Naming convention for procedures, from the ICAO bible...


9.5.3 Duplicate procedure identification

9.5.3.1 A single letter suffix, starting with the letter z following the radio navigation aid type shall be used if two or more procedures to the same runway cannot be distinguished by the radio navigation aid type only. For example:
VOR y Rwy 20
VOR z Rwy 20
9.5.3.2 The single letter suffix shall be used when:
a) two or more navigation aids of the same type are used to support different approaches to the same runway;
b) two or more missed approaches are associated with a common approach, each approach being identified by a single letter suffix;
c) different approach procedures using the same radio navigation type are provided for different aircraft categories;
d) two or more arrivals are used to a common approach and are published on different charts, each approach being identified by a single letter suffix. If additional radio navigation aids are required for the arrival they shall be specified on the chart's plan view. For example:
ILS y RWY 20 (“CAB VOR Arrival” shown in the plan view)
ILS z RWY 20 (“DNA VOR Arrival” shown in the plan view)
9.5.3.3 As some avionics systems are capable of loading only a single approach, States should ensure that the preferred approach is identified using the z suffix.

Alpha…

That's a great explanation and answers a lot!! Thank you!!

I guess my question at this point would be….

If it's up to us to fly either approach, why did the approach controller advise us, "Well then you flew the correct approach"?

Some have said we would be correct in flying either approach in absence of a specific approach, but to me, what the ATC guy said would imply that had we flown the Y approach procedure, it would have been the "wrong" approach.

So far, I've not found nor have I been shown here any documentation in black and white saying something to the effect, in the absence of a specific procedure, you can fly any procedure (ILS specific in this case) you want".

Maybe there is no such thing. Then again, I may be so blind as to not see something already discussed here. :-)

K

That's because there is no reference because the ILS Z is not the preferred procedure. If it was, it would be written somewhere.

As AC said, "they just say "Cleared ILS approach". They do this for a few reasons but the main one is that they just don't give a rats which one you fly."

Put it another way. On any approach where there is currently a Y and Z option, and ATC hasn't nominated the Y or the Z, would in make any difference to what the aeroplane would do if you flew either? The answer is, of course, no.

Well, technically, it should be. That is part of the Jeppesen naming convention for approaches. The preferred approach should be named the Z approach.

In Australia, no, in other parts of the world, absolutely.

If I were flying into another part of the world where they had two separate approaches for the same runway, and I wasn't told which one to fly, I'd be asking! A close look at the Australian case and yes you can see there is no difference between the charts, so it makes no difference - but that shouldn't be a default position to take, as we are perhaps being a little unique in that regard.

The reason nothing is written about which to fly when told "cleared ILS" is because the controller should be clearing you for the specific approach - the ILS Y, or the ILS Z. That is what normally occurs when there are separate approaches for the same runway (for example, Japan). It is really no different to being cleared for an ILS vs NDB at MEL - they are different approaches. As are the ILS Y and ILS Z. At the moment, with only the DME the difference between the two, it doesn't matter, and we're assuming we're all on the same page. But it is an assumption that doesn't hold with the global reasons for having more than one ILS approach to a runway.

Thank you Comfy Chair!!

After some more research, a colleague of mine gave me a reference I've been looking for CONFIRMING that the Z is indeed the preferred approach!!

It states:

"If a state identifies two or more approach procedures which are based on the same NAVAID, they are shown with a figure or a letter suffixed as, e.g. ”A”, ”B” but ”One”, ”Two”. Where approach procedure designator suffixes are officially published with "A" or "B", "One" or "Two" respectively ”Z”, ”Y” or ”X”, the primary procedure (usually the standard procedure with the lowest minima) will be identified with the suffix "Z.

So…. now the only question I have is again…. in absence of a procedure to fly, which one would you legally be required to fly? I still say it is wise and prudent to ask for clarification. How can you not… especially as the controllers down under can be a bit demanding…. good, but a bit demanding.

Kap

Let's get a couple of things straight here.

1. Are you based in Australia? It would help if we know who we're dealing with.

2. That text DOES NOT APPLY to Australia (not that I can find in our AIP, anyway!). Where'd it come from? Alpha Centauri posted the Australian AIP reference that covers this exact issue.

Nothing in particular! Fly whichever ILS you like; they are both exactly the same (the DME used is irrelevant). The initial replies from ATC indicated that. I suspect that he finally caved in and gave you an answer that wanted to hear.

