What is the meaning of ILS Z and ILS Y on terminal procedure charts? Quentin Moore

For some bizarre reason, the aviation oganisations concerned with making the rules for such things as ILS approach naming conventions decided to start at the end of the alphabet and work backwards, which is why you have things like Z and Y, rather than A, B, C, etc. That means Z is usually the primary approach, Y is the next one, then X etc, so in normal circumstances, you'd use the Z approach.What they actually are, is different procedures to the same runway which may or may not use the same ground equipment and may include a lot of the same navigational approach points, but perhaps at different heights, or occasionally they can simply have a different missed approach procedure because of terrain, or for aircraft types that do not climb so well, or are noisy on their climb. Another reason for different ILS procedures to the same runway will be to allow different descent rates on different aircraft types. If you have the Aerodrome Booklet for a particular airport, you'll see that it will usually clarify the reasons for such differences in the first few pages of its notes.Sometimes there will be an equipment-related reason, because to legally allow ILS approaches of various categories, there has to be a minimum amount of equipment serviceable at the airport, plus sufficient back up systems too if it is a very high category approach, so there are occasions where if an airport is servicing equipment, it may not be able to offer its normal full range of ILS approach categories. When that happens, they may prefer to keep aircraft further above obstacles and terrain peaks, by having them come down on a different profile, because some signal equipment can be adversely affected by the terrain if one or more of its gizmos are not available, which can occasionally lead to short, but rather alarming erroneous deflections on the needles; even the radar altimeter can occasionally freak out over some terrain on an approach.As noted, different descent rates will of course mean that the aircraft is at different heights above the ground on the way in to the airport, and beyond safety considerations if the airport temporarily has reduced equipment owing to maintenance, that can also be useful if there are perhaps noise abatement rules at night around the airport. So you may find that a noisy military jet, or an old 737-200 with turbojet engines will be mandated to fly an approach that has a different descent profile, so there is likely to be less throttle required on the way in, thus making its approach quieter. In short, there can be a lot of different reasons for using Z, Y and X etc.Since a different descent rate is not immediately apparent on a top-down diagram for an approach chart (unless there is a profile view on the chart too), it is helpful to make it very obvious that there are differences, by naming the approach with an additional identifying letter.Al

Adding to Al's (great) response; if an approach procedure is Z, Y or X or whatever, it means the final approach course is within 35

Thank you very much for your explanations. Never a day goes by I don't learn something new! Quentin Moore

At least in the US / FAA system the "Z", "Y" convention was designed to allow FMS or other computer database systems deal with multiple approaches of the same description (ie, two ILS 35 approaches). In the FAA world, the suffix "A", "B", "C", is given to approaches of any type which are "circle to land" that is, not assigned to any particular landing runway (thus always non-precision). In FAA system, the letters are assigned uniquely for each CITY (not airport) regardless of type of main nav aid the approach is based on. So you might have a VOR A approach to airport 1 and and an NDB B approach to airport 2 in the same city. To avoid confusion they decided to go from "Z" down when there are multiple approaches of the same type/runway.scott s..