Hi. I wanted to specify, but then the browser crashed on me (twice) and I had to hurry to finish that post before having to leave to catch my bus.You may notice, that most everywhere there is a CAT II or III approach available, there will also be a CAT I ILS approach published. That is, because of when Low Visibility Procedures are used, there are constraints on some things, like usage of holding points, clearance areas and so. You might have noticed that CAT II or CATIII holding points are marked further away. These and other limitations mean, that when LVP ops are in place, the capacity of the runway is lower. Therefore CAT I approaches are generally used, when weather allows. With that, you need higher minima even on II/III equipped runways, so you can for ex. recognize if ILS signal is corrupted by, say, departing aircraft, or aircraft on a holding point too close to the centerline.

thank you very much, so what your saying is that CATII AND III uses for low visibilty, and ils approach CATI uses when the weather is good? thanksss!!

Pretty much. Of course as long as you are in LVP ops (low visibility procedures) you might as well use highest rating available on this or that runway.CAT I is used as long as practical with the weather, might not be exacly good, but if you have, say, one mile of ground visibility and 500ft cloud cover (otherwise dreadful weather to, say, drive in, usually) you might want to stick to the CAT I.

thanks!(: