Definition
A naming convention used when more than one instrument approach procedure of the same type serves a single runway end. To distinguish them on charts and in clearances, each procedure is identified by a letter suffix in alphabetical order from the end of the alphabet backward — for example, ILS Z RWY 13L, ILS Y RWY 13L, ILS X RWY 13L. The suffix letter has no operational meaning; it is only a label that keeps the procedures from being confused with one another.
Plain English
When the same airport runway has more than one instrument approach of the same kind (say, two ILS approaches to the same runway), each one is given a different letter at the end of its name so pilots and controllers can tell them apart.
Context Anchor
Seen on instrument approach chart titles, such as RNAV (GPS) Z RWY 27 and RNAV (GPS) Y RWY 27.
Why Pilots Care
Correct identification prevents selecting the wrong procedure, which could result in incompatible minimums or missed approach instructions.
Intuition Check
Do not assume the procedures are interchangeable just because they are the same type and go to the same runway. The added letter is a warning that the charts are different procedures with similar names.
Example Sentence 1
Tower cleared us for the ILS Z RWY 13L, so I made sure to load the Z procedure rather than the Y, since they have different initial approach fixes.
Example Sentence 2
The RNAV Z approach was chosen after the pilot determined that the RNAV Y minimums exceeded the aircraft's approach category limits.