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The RNP approach for LGIR runway 27 has the altitudes on some of the points underscored.

LGIR RNP approach

I had the impression (and I'm afraid I'm wrong) that these are "crossing restrictions" and the controller can use the "Cancel Level restriction at IR402" in order to waive the restriction. Though this phraseology in ICAO Doc 4444 16th edition is only found under the Clearances on a SID and Clearances on a STAR sections but not in approaches.

Furthermore, I tried to search for the ICAO chart legends that unfortunately don't go in any further detail. Annex 4 for example has this Annex 4 altitude legend

Unfortunately I don't have access to the Aeronautical Chart Manual (Doc 8697) in order to see if there are more details there.

While searching, I found out that the Belgian AIP goes one step further and has an (on SID/STAR) comment next to each of the altitude descriptions. Curiously, the Upper and lower limit got away with it.

To make things even more complex, conventional procedures have similar altitude depictions but on the route and not on the point. From that, I got the impression that this is a MEA (minimum en-route altitude) or something similar, thus different from having the altitude on the point. For reference, older VOR procedure, same airport.

LGIR VOR approach

My question: what are the altitudes depicted on the two approach charts? Is it the same concept drawn differently (conventional vs RNP) or is it something else?

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  • $\begingroup$ What do you mean by "semantics" and "drawn differently"? An altitude with a line under it always means "at or above", per the table you posted in your question. Are you asking why the number values themselves are different? $\endgroup$ Feb 1, 2023 at 17:03
  • $\begingroup$ @MichaelHall No sorry, I wouldn't go in all that trouble just for that. I want to know why the former is on the point and the latter is parallel to the route. Hence the "semantics" part. For example why would the Belgian AIP say that the underscore means at or above only on SID or STAR? That pops the question, what does it mean elsewhere? $\endgroup$ Feb 1, 2023 at 17:39
  • $\begingroup$ I would guess that it means that if you are not on the published SID or STAR that the altitude restriction does not apply, but I don't see any real reason to emphasize that point. Anyway, it doesn't imply that the underscore means something different elsewhere, only that it applies to the SID or STAR. Those are two very different things... $\endgroup$ Feb 1, 2023 at 18:59
  • $\begingroup$ Remember that an RNP approach is not simply an overlay of the existing VHF-based approach, it is an all-new, “clean sheet” approach. At almost 20 NM, your VOR positional error will be roughly 40% greater than your RNP positional error. At the FAF the VOR will be about 20% less accurate than RNP. Thus, the RNP’s altitudes reflect the improved clearance from terrain that the system provides. I also suspect (not sure) that expressing them as crossing altitudes is more aligned with the logic in modern FMSes. $\endgroup$
    – Max R
    Feb 2, 2023 at 16:10
  • $\begingroup$ @MaxR Thanks adding this bit of information. The last sentence of your comment is what interests me: why not make a legacy procedure compatible to the modern FMSes? I reworded the question with the hope that it became more clear. $\endgroup$ Feb 2, 2023 at 22:40

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This is merely a difference between charting styles from old to new. This happens when a country changes their style which only affects newer procedures. The VOR RWY 27 was last updated in 2016: enter image description here

The RNP RWY 27 was updated at the end of 2022: enter image description here

In both cases, these are crossing/segment altitudes. If you look at the coding for the RNP, you will see that these apply at each waypoint: enter image description here

*Edit: I highlighted the MAP, but that does not get shown in the plan view as it depends upon the aircraft category.

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  • $\begingroup$ I appreciate the effort but unfortunately this doesn't answer the question. Plus it's more of a speculation as it doesn't cite any official documents. LGAV has conventional procedures updated on July 2021 and they still have the altitudes on the route, not on the point. $\endgroup$ Feb 4, 2023 at 10:28

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