Jeff;
It's very simple and explained very well in Davies. The L/D curve for a jet transport when Davies wrote back then and still today, is very shallow. The airplane will slide along the curve left and right, very easily where as for propeller-driven aircraft the curve is much more "U" shaped mainly due to the thrust and drag offered by the large disc(s) described by the props.
Thank you PJ2, I will try to find your reference. I did a very simple computation a while ago, but I have no experimental data to validate this, do you think it could be plausible ?
This would suggest that it takes a little bit more than 30 s to exceed the MMO, a little bit more than 1 min to reach Mach 0.88, and more than 2mn to reach Mach 0.89 (with a N1=0.95 and a 2° descent)
Jeff