Hi Schmid (Mr. Schmid?)

What I meant was that in flight level change, it is my understanding that it will adjust pitch to maintain speed.  If you are crusing at 300 kts and descend by hitting that button, you will descend at a rate that does not alter your airspeed, bur rather your pitch.  You can steepen that descent with the speed brake, but the speed shouldn't change.  In other words, the controlling factor is airspeed, not descent rate.

With V/S, if you set it at at -3000 you are going to get get whatever speed you get at that rate of descent, and you will find yourself having to reduce your descent rate and/or employ brakes to keep your speed from running up (and, for example, busting your 250 kt limit).  So, yes, V/S can affect your speed incidentally to the descent, but it is different from flight level change in that descent is locked into your desired speed.  Contraiwise, in V/S the controlling factor is descent rate, not speed.  They're really opposite.

I usually descend using flight level change unless I am flying a full procedure without ATC, in which case I let VNAV handle the descent.  When I get further down and want to control my speed and descent more precisely myself, I use V/S.  Or, I'll switch to V/S, perhaps with speed brake, for a limited time to get down quicker when ATC wants me at a certain level within 30 miles.

Yes, a 3 degree glide slope is typical.  But going 300 kts while on that 3 degree glide slope is not.  You should not even be above 250 at that point, and, depending on where you are in the approach, a darned sight less than that.

Look at p. 31 of Part III of the manuals, Normal Procedures.  You should already be at Flaps 5 before you intercept the localizer.  That's your vref for flaps 30 + 40 kts.

Let's say you're coming in at a landing weight of 200,000 lbs.  According to p. 12 of Part IV of the manuals, your vref for flaps 30 is 133.  (Or look on your MCU.)  So you should be at 173 kts with flaps 5 when you intercept the localilzer (flaps 30 vref (which is 133) + 40 = 173).

It should not be a question of this add-on performing differently, but giving yourself plenty of space to slow down.  I am just having trouble imagining any circumstance where you would be on the 3 degree glideslope at anywhere near 300 kts.  I respectfully invite you to consider that this is the problem, not the way the speed brake works.

How far out from the runway threshold are you turning onto the extended centerline?  Why are you going 300 kts at the point?  I'm sorry if I'm not understanding your question, but I bet if you give yourself more room to slow down, you will find your speed problems disappear.

Are you following FS ATC, or what?

Tim