A330-A340 Ground Roll Guidance Command Bar

[EK380]

After doing some A330-A340 Low vis ops in the Sim, if have the following question which is not answered in the FCOM's.

This must be applicable to A320 series as well.

Take off from a runway with an offset ILS.
Select the LS button for your take-off

How does one explain the different behavior of the Ground Roll Guidance Command bar (Yaw bar) and the LOC on the LS scale...?

At the beginning of the RWY the LOC and Yaw bar will be almost centered, During the T/O the LOC will run away (which is normal for an offset LOC) faster then the Yaw bar....

I had always thought that the Yaw bar (official name, Ground roll Guidance bar) was presenting exactly the same as the LOC...


Anybody knows how the Yaw bar really works?


Thanks

[kijangnim]

Greetings,
My understanding is that the Yaw Bar is using the ILS LOC during the ground roll, and for low viz it has to be at least a CAT II ILS.
The YAW bar doesnot have the same sensitivity, as the LOC does, nor its scale.

To be confirmed.