Why have ILS X / ILS Y / ILS Z ? (and which IAP types are missing in GPSs)

Peter wrote:

My original Q (today) was triggered by reading a supposedly definitive document on this stuff which states

Only the ‘Z’ variant is coded in the GNSS database, and the procedure identifier does not include the ‘Z’ character

So it looks like there has been a change within the last few years at most.

We are in the middle of a major rewrite, Edition 3.0, which is picking up a lot of this stuff.

Slightly different, but similar; you will sometimes find several procedures in the database with the same name. The thing that differentiates them is the category of aircraft. IIRC the first is for the higher category aircraft, and the second (last) is for the lower category aircraft. You may find that when the procedure turns are different based on the category of aircraft like is the case for Dijon LFSD VOR RWY 17 for example.

Aviathor wrote:

Slightly different, but similar; you will sometimes find several procedures in the database with the same name. The thing that differentiates them is the category of aircraft. IIRC the first is for the higher category aircraft, and the second (last) is for the lower category aircraft.

I would say that for overlay approaches this is the norm rather than the exception and, indeed, the adage “Lowest is Slowest” normally applies.

However, I have found one example where that is not true, the categories are upside down. That is procedural approaches to London City. I am going to guess wildly that

1. No-one has ever flown the procedural approaches to London City
2. Anyone who has, maybe in the sim, has been flying a Cat B or C aircraft, which would be in the middle anyway

…so I may be the first person in history to spot the anomaly.

It also depends on the type of approach. Some RNAV approaches are not in the DB, in the US for example, those that are RNAV (RNP) won’t appear in most GPS database.

NCYankee wrote:

those that are RNAV (RNP) won’t appear in most GPS database.

Do you mean those in the States or worldwide? I know that the FAA are opposing the RNP designation, but it is going to become very widespread in Europe and the rest of the world and is supposed to be mandatory by 2024, so does that mean that Jepp DBs won’t have any RNAV approaches because they are all designated RNP?

I had heard that this was just a problem for Boeing but that may just be hearsay.

Timothy wrote:

Do you mean those in the States or worldwide? I know that the FAA are opposing the RNP designation, but it is going to become very widespread in Europe and the rest of the world and is supposed to be mandatory by 2024, so does that mean that Jepp DBs won’t have any RNAV approaches because they are all designated RNP?

I had heard that this was just a problem for Boeing but that may just be hearsay.

Just in the US. The RNAV designation is being kept, US got tired of changing names to suite ICAO’s latest whim and is not going to adopt the new approach naming and will file an ICAO difference. With some 11,000+ charts to update, no thanks. Just because the new kids on the block want to name things their way. We already changed everything once from GPS to RNAV (GPS). RNAV (RNP) is the terminology for the GPS procedures that are Authorization Required and carry a RNP value and use Baro VNAV for vertical guidance. To confuse matters, there are two Nav Specifications, RNP APCH (US will retain approach names of RNAV (GPS)) and RNP AR APCH ( US will retain RNAV (RNP)). The Nav Specifications will be used, but the approach names will remain unchanged, although the RNP equipment block will get adopted, or something equivalent.

Does that mean that the RNAV specification is not technology independent? Presumably an RNAV (GPS) approach cannot be flown using GLONASS, Galileo, DME/DME/DME, DME/DME/INS, or any future technology such as eLoran, because it has GPS in the chart title?