I'm wondering how is the right choose when an runway has ILS x, Y, Z. What should I consider in order to pick one of those for landing?
When an airport has multiple ILS approaches to the same runway (usually labelled ILS-Z, ILS-Y, ILS-X, ...), they all end up using the same ILS for that runway, but they differ in how you fly the approach until final.
Consider e.g. KTUP, which has an ILS-Y and ILS-Z approach to runway 36. Here is the chart for the ILS-Y:
This ILS approach starts at an RNAV waypoint GAGYE and therefore lists "RNAV 1-GSP required" as a requirement. The ILS-Z approach looks completely different:
This ILS approach has two IAFs: CEVSI on the OTB 12-DME arc and the VERON NDB. It therefore lists "ADF or DME required" as a requirement.
So, which one should you choose?
- During flight planning, you should plan to use the approach that best fits your aircraft's equipment. If your aircraft does not have ADF or DME installed, ILS-Z in the example above would not be possible. On the other hand, if your aircraft is not RNAV approved, you cannot fly the ILS-Y. If you can do both, then file a route that ends up at whatever IAF works best for you.
- During the flight, it should be clear which approach you should fly based on which waypoint you end up at. In the example above, if had filed a route to the VERON NDB, then you would fly the ILS-Z. If you filed a route to a beginning of a STAR, then you fly the matching approach from the last waypoint of the STAR (KTUP in my example does not have STARs, but other airports with multiple ILS approaches might). This would usually be the RNAV variant, since most STARs are RNAV anyway.
- If still multiple approaches are possible, ATC should clear you for one in particular, e.g. via "Cessna 12345, cleared ILS Yankee approach runway 36".
ATC will assign the correct approach.
There are many different reasons some airports have multiple different ILS procedures for the same runway:
- Some have separate procedured depending on whether ATC will provide vectors to final approach or the pilot needs to fly a procedure turn
- Some have different procedures because of terrain/obstacle clearance - different aircraft with different performance may need to follow different routes to stay clear of terrain
- Some have different missed approach procedures depending on the traffic flow in the terminal area
- Some have almost identical procedures, where the only different is which DME-station is used for reference
- Some even have two separate glideslopes with different angles/aiming points due to noise abatement (Frankfurt is an example)
Some procedures are limited to certain times of day, aircraft type or operator approval - in which case it is probably pretty obvious which one you should fly. In other cases (for example identical procedures with different DME-stations) which procedure to fly will depend on the actual status of ground equipment.
In any case, ATC will tell you which procedure to fly. When a runway has more than one ILS procedure (typically designated X, Y, Z) ATC will tell you to expect and clear you for that specific approach. For example: Expect ILS-zulu approach runway 27 and Cleared ILS-yankee approach runway 05