We discontinued Cat.IIIa operations in 2002.
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It required Autopilots A, B and C operating and Autoland.
"Decision" to land or missed approach was made at "Alert Height" 50 ft.
All I recall, is that we needed 200 (or 250?) meters RVR, there was no DH.
The planes were equipped with LRCU as required for Cat.IIIb., but no 747 Classic was ever Cat.IIIb.
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And for Cat.II requires 2 autopilots to operate, for 100' DH.
If only 1 autopilot operating, our Cat.II increased and becomes 150' DH.
Happy contrails