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When should we use CAT II or III during landing in bad weather? What we have in our country the standard shows 800m and RVR of 550 m for the ILS, which category procedure should be used if the visibility below or above?

On the other hand nothing tells us if it is CAT II or CAT III inside the charts except at the large airports.

And regarding the DH during the CAT III is it always 50 RA ?

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If both are available for the runway you're using, it's often easiest just to set up the Cat III, because you have the least risk that way of a random cloud between the Cat II DA and the Cat III DA interfering with your approach.

That said, there may be reasons to prefer the slightly higher mins of the Cat II, and those are typically specific to your equipment and your procedures. Perhaps crosswinds, either steady or gusts, make the Cat II more desirable, or if your equipment has a limitation on "XYZ must be operative for Cat III approaches" and that happens to be deferred on MEL for you, then the Cat II is the better (only) option. But generally, if both are available, setting up for the Cat III is usually preferred unless some specific factor drives you to the Cat II.

Separate from this, some runways only have a Cat II approach and not a Cat III available. This is typically driven by runway lighting differences, although other equipment may drive this as well (specifically, a backup Glideslope transmitter, IIRC).

As much as I hesitate to answer "always," I can't think of any Cat IIIA DA that's other than 50' RA, and I can't think of a reason it would be, since you're over the runway itself by that point. Cat II DA's can be slightly above or below 100' RA since the terrain you're over when you reach 100' above the TDZ (on glideslope) may be slightly above or below the runway elevation, but that's a Cat II thing, not a Cat IIIA deal.

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    $\begingroup$ CATIII DH's can vary from 0-50 ft, with CATIIIA generally having a DH of 50 ft and CATIIIB having a DH below 50 ft (generally 0 ft/no DH). Some regulators prohibit approaches without DH, so you'll see CATIIIB-approaches with DH varying between 0-50 ft). $\endgroup$ – Waked May 8 '15 at 20:04
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    $\begingroup$ Other good reasons to choose CAT I -vs- II -vs- III are legal authorization to conduct the approach and aircraft equipment limitations. $\endgroup$ – Lnafziger May 8 '15 at 21:25
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    $\begingroup$ From a historical standpoint, I find it interesting that "choosing" is now involved. In the "good old days" (not really, but it's convenient to remember only the good things) the question was simply whether or not we had the minimums for the equipment we had. The only thing that was going to change from a practical standpoint was the DH, and that was just a verbal call. $\endgroup$ – Terry May 8 '15 at 21:57
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    $\begingroup$ @Waked I was thinking of Cat IIIA, but you're quite correct that IIIB is a different set of considerations. $\endgroup$ – Ralph J May 9 '15 at 1:35
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    $\begingroup$ @Terry When you have a Cat III capable airplane & a Cat III runway, there is the choice of Cat I or II or III, with I being a bit less setup, and III having the lowest mins. Which opens up the question of why/when you'd want Cat II. Obviously, if you lack equipment or certification for Cat II or III, there isn't much of a choice to make there (other than, am I following or breaking the rules today, I suppose). But yes, in a jet capable of all of the above, there IS a choice to make. $\endgroup$ – Ralph J May 9 '15 at 1:44

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