wims wrote on May 26^th, 2011 at 3:47pm:

mon/13 is not an actual fix, its just an indication that you have to fly a dme arc at 13 dme from mon. You have to handly it. The plane will not turn automatically.

Thanks for the info.
Probably the angle was to much for the aircraft and it couldn't line up correctly.

For my earlier mentioned problem , flying another direction with HDG  for a while and then trying to return to the fp ,I found an answer here at the forum, posted by Sjef Gielen :

I allways fly the plane in heading mode because the DME Arc is not in the STAR, as soon as I've flown the ac to the first waypoint after the DME Arc I want to switch to LNAV (Which is allready armed on the runway), but as soon as I want to do that the aircraft turns 180 degrees and heads back to the previous waypoint which isn't on the legs page anymore. I found out that switching to LNAV right before a waypoint works fine, but when, say, flying halfway inbetween two waypoints it allways turns back to the previous one.


I also found the follwing about flying a DME ARC with a FMC.
This translated by Google in English.
Don't know if it can be done this way with the CS FMC.

How do you now fly a DME Arc ?
Eg : the  APP for Madeira uses a DME Arc.

Visit the FIX page on the FMC, and enter at the FIX position the name of the circle drive where the DME is based on.

On the RAD / DIS below enter "/ 10" if it is a 10 DME circle, or / 12 if it is 12 DME arc.

On your map will display a circle that you can fly by hand, also called poor man's DME arc.
It is NOT part of your flight plan, so follow your autopilot in LNAV NOT that circle

Do you want the autopilot fly this in LNAV mode then you need to add  points to your flight plan

Set the Navaid used as the basis for SPL circle and the circle begins at 10 DME distance on the 180 radial (due south of the VOR) and ends on the 090 radial (due east)

Add the following points to your flight plan:
SPL180/10 SPL180/10
SPL170/10 SPL170/10
SPL160/10 SPL160/10
etc t/m etc to
SPL090/10 SPL090/10

Possibly with height restrictions.
Press execute and the points are in the flight plan.

In real life one will often use the manual method using the RMI instruments or to circle the fix page.