Categories of the ILS
The ILS comes in flavors beyond the CAT I version we normally fly. Knowing how they work and how they affect us can increase our level of operational safety.
At the risk of getting the metaphorical "hook" for expressing the thought, we are about to engage in herding CATs. Most instrument pilots are lucky and only have to deal with one category of ILS approach during their lives. The Category I, or "CAT I" is usually the only kind of instrument landing system approach minima you can fly down to as a single pilot operator in the IFR world.
It is easy to get caught up in the minutia of what makes a Cat I different from a Cat II and so on, but there is only one really basic thing you need to remember about them: The category of ILS approach refers to how low a visibility you can legally fly your airplane down to during the approach.
Specialized Training Required
Any ILS category other than CAT I requires specialized crew initial and recurrent training as well as special operating manuals and rules for the owner operator of the aircraft being used. Now that we've split the question of CATs into two parts lets split it into four more. These four things determine if you can fly the CAT II or CAT III approach, where you can fly it and in what sort of aircraft/equipment. The FARs for this CAT question are designed to tell you what kind of flight crew you need to fly the approach, the kind of airborne equipment you need and what things and equipment you need on the ground in order to do one.
Air Traffic Control also has a rule book telling the controllers how to handle these different kinds of approaches and low visibility operations.
Even though you may not ever wish to jump through the hoops to get CAT II certification for your airplane and yourself (it can be done), it is still handy to know what CAT II and CAT III approaches are and what is going on when those approaches are taking place.
For example, do you remember those ILS hold short lines you sometimes find at airports near the end of the runway? ILS critical areas become even more critical during CAT II and CAT III approaches and some airports even have specific and different hold short lines when these approaches are being done.
This cat is the one you are most familiar with and it may be the only one you are qualified to fly. A CAT I approach is your basic, run of the mill, "two hundred and a half" ILS approach. Minimums can be higher for this approach, but not lower. It can be hand flown, meaning no autopilot is required and it can be done with the onboard equipment found on most General Aviation instrument qualified aircraft.
Even though a lot of people talk about ILS minimums using a ceiling and say things like "two hundred feet and a half mile," you smugly remember that ILS minima are predicated on visibility, more specifically RVR (runway visual range).
As a general rule, figure that you can fly a CAT I if the RVR is at or above twenty four hundred. Also, remember that the CAT I approach is the lowest approach you can fly without at least one operating RVR unit dedicated to your landing runway.
Where Fools Tread
Unlike airline and air taxi operators who have to have the weather at or above legal minimums before they can begin the approach, you can legally go down for a look in even the most rotten conditions. (But if those who fly for a living and take checkrides every six months can't do it, should you really be there?)
On a CAT I approach, the missed approach point is normally based on a decision height that is, in turn, based on a barometric altitude. Radio altimeters can be used as a back-up for situational awareness, but the actual minimums are based on a barometric altitude.
A good way to think about Category II approaches is that, in general, you need two of everything. You need two fully trained and certificated pilots, you need two full sets of flight instruments and you need two independent ILS receivers.
(FAR 91.193 allows the FAA to make some exceptions for CAT II ops, so a single pilot flying a Category A [less than 91 knots on the approach] airplane can get CAT II approval for nonrevenue operations.)
CAT II approaches can go down to 1200 RVR. An autopilot coupled to the ILS must be used. There are specific procedures required of the flight crew including verbal call outs. A CAT II and above approach is more like theatre than anything else. The higher category ILS approaches are all about giving and hearing the proper call-outs at the proper time.
The minimums (which on any ILS approach also means the missed approach point) on a CAT II ILS are predicated on a radio altimeter altitude or RA. Minimums are usually around one hundred feet above touchdown.
A CAT II or CAT III approach is legally the captain's approach, meaning that he or she is in charge of conducting the approach and making the decision to land or miss. The copilot has quite a few responsibilities on these approaches as well and actually is the harder working person on the crew when a CAT II or CAT III is being shot.
This is the lowest approach where you will find a "decision altitude" (DA) meaning that the captain has a decision to make about whether or not to continue to a landing. On the CAT III approaches the term changes to "alert altitudes" meaning that if everything is going normally the airplane is merely telling you (or alerting you) that it is going to go ahead and land itself even though you quite literally cannot see the runway.
Alert altitudes are not published on approach charts but are normally established by the operator's Flight Operations Manuals at 50 feet RA.
A Category III approach can take you down to an RVR as low as 300 feet. That's right, RVR 300, baby! Can it go any lower than that?
Well, actually, yes. It can go down to zero and still be legal. There are three kinds of Category III approaches, CAT IIIa, CAT IIIb and CAT IIIc. When you get down to flying a bird capable of CAT IIIc it can actually land, stop and then taxi in with zero visibility.
A CAT III boils down to this: The airplane does everything including the landing and roll-out and you just watch and feel your face twitch a bit every now and then.
CAT IIIa means the airplane can auto land and have auto throttles. CAT IIIa minimums are normally RVR 700.
CAT IIIb mans that the airplane can do everything a CAT IIIa airplane can do plus it has the ability to stop itself after landing on the centerline of the runway. I have done more than a few of these in the Boeing 767 and 777. Usually my biggest problem during the entire process is figuring out a way to taxi to the gate. I have had to call for a follow-me vehicle to get to the gate.
Zero Visibility? Not Happening
CAT IIIc means an actual zero visibility landing because the airplane not only can land and stop itself, but can also automatically taxi in to parking. These approaches, as far as I know, are listed as "Not Authorized" or "NA" on the approach plates because there is no way for emergency people or equipment to find an aircraft in distress if the weather is zero-zero.
This system of ILS categories is based on fairly old technology and will soon make way for the newest of the new GPS low approaches but they continue to be the standard around the world.
There is a lot more to the world of CAT II and CAT III approaches, such as what you should see on break-out, standard call-outs, etc, but that information gets fairly aircraft and operator specific and will be there for you to learn when and if you get CAT II or CAT III certified.
Relax in the knowledge that even some airlines refuse to use this stuff. They have figured out that the very few days when the weather is below two hundred and a half aren't worth the trouble and expense of doing the CAT II and CAT III thing.
I think they have a good point.
Kevin Garrison retired as a captain from a major airline, but isn't sure why; and also was the author of the "CEO of the Cockpit" column on AVweb.
This article originally appeared in the June 2012 issue of IFR Refresher magazine.
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