Hi all,

I posted the following in the Navigraph forums, but they suggested to post here as they don't believe the issue is on their end.  I often see a very slight difference between the charts and the FMC, but at KDFW the difference is quite significant:

I am wondering why, particularly in the US, there is a large discrepancy between the charts and the Navdata in terms of bearings?

For example, the JPOOL9 SID from KDFW has:

NELYN - 177 - ACT

ACT - 193 - HOARY

HOARY - 204 - SAT

Whereas, the PMDG 747 FMC with the same SID (Cycle 1710), has:

NELYN - 183 - ACT

ACT - 198 - HOARY

HOARY - 209 - SAT

I have linked two pictures from my drop box that illustrate the difference:

Chart

FMC

Why is there this discrepancy?

Thanks for any help you can offer,
Rudy

 

Hi Rudy,

Is your magvar data up to date?

Ugh... that color. Why?

Simon is exactly correct.  The navdata provides each waypoint as a specific location identified by lat/lon coordinates.  The magnetic bearing between them depends on the deviation.  A fix to fix segment is not defined with the bearing between them, only the two fixes.

Hi Simon,

I do have updated magvar data from here (as well as his ILS and Navaids updates):

https://www.aero.sors.fr/navaids.html

Dan - the green was the first colour that I picked to show that post was copied from another place.  I'm assuming it's better than this

Cheers,

Could it be that the FMC bearings between those waypoints are true bearings and the charts bearings, the VOR radials, are magnetic? According to the charts for KDFW the variation currently is 3.8 deg. It is possible the current VOR radials differs from the original magnetic bearings when it was constructed due to the variation changing every year. This then could explain the 5 deg difference between the FMC and the chart bearings. Just my thoughts, I am still learning.

I am pretty sure all bearings on the charts and in the FMC are magnetic - they would be updated as variation changes.

A strange issue indeed.

There is no mystery here, the FMC is going to fly from point A to point B defined by coordinates.  Whatever the heading on that track happens to be does depend on both the true course and the local magnetic variation in the simulation world. OP provided a 5 deg difference between charts and FMS, this isn't any surprise and is pretty common in the simulator world.  This is the reason we fly from point A to point B and only rarely see something like "fly heading until" because the "until" in that clearance becomes a floating waypoint that can be many places.