An Airbus has a “GS hold” mode. I don’t know what it’s used for, however.
I think you mean the GS mini Function :
Most instructors recognise that we’re talking about NPAs and a 100ft discrepancy on approach isn’t going to make much difference with a 350ft MDH. 
As Josh says, half estimated GS + a zero; the maths is easier. From there onwards range vs. altitude +/- 100ft at each range check is more than adequate.
AeroPlus wrote:
Most instructors and examiners don’t like people asking too many questions.
Because they don’t know the answers themselves…
Even in a light twin increasing IAS could put you close to gear and flap limits?
You actually decrease the IAS, to hold a fixed GS, on a descent into any wind shear.
So the danger is getting too slow, not too fast.
An Airbus has a “GS hold” mode. I don’t know what it’s used for, however.
Most instructors and examiners don’t like people asking too many questions. I have in the past always been the person that wanted to know the WHY. Now I have been doing a lot of IFR flying, but for the MEP I had to go through an initial MEP/IR skills test doing it all as I also learned to do it when I just started flying IFR and went for the initial SEP/IR skills test. So, I opted to just go along with them which is already quite difficult as one instructor to another examiner might have a completely different view on how things MUST be done (according to them) such as when to lower the gear and you name it. So, my short answer: no, they did not tell me why 8% and I also did not ask :-)
AeroPlus wrote:
That is what was in the IR syllabus of the training school (+8%)
Did they tell you why? The exact ratio of VS in fpm to GS in knots for a 3° glideslope is about 5.3, so If you wanted the exact VS you should add 6%. But there is no point in trying to achieve that kind of precision as GS will change during the approach in any case.
Varying IAS would just lead to an unstable approach in a big jet surely especially with inherent speed instability away from target Apporach speeds? Even in a light twin increasing IAS could put you close to gear and flap limits?
Ps I use 5 x GS and plate VS as well. On my IRT wind was calm which was actually harder work than strong wind I found!
Half your ground speed and add a 0 to the end. I can’t multiply by 5…
I use GS x 5 and 300 ft per NM … somehow my (already bad) math doesn’t work well in the airplane.
