Thanks for the input people.
And for exploding one of my misaprehensions...
apart from any other reason, a modern clean airliner will not descend >3degreees without gear / speedbrake
So an airliner will descend at shallower than 3 degrees unless some positive drag is introduced
. This rather negates my question because the aircraft in idle descent will tend to be coming at the glideslope from beneath anyway.
They really don't want to come down do they?
Reminds me of the old joke "If we lose any more of these engines we'll be up here all night!".
