Thursday, September 30, 2010

Y or Z?

A CJ2 flight into Jackson Hole, WY brought up an issue that confuses many instrument pilots- what does an X, Y or Z in the approach title signify?  Examples are the KJAC ILS or LOC Z Rwy 19, or the KSFO RNAV (GPS) Y Rwy 19L.

The answer is fairly simple, but like many answers, it only begs further questions be asked.  When there exists more than one approach of a given type to a specific runway, the approaches will be titled uniquely by including Z, Y, X, etc… in the title.  Thus the presence of an RNAV X Rwy 9 approach at an airport implies there are at least three RNAV approaches to runway 9.  If only two exist, they will be Z and Y; the convention starts at Z and works backwards into the alphabet.  Letters at the end of the alphabet are used so there is no confusion with the labeling for circling-only approaches, which are named with letters starting with A, moving into the alphabet  (The first circling-only VOR approach at an airport is titled VOR-A, the next is VOR-B, etc…).

So why would there be more than one approach of a given type to the same runway? Wouldn’t the FAA just make one approach as “good” as possible?  I’ve identified three broad reasons this isn’t always a viable option- for this post we’ll look at the reason Jackson has two ILSs.

Here, the difference between the Y and Z can be subtle to spot.  Looking at the profile and plan view of the charts, the Y and Z ILS seem to be identical procedures.  A careful look shows the ILS minimums are different, though. The Z approach brings a plane down to 200’ above TDZE, and requires ¾ mile visibility, while the Y only goes down to 612’ above TDZE, and needs 2 miles visibility.

So why would anyone shoot the Y? The answer lies in the notes.  On the Z approach is stated “Missed approach obstructions require a minimum climb gradient of 245 feet per NM to 11200; if unable to meet rate of climb, see ILS or LOC Y RWY 19.”

The standard gradient for a climb that is part of an IFR procedure is 200’ per NM.  If this gradient will not ensure adequate separation from terrain, a higher gradient may be required.  This is commonly seen on departure procedures, but as an aircraft starts a missed approach from a few hundred feet up, and from a point often before the runway starts, it’s uncommon to see a missed approach that requires a non-standard gradient.


 
By starting the missed approach 412’ higher, and almost 1.5 miles further from the runway, an aircraft conducting the Y approach will maintain adequate separation even if only climbing 200’/ NM.

1 comment:

  1. Thanks for this clarification. I'm going into BTV tomorrow and it has Z and Y for 15. This is the second time I've seen these destinations and it was really bugging me that I didn't have a clear answer. Now I've got one. (I'll read Part II next).

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