B777 Cat 2 & 3 Approaches

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Why is it that an Alert Height is called on a Cat3 approach and not on a Cat2 approach?

Why does one need a 90m visual segment from the cockpit, yet a RVR of 150m is required for take-off? Surely the 150m requirement overides the requirement of 90m.

Thanks in advance
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[kijangnim]

Greetings,
Alert height is to do with fail operational status enabling to continue a CAT III approach, however the approach is degraded form CAT III B to CAT III A with auto land capability, and lower minima (50 FT 200 Meters which can be viewed as an improved CAT II) providing that the weather is equal to or greater than the above.
this should reflected in the AFM.
For the 90 meters it is missunderstood by a lot of operators, as long as you have your LVP takeoff published minima you dont need anything, but sometimes fog is patchy then Pilot viz assesment of 4 center line light (90 Meters) is to be done. :ok: