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According to @RalphJ's first comment on my earlier question about ILS approaches in an E-Jet sans pitch trim:

Are you sure that Cat III approaches are allowed in that condition? Typically, Cat III requires basically everything to be operative, unless there is some pretty unusual redundancy in the aircraft. Wouldn't be surprised if one channel out removes Cat III, and then losing the 2nd channel takes out Cat II. Just a guess, but the list of things required for Cat II that aren't needed for Cat III, while not empty, isn't very long in most cases. [Emphasis (both bolding and italics) in first sentence present in original; emphasis in second and last sentences mine.]

"Requires basically everything to be operative" seems like basically the highest bar possible in terms of functional functionality, so that's kinda difficult to square with it being possible for something to not be required for an ILS-III approach despite being required for an ILS-II approach.

What sort of things would an aircraft require to be operative for an ILS-II approach, yet not need in working order for the more-demanding ILS-III approach?

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If approach lights (ALSF-II) go inop when inside the FAF, it's a required go-around Cat II, because at 100' you're looking at them + the runway. Same failure during a Cat III, you can continue, because at a 50' DA the approach lights are all behind you & only the runway lights matter.

Not all OpSpecs may offer that relief, but I've seen one that does. AFAIR, that's what I had in mind with that statement in the other thread.

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  • $\begingroup$ Do you need even the runway lights if you're going Cat IIIb/c, where there's no DH at all and you ride the ILS all the way to touchdown and beyond? $\endgroup$
    – Vikki
    Dec 15, 2021 at 20:20
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    $\begingroup$ IIIB you still have to steer off the runway, so probably required there. IIIC, no idea what those OpSpecs look like. In really, really low vis, the taxiing can be as challenging as the approach, and with less automation to help out. $\endgroup$
    – Ralph J ♦
    Dec 15, 2021 at 23:23

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