I received this Q by P Message:
<Abeam points
How long does it take to get the abeam point into the FMS?>
A: On 737 3/4/5, around 40-60 seconds before the descent profile is 're-calculated'.
If abeam waypoint needs spelling to crew, longer!
To expand on Cough's post - not all 737's have this 'abeam points' function, depending on the mod state of the FMS. Therefore it will be nearer to 60 seconds on these a/c as the point has to be 'constructed'. As he says, of course, 'abeam SABRE' is no longer based on a DME range in his scenario, but 90 degrees abeam SABRE on the track to LAM, somewhat more difficult to calulate by DR!
It is 'seductive' for crews in this scenario to wait for computed descent point to appear: hence a re-inforcement of the trial of 'descend now' if that is what you want? If you cannot bring yourselves to make this non-CAP call, how about giving that minimum descent rate or "Descend Fl xxx to be level by YYY, leave now"? Another R/T call we get elsewhere. Then you would trigger those 'seduced' into actually starting down while the great computer machine does its bits with bytes.
Let's not forget those colleagues of ours (me not so long ago!), stumbling about Europe using antiquated navigation equipment, non-FMS, where all descent planning had to be based on DR, and 'abeam' points most certainly need a DME reference if available.