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There are cases with instrument approaches where there will be multiple approaches to the same runway, in many cases uses some of the same nav aids and positions, but differentiated using alpha characters.

One example are the ILS Y or LOC Y RWY3 and ILS Z or LOC Z RWY3 approaches at Raleigh Exec Jetport at Sanford-Lee County in Sanford, North Carolina. These approaches both use the LOC/DME I-TTA there, and some of the same positions.

It seems that the differences are subtle in the overall procedure on things like the missed approach procedure and how positions are identified, but I'd love a textbook answer that explains it more thoroughly.

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ILS Y and Z (click either image for higher resolution).

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    $\begingroup$ In this case it seems the difference is the minimum altitude at AMIRS (due to the surrounding obstacles). Y approach which requires GPS and radar contact has lower minimums. $\endgroup$ – mins Nov 20 '15 at 7:21
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The naming convention is explained in Chapter 4 of the FAA's Instrument Procedures Handbook:

When two or more straight-in approaches with the same type of guidance exist for a runway, a letter suffix is added to the title of the approach so that it can be more easily identified. These approach charts start with the letter Z and continue in reverse alphabetical order.

The TERPS explanation (section 161) is a little more specific:

d. Duplicate identification 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.”

The Handbook gives the example of two approaches at Chicago Midway (KMDW), which has the RNAV (GPS) Z RWY 13C and RNAV (RNP) Y RWY 13C approaches. They're both straight-in GPS approaches to the same runway, but one requires RNP and the other doesn't.

As the question itself and this comment note, there are plenty of other possible differences between approaches to the same runway; RNP is just one example. Having multiple approaches allows an airport to accommodate aircraft with different equipment and capabilities or, as section 140 of the TERPS puts it, to "gain an operational advantage":

To permit use by aircraft with limited navigational equipment, an IFP [Instrument Flight Procedure] should be formulated using a single navigation source whenever possible. The use of multiple navigation sources of the same or different types may be permitted to gain an operational advantage

I have no idea how to find out why a specific runway has multiple approaches or how the approaches were designed; presumably it's something that the FAA and the airport operator (and maybe even the airlines) work out based on local runway use and traffic requirements.

In the example in the question, one plate requires GPS, the other DME or radar. So a non-GPS equipped aircraft can still use the ILS, but with different ways of getting to it, different minima, etc.

Note that there's a similar naming convention for circling approaches:

Approaches that do not have straight-in landing minimums are identified by the type of approach followed by a letter. [...] As can be seen from the example, the first approach of this type created at the airport is labeled with the letter A, and the lettering continues in alphabetical order.

Their example is Rogue Valley International - Medford (KMFR), which has the following circling approaches:

  • VOR-A
  • LOC/DME BC-B
  • VOR/DME-C
  • RNAV (GPS)-D

There's no explanation that I could find in the Handbook or TERPS to explain why circling approaches must have a letter even if the navigation systems are different. But as @RalphJ mentioned in a comment, this provides a clear way to identify multiple circling approaches using the same guidance, e.g. a VOR-A approach from the north and a VOR-B one from the south.

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  • $\begingroup$ Another reason you might have a "Y" and a "Z" approach could be the use of different navaids to define fixes on the approach or missed approach. For instance, one version of the approach might define the FAF based on an NDB, while another version defines it as a DME fix. An aircraft equipped with one but not the other could fly whichever version it could use. Alternatively, missed approach holding or holding-in-lieu-of: holding at a VORTAC works if equipped with a VOR, but can not be done if only TACAN equipped. $\endgroup$ – Ralph J Nov 21 '15 at 1:25
  • $\begingroup$ While this says when they use multiple letters, it doesn't really explain why which is the question (other than the RNP / non-RNP example, which doesn't apply to the situation they present). $\endgroup$ – Lnafziger Nov 22 '15 at 16:19
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    $\begingroup$ @Lnafziger Isn't that exactly what the quote explains? If there are multiple approaches to the same runway that use the same guidance method then there needs to be some way to tell them apart. $\endgroup$ – Pondlife Nov 22 '15 at 16:22
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    $\begingroup$ As to 'why circling (only) approaches must have a letter,' it's because they don't have a runway ("VOR22") and they need something more than "VOR" as an approach title, so they get a letter. Each circling (only) approach gets a different letter, so you could have, say, a VOR-A approaching from the north, a VOR-B from the south, and then a VOR/DME-C from the north, etc. The ILS-X/Y/Z Rwy ## or RNAV-X/Y/Z Rwy ## differentiate different straight-in approaches, while the VOR-A/B/C etc are for circling-only approaches. $\endgroup$ – Ralph J Nov 23 '15 at 1:08

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