Definition
An additional letter appended to a standard instrument procedure designator to distinguish between two or more procedures of the same type serving the same runway or to indicate a specific variation of that procedure. Supplementary letters are added in alphabetical sequence (Y, Z, etc.) when more than one procedure of the same type exists for the same runway, and are used in published procedure titles such as 'ILS Y RWY 18' and 'ILS Z RWY 18.'
Plain English
An extra letter added to the name of an instrument approach when more than one approach of the same kind serves the same runway, so each one has its own unique title.
Context Anchor
Seen when learning how published airway and route names are built on instrument en route charts and in FAA route descriptions.
Derivation
Supplementary' comes from the Latin 'supplere,' meaning to fill up or complete. The letter 'supplements' the standard designator by completing it with what is needed to keep one procedure distinct from another.
Why Pilots Care
Ensures the pilot selects and briefs the exact procedure intended by ATC or the published chart rather than confusing it with a nearly identical alternative.
Intuition Check
Do not read a supplementary letter as a decorative or optional extra. In this FAA context, it has an assigned meaning, and its position before or after the basic route name matters.
Example Sentence 1
Cleared for the ILS Z RWY 27 approach, the pilot confirmed she had the correct chart, since an ILS Y RWY 27 also existed at the airport.
Example Sentence 2
When two ILS procedures serve the same runway, the supplementary letter Y or Z appears after ILS to keep the procedures separate.