Hi Guys,

 

I've been doing plenty of reading on approaches today and I'm currently a little bit in the dark over ILS approaches and their categories.

 

I understand that the category is determined via the equipment both on the ground and in the aircraft and the pilots training, but what I'm not sure about is the practical differences between the three categories (I know there's more than 3 but I'm simplifying). When for example does a Category I approach become a CAT II (assuming that the hardware on the ground and plane is CATIII)?

 

Is it only weather/visibilty influenced or are there other factors??

 

And in terms of flying the actual aircraft, how does a CAT I approach differ from a CAT II approach? What would a pilot do different between a CAT I and CAT II or CATIII?

 

Many Thanks for your help.

 

Dan

The basic difference is minima. In CATI you will fly using your barometric altimeter for minimums, and at the least they will be 200 feet AGSL. Cat II you will use the radio altimeter, and they will be at the least 100 feet AGSL. CATIII has various sub catagories, but the biggest and baddest of them all, CATIIIc, will have no minima, meaning you can in theory fly all the way to the ground without ever seeing the runway (on autoland of course).

 

There are other things, I know I am over simplifiying here. CATII and above you will want to have both right and left radios tuned to the ILS.

So its mainly a Decision Altitude parameter or am i simplifying it too much; The higher the category, the lower and closer to the runway I can fly without actually having to see the runway before having to execute a missed approach?

When for example does a Category I approach become a CAT II (assuming that the hardware on the ground and plane is CATIII)?

 

It at the discretion of captain. Visibility is the main factor. Then it must be a brief for CATII completed.

 

What would a pilot do different between a CAT I and CAT II or CATIII?

 

On CATI and CATII you have to see runway at DA(DH), on CATIII RVR should be not less then specified in charts.

For CATII/III it must special crew certification and for CATIII HGS and/or autoland must be installed.

Shoot for the highest minimum approach you can. So even if you are flying the NGX, into a CATIII runway, but it is a beautiful sunny day, and you can see the runway for miles and miles, dont bother shooting a CATIII (unless you are in need of practice and currency). Only go up to a CAT II if you aniticipate you will need that extra room for seeing the runway, and obviously CATIII is even more so.

 

And there are special approches, like take a lot at approach plates for KLAX, where there is CAT I ILS approaches that have lower minima if you have certain equipment installed, such as the HGS.

Thanks for the advice guys, most helpful as always :-)

I feel this could do with some more info.

 

As correctly stated, the most basic difference is minima, and then equipment both in airplane and on ground, that would allow to use said minima.

 

Any basic ILS is CAT I. There is not much to tell about that.

 

CAT II and CAT III are considered low-visibility procedures. Any airport, that will have these kinds of ILS installed, will have to fulfill some more requirements to be able to use them, for example, sufficient airport and runway lighting, including sufficient backup power, that will be able to kick in in required amount of time.

 

When airport fulfills those requirements, it will have to lay out guidelines for low visibility operations. For example, operation of airport vehicles on active movement areas will be prohibited or limited. Different holding points will be used, to ensure enhanced protection of the ILS signal (ILS signal may twist around large metal objects such as airplanes, this is OK for CAT I as you have time to react, but not OK for CAT II or III), enhanced separation will often be enforced etc.