Category II and III operations: General operating rules

Cat II and Cat III ILS approaches let aircraft land in far lower visibility/ceiling than a standard Cat I approach, so § 91.189 layers on extra requirements.

• Crew: a PIC and a second in command, both appropriately authorized and rated.
• Equipment: the controlling pilot's panel must have the right instrumentation for the flight-guidance system, and each required ground component and the related airborne equipment must be installed and operating.
• Authorized DA/DH: the decision altitude/height you use is the highest of the one published, the one prescribed for the PIC, and the one the aircraft is equipped for.
• Continuing below DH: you may not descend below the authorized decision height unless you're positioned for a normal landing in the touchdown zone and at least one specified visual reference (approach light system, threshold/its markings/lights, or touchdown-zone markings/lights) is distinctly visible — with the approach-lights-only limit at 100 ft above TDZE. If those conditions aren't met, execute an immediate missed approach.

Cat III "no-DH" landings require operating under a specific letter of authorization. Air-carrier/commercial operators (Parts 121/125/129/135 and subpart K) instead follow their ops/management specifications.

Summary: Cat II/III low-visibility ILS approaches require a two-pilot crew with proper authorizations, all required ground and airborne equipment operating, use of the highest applicable DA/DH, and specific visual references at the decision height — otherwise an immediate missed approach; commercial operators follow their ops specs.