If we get every man and his dog now asking which one to fly because of this I'll spew... :*

The only reference to it in the Jepp is in Terminal AU-18 (3).

The only time I would imagine a controller clearing you a specific approach (with reference to Sydney) is if you had not specified DME equipment on board your aircraft in the flight notification, obviously then you could only use the PRM - Y with use of GNSS.

Cairns has a LOC-Y RWY 33 and a LOC-Z RWY 33. The LOC-Y is annotated Cat A & B, whereas the LOC-Z is annotated Cat C & D. It's the only approach which comes to mind, where there is a difference between the Y & Z version other than the DME used. The MDA, Viz, Mapt and Mapt speeds are different.

Any time I've been cleared for said approach the phraseology has specified "LOC-Z".

All good unless you fly outside our protected bubble in Oz :D

You know what would remove all this confusing rubbish, this "we have to different" business?

Remove from the AIP that reference that prohibits use of GNSS unless specifically authorised, and replace with

"GNSS may be used in lieu of DME in all instrument procedures. When using GNSS in lieu of DME on an ILS procedure, care must be taken to ensure that the correct reference waypoint is used."

The safety implication for using the wrong reference on an ILS is... a 'failed' GS integrity check and a missed approach. You'd do it once, learn from the error and not do it again.

Sorry for the rant. But sheesh, way to make something easy into 20 extra pages in the AIP!

There are many different Ys and Zs about. Lots of VORs, NDBs eg YPPD 14 VOR, YPKA, YBRM.

There is a small problem with your theory. Where there are 2 ILS approaches available, one will be referenced off a VOR/DME and the other will be referenced off the ILS/DME. Only the plate referencing the VOR/DME will allow GPS in leiu of DME. The other plate does not allow it. The reason is that DME's are not stored in navigation databases as a stand alone point.

The reference waypoint for any NDB/DME or VOR/DME is the location of the NDB or VOR respectively. They are deemed to be co-located and the difference in physical location is small enough not to cause a problem.

The reference location for any ILS installation is the localizer antenna array. This could be several hundred metres from the DME and as such GPS in leiu of DME is not allowed.

The saftey implications of using the wrong reference on an ILS are fairly significant I would have thought...in the case of YSSY ILS 34L the reference point giving you distance would be some 2000m from your intended touchdown point....the figures would be way out. Are you going to trust the GS, or the GPS distance numbers, which won't match those on the plate?

Alphacentauri

Quick question - what determines the -Y and -Z?

For example, most ILS-Z use the ILS DME and the -Y uses the VOR/DME. Brisbane is the other way around - would seem a potential 'gotcha' to me....

UTR

UTR,

Brisbane was one of the first to be published and unfortunately predates the naming convention we have today.

Unfortunately, it's not easy to change under the current data change rules. We can't simply swap them round in one cycle, it would take 3 cycles. The -z would have to go to an -x, then the -y would have to swap to the -z, then the -x would go to a -y....over 3 cycles. Bit confusing and probably safer leaving it as is.

Alpha


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Capn Bloggs,
A question though- In many of the ILSs overseas the difference in the Y,Z etc lies in the missed approach path. I am not sure if this is the case in Syd.
Would it not be prudent for ATC to specify the letter suffix to ensure the correct missed approach is flown?

This is getting really complicated, but the fundamental was the date on the chart, correct?

Guys, it's not that hard.

As has already been pointed out, the only ils in the country that has a difference between -y and -z is Cairns. All others are exactly the same. In the case where they are the same, and no confusion can exist, ATC are not required to specify the suffix. (refer aip)


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The "date issue" may relate to this week's update for CAT II approaches.

Alphacentauri, not quite. (Yes it's not allowed, as i said.)

For any ILS, there are two reference points in the GNSS database. One is the LOC transmitter as you say. The other is the GS transmitter, which is (approximately) where the DME is. It is possible to select the correct reference point such that the GPS mimics the DME, although of course that would be illegal in Aust! (Not always elsewhere...)

Yes, if you picked the wrong waypoint it could be 2000m or more away from the DME. So? The sole purpose of a DME in an ILS installation is for an integrity check. If it's 2000m or more out, the integrity check will fail. The AIP (rightly) provides for this, but there is zero safety implication associated with an erroneous distance readout unless the GS signal is faulty. Hence the need for this integrity check.