In all instances when you're in Descent Mode but under the glide path, VNAV will command a 1250ft/min descent in the hope of capturing the idle path. It'll do this in IDLE/HOLD or THR, along with VNAV SPD. When within 150ft of the idle path, it'll transition to IDLE/VNAV PTH.
I have been trying to find anything in writing that confirms that when in a Descent Mode and under the glidepath, VNAV will add power to maintain a 1250'/min descent rate. I agree with McNugget, but all I can find in reference to a 1250'/min descent deals with an
early descent -- not one where you are on path and then go below path.
If you were on path (IDLE||VNAV PTH), descending at 300 knots (FMC-commanded) and then you speed intervened and set 340 knots in the window, the airplane would, of course, pitch for speed and go below the path. Presuming the above quote is true, at what point would thrust be added to attempt a 1250'/min descent? Anyone have any reference that the logic is set up to do this? Thanks.