed at SEEDO not the bearing.

the ILS frequency and Course is provided in the FMC when you press the INIT REF after passing the T/D pseudo waypoint

[AlfaMike]

I think he's referring to the bearings that are always 172° (HERVI, FONIK, SEEDO, COCAS and PW17L).

[Litjan]

Hi, we are investigating some subtle differences between published bearings and bearings we have in the simulator. In the real world, a discrepancy of ca. 1-2 degrees would not cause much concern and is explained by old IRS magnetic variation database.

In X-Plane things get a bit more difficult - especially when flying longer flights where the Mag Var changes substantially. I think we are calculating magnetic track when first loading the route (True Track + Mag Var). But for this calculation we are taking "present mag var" - there is simply no way to read the "destination mag var" easily in X-Plane, though I have some ideas (after all I am the one who supplied LR with an updated MagVar table a year ago, or so).

So when you get towards the destination, you will see this discrepancy - however when flying LNAV, this should have 0 impact, because you are flying between waypoints that are fixed geometrically.

Jan

 

[TDPlane]

@cmbaviator: those numbers are really headings c.q. bearings. Not speeds.

Edited May 23, 2016 by tdplane

[Tchou]

31 minutes ago, tdplane said:

@cmbaviator: those numbers are really headings c.q. bearings. Not speeds.

And the ILS bearing and frequencies are in the approach ref page as soon as you entered an ILS final, even on the ground before taking off… ;-)

[TDPlane]

Just checked again. Even on the ILS of KMCO17L, on final (bearing 185-186), the fixes still read way off with their 172. And for KATL27R after programming the FMS, I get 263 where they should read 275. Stock navdata 1509 ...

 

 

[cmbaviator]

14 hours ago, tdplane said:

@cmbaviator: those numbers are really headings c.q. bearings. Not speeds.

 

I just got it, I saw : SEEDO :172/2500A, I thought you were referring to that 172 which is speed^^

[TDPlane]

In my book it's a bug ...

[mokny]

 @Litjan

You can grab the latest magnetic declination model here as a database (csv exportable): http://www.ngdc.noaa.gov/geomag-web/#igrfgrid

[Graeme_77]

There's certainly something happening with the FMC computed courses. A couple of screenshots from the Turin (LIMF) SIRLO7A departure, the key waypoints being MF403 and SIRLO. On the charts it's course 104 from the CSL. I've tuned  the CSL and also drawn a fix on radial 104. As you can see from the pic on the ground that fix line parallels the course line on the Nav display, but the FMC shows course 107 rather than 104.  In the second pic I've just passed MF403 (odd config to keep the speed low) The heading pointer, fix line, and on the P2 VOR display the actual radial all show the correct thing of 104, and unlike the first pic where the FMC said 107, it's now 103.

I'm happy with that 103 vs 104 as one expects a certain error as mentioned earlier in the thread, but it's the fact that the course shown on the FMC changes from 107 to 103 as soon as the waypoint sequences. I understand it may be using source location magvar for the initial FMC display, then actual magvar as it sequences, but it doesn't change by 4 degrees in such a small area.

 

[cmbaviator]

any way it's not a big deal if you enter the correct ILS course provided in the charts, but could be a big deal if you perform an LNAV approach

[TDPlane]

Maybe not a big deal. True, the ILS will bring you down. But it's just not appropriate ... 12 degrees difference with the true or magnetic bearing ... Then again, maybe it's something with the navdata itself.

Edited May 25, 2016 by tdplane