Generally Z is used for the procedure with the lowest minimums, but this is not a rule and there are many counter examples. This is all TERPS states regarding the assignment of an Alphabetical suffix:
d. Alphabetical suffix. When more than one procedure to the same runway uses the same type of navigation system for lateral guidance within the final approach segment, differentiate each procedure by adding a non-repeating alphabetical suffix using the letters “S” through “Z.” Suffixes are normally assigned in reverse order starting with “Z,” but may be assigned as needed to meet operational needs (for example, all RNAV (RNP) approaches at an airport assigned “Z” suffix, all RNAV (GPS) approaches assigned “Y” suffix, etc.).
The Z is Category C only
LFLP is another counter-example where the Z plate is for A B and C:
And sure enough LFLP came up previously.
Also various posts further back state that Z is the default and they count back from Z. That would perhaps explain why the KLN94, and Foreflight, display Z only for EGMD.
The Y was not in the FF database and not in the KLN94 database.
Got this again today in the KLN94 database. The FF plate could not even be loaded.
Peter wrote:
It would take a procedure design specialist to work out the “why”…
The Z is Category C only. The Y is category A/B only and has shorter base legs so if flown at 180 Kts, the aircraft can be in a continuous turn from downwind to final on the base leg in worst case wind conditions. Note the speed limitation at the Straight In IAF/IF. TERPS would want the base leg to be 8 NM and would never allow the the TAA altitude to be below an IAF segment. Also there is a lower GP limitation on Category C verses Category A/B, making the descent angle lower on the Z resulting in a higher DA/DH.
Bathman wrote:
The NDB seems to be much more reliable
I just quote this for history :D
There are many differences but they are basically in the same place. As I say, I am amazed that the CAT A/B/C was not pointed out before! Especially as both plates do CAT A/B which is all the KLN94 is ever likely to be used for.
It would take a procedure design specialist to work out the “why”…
Peter,
I see both approaches in the ForeFlight procedure advisor and in the Jeppesen approach charts. The Y appears to be for category A and B while the Z appears to be for Category C only. There are many differences in the two procedures, it appears they are driven by the speed differences of category A and B in comparison to Category C.
Is there any consistency in the CAA identification of RNP plates?
In the States they are, I believe, listed in logical reverse sequence in order of Category I.e. Z for Cat A & B; Y for C and so on. etc. Thus making it easier to select the right plate (especially in a hurry) without having to peruse it first.
At Lydd, the Cat A & B is identified as Y.
I got LONRU – Z
The Y was not in the FF database and not in the KLN94 database.
