Strictly speaking, you shouldn't be using the Glide Slope for primary vertical navigation above the Glide Slope Intercept Altitude. The Glide Slope is only fully flight checked to GSIA, it might be checked further in some cases, but you can only be sure that it has been checked to GSIA Out further, it is flight checked below the glideslope for proper fly-up indication, or in other words they check that you don't get an above GS indication when you are below the GS.
So, you're out there on the localizer and you have a stepdown coming up, and your gs needle is coming alive, soooooo.......do you dive for the next altitude then level off again? or do you follow the glideslope? Again, you're not supposed to be using the GS for *primary* vertical guidance above GSIA, but what's wrong with using it as a "guide" for your descent that you're going to make anyway? Personally I don't see any compelling practical or regulatory reason not to use it as a guide for your descent.
There are a couple of objections which come up. One it the fear that you might get on the wrong lobe of the glideslope. Well, let's think about it. An ILS would not have a GS lobe below the correct GS, It would be a disaster, planes would be hitting the ground at the inner marker one after the other. Additionally, that's why they flight check for fly-up indication, to make sure that there's no false GS below the correct GS. There are GS lobes *above* the correct GS, the first one is has a 6 degree slope and has reverse GS needle sensing, you should be able to tell if you're tracking that one. The next one is at 9 degrees and has corect sensing. If your normal final approach speed requires a 700 ft/minute descent to follow the 3 degree GS, the 9 degree GS will require better tan a 2000 ft/min descent to follow it. That should be your tip off.
The false GS shouldn't be an issue unless you're really, really unaware of your descent rate.
The other objection is that the GS might take you below an intermediate stepdown altitude. This is a little more credible concern. I understand that there are ILS approaches that are like this in the US. I don't know where, but I was told by a flightcheck guy. I assume he knows what he's talking about.
Well, here's the deal, like I said, don't use the GS as your *primary* means of vertical navigation. Presumably, you have navigation equipment sufficient to identify the stepdown fixes and determine that you are above minimum altitudes at all times, right? If not, you shouldn't be flying that approach. So, you use that equipment (DME, Altimeter, etc) as your primary guidance to ensure that you're above minimum altitudes, and you use the GS as a guide to help you achieve a nice smooth descent.
Disclaimers:
1) There are ILSs with *mandatory* stepdown altitudes. IAH is one. Like RJET said, those are self explanatory.
2) If you're doing an oral or a checkride, the best answer is that you intercept the GS at GSIA. If the examiner is enlightened, he can then explain to you the same thing I just explained. If he is unenlightened, that is the only correct answer.
3) You are not inside the final approach fix untill you have passed the GSIA on the GS, intercepting the GS 10 miles out does not constitute the FAF
4) Your company policy may vary.
regards
