Barankin
Indeed, as you say, you should not descend prior to reaching the IAF. Out here, ATC sometimes specify it in the transmission "Upon reaching YXX VOR, you are cleared for the .... approach/ upon reaching DUBAG waypoint, you are cleared for the .... approach". Even when this is not the case, it is implied that before reaching the actual approach you are only "pre-cleared" for it and the clearance only comes into effect once you reach a point which is part of the approach (usually an IAF). Therefore, before reaching it, you should maintain the last assigned altitude.
Once the IAF has been reached, there is no need to refer to any document except the approach plate. It will show you exactly how much you can descend on each leg. You can intercept the glideslope (whether actual, virtual or calculated) from "underneath" by maintaining altitude until you start to close in, or you can descend to the minimum altitude for each segment. Usually it's the first option, but depending on the type of operation, on the approach itself and on other circumstances, you might go down the steps instead. Or a combination of the two. In both cases, you must ensure that you are above the relevant minimum altitude. There are exceptions to this - for instance, there are ILS approaches on which the glidepath will briefly dip you under the minimum stepdown altitude at the FAF - but that's where that altitude restriction ceases to be applicable anyway, so it's not really an issue. Otherwise it wouldn't be designed that way.
All that being said, if the minimum altitude over the FAF is 3000' and it was also that at the IAF, as in your example, you naturally wouldn't descend between them.
If your question is whether it is a huge violation to descend below any minimum altitudes, that is another discussion. Is that what you meant?
Last edited by Stuka Child; 14th Jul 2021 at 23:46.