Difference Between CAT I, CAT II & CAT III....

Fri Jan 21, 2000 2:44 am

What is the difference between CAT I, CAT II & CAT III approaches???

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Joined: Thu Jul 29, 1999 8:12 am

RE: Difference Between CAT I, CAT II & CAT III....

Fri Jan 21, 2000 3:29 am

Well, there is many ways to ask and answer this question. I hope JETPILOT or BUFF will backstop me on this, but here's how I unerstand it.

Basically, the differences are in the amount of redundant equipment on board the aircraft. Cat IIIB requires three separate Flight Management Computer systems with three independant power sources, but it will allow a 0/0 landing.

Cat IIIC restricts you to a minimum Runway Visual Range and a minimum Decision Height, I think 50/50 because the aircraft is only required to have three systems and two power sources.

Cat IIIA is allocated for aircaft who can manuver on the ground in 0/0, and requires the AIRPORT to have a wire inserted into the runway/taxiway so the aircraft can taxi without visibility. Heathrow is the only CAT IIIA I'm aware of.

CAT II and CAT I are just further degredations of the RVR and DH minimums.

I think this sounds right!

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AIM References..

Fri Jan 21, 2000 3:57 am

If you'll follow the link below...'ll take you to the FAA's AIM site. Just scroll down to "ILS Categories" and all the criteria are right there.

ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.

RE: Difference Between CAT I, CAT II & CAT III....

Fri Jan 21, 2000 6:58 am

Thanks much for the info and the reference to the site. It was stunning to learn about the 0/0 CAT IIIC approach, that must be quite an experience to go through such a landing, not seeing anything!!!

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Joined: Wed Dec 22, 1999 10:28 pm

RE: Difference Between CAT I, CAT II & CAT III....

Fri Jan 21, 2000 7:22 am


actually there is no commercially applied CATIIIc Landing in the world. Although the technical standard is defined, the use of it would be so limited, that there is no commercial justification to spend all the money needed to meet the specification.
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RE: Difference Between CAT I, CAT II & CAT III....

Fri Jan 21, 2000 9:53 am

That's a really good link for generic answers to this question, as it comes up fairly frequently. Another way to think of the variable systems is just about everyone can do a Cat I so long as the weather required on the plate exists (except in Canada - later); many can do Cat II, but only with special training and special airport installations; few can do a Cat III primarily because there are very few Cat III installations (more in Europe than North America) but also because training, aircraft and ground equipment (as Jim referred to above) must also meet very high standards.

With regards visibility limits for an approach, in most of the world if the approach plate says 1/2 mile or 2600 RVR, then that is the minimum visibility that must be present to carry out and complete a Cat I ILS. In Canada however, these visibility listings are advisory only. The pilot may do the approach and if the "required visibility reference" is there at DH, then he/she can land. If not, a Go Around must be executed. Only if the visibility goes below 1/4 mile AND the RVR goes below 1200' must the pilot refuse the approach clearance. Oddly enough, the same approach ban (1/4 mile AND 1200 RVR) apply to completing a Cat II approach.

Many ask: Wherein lies the difference? It is a question that is being addressed by the regulators much to the consternation of many Canadian operators. There's a good chance a Cat I ILS will be restricted to 1800 RVR very shortly, and non-precision approaches to 1 mile.

But that's food for another thread!

Best Regards,

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Fri Jan 21, 2000 10:52 am

I think the really neat thing about these low visibility landings are the aircraft that have HUDs (in place of some other equipment). I think that the Dassualt Falcon 900, Bombardier Global Express, 737NG?, and MD-80/MD-90? offer it. It must be neat to hand fly the approach all the way down instead of having the autopilots do it. Incidently, CAT I was the first. CAT II was created for GA airplanes that were more maneuverable and had less inertia than the big jets. Way back, I can remember flying on a friend's Piper Cheyenne into Atlanta, and the conditions were below CAT I. All these big jets (727s, 737s, and DC-9s) were being diverted while we landed, almost having the entire airport to ourselves. OF course, nowadays technology has evened things out.
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Joined: Fri Mar 16, 2007 1:29 pm


Fri Jan 21, 2000 2:13 pm

Also the advent of EVS (Enhanced Vision System) and Synthetic Vision over the next 10 years will revolutionize bad weather flying. This month's Pro Pilot Mag has two good articles on the subject.

for those interested!

Best Regards,


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