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CatIII A, B, And C

Sat Dec 13, 2003 3:48 am

I'm not as smart as some... can someone concisely detail the differences in the 3 Category III classifications?

All this talk of fog in Delhi has got me thinking... is it possible to build a truly all-weather airport?

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RE: CatIII A, B, And C

Sat Dec 13, 2003 3:56 am

CATIIIc is 0/0 vis. the others have a slightly higher vis limit, i dont recall the exact nubmers but they were posted here a while ago.
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RE: CatIII A, B, And C

Sat Dec 13, 2003 4:18 am

I searched but didn't find it.

Basically, Delhi just got a Cat IIIa system working on one or two runways, but should have gone for Cat IIIc.

Is there a difference in the onboard avionics between the three, or just the certification?

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RE: CatIII A, B, And C

Sat Dec 13, 2003 4:23 am

some copy+paste from

ILS Categories -
Cat.I - 200 feet DH - 2,400 feet (or 1,800 feet) RVR
Metric: 800 metres of 550 meters RVR...
Cat II Restricted - 150 feet DH - 1,600 feet RVR
metric: 500 metres RVR
Cat II - 100 feet DH - 1,200 feet RVR
Metric: 350 metres RVR
Cat.IIIa - 700 feet RVR - no DH (alert height generally 50 feet)
Metric: 250 meters RVR
Cat.IIIb - 600 feet RVR - no DH (alert height generally 35 feet)
Metric: 175 metres RVR
Cat.IIIc - zero ceiling, zero visibility - "blind" landing...

RVR is Runway Visual Range, basically a distance in feet that the pilot can expect to see forward in his airplane.

The ILS equipment at the airport must be certified for it, as well as aircraft type (actually individual aircraft) and crew have to be certified.

Alert Height (AH) is not like a Decision Height (DH) -
At "DH" (obtained from radio altimeter for Cat.II) you have to make a DECISION to land or go-around...
In Cat.III operations, there is no DH... but you have to make a decision to land based on "what you see"... pilots find the DH "decision" very convenient for Cat.II, but did not exist for Cat.III...
So in "pratical operations", the AH is used somewhat like a DH, but is not regulatory. In other terms, we expect to "see the runway" at that point... which is about 50 feet radio altimeter, just about where the runway threshold is located, in Cat.IIIa minimums. In Cat.IIIb, happens at about 35 feet...
Many 747 are equipped for Cat.IIIa operations (not Cat.IIIb), although most of the "Classic" 747s (with 3 autopilot channels) have the LRCU that is required for Cat.IIIb... LRCU = landing roll control unit... keeps the nose wheel on the center line, using the localizer...

i think that sums it all up perfectly. good thing we have such knowledgeable people on the boards here like those in the thread i copied it from.
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RE: CatIII A, B, And C

Thu Dec 18, 2003 11:46 am

The very first airplane certified for Cat IIIa was the L-1011, and the 747 and DC-10 never came stock from the factory with that level of certification (I read that somewhere, and I'm juessing it refered to the -100, I'm not sure about the -200/-300 and obviously not the 744)

No airliner has ever been certified for Cat IIIc, because you would need some way to taxi the airplane in the literally zero visibility, so Delhi would for now be wasting their money for Cat IIIc. Especially since embedding something in the pavement for the airliners to follow would be expensive.
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RE: CatIII A, B, And C

Sat Dec 20, 2003 12:36 am

Actually, the wide use of GPS and the increasing availability of ground-based differential GPS transmitters could make ICAO Category IIIc operations possible at most airports. Such a GPS setup could allow an airliner to know its position within one feet (less than a third of a meter!).

I can see by 2010 many airports installing special transmitters that generate a signal for both the US-based GPS system and the European based Galileo system that will allow essentially totally-blind takeoffs, landings and ground operations.
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RE: CatIII A, B, And C

Sat Dec 20, 2003 1:19 am

There is one serious problem with zero-visibility landings and/or ground ops. In the event of an accident or problem there would surely be serious issues with the ability of fire or rescue crews locating debris, pax and even the entire plane !

Just my 2 cents.

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RE: CatIII A, B, And C

Sat Dec 20, 2003 3:43 pm

In Cat.III operations, there is no DH...

AA's B738 CAT-III ops utilize a 50 foot decision height.
*NO CARRIER* -- A Naval Aviator's worst nightmare!
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RE: CatIII A, B, And C

Sat Dec 20, 2003 11:52 pm

Some good answers.

Aditional info:

CatIIIB must have rollout guidance, meaning the aircraft stays aligned on the runway after touchdown, if the aircraft drifts off the centerline after landing, that's considered a CatIIIA landing. At least that's how we do it at FedEx.

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