I will append a short handout that I developed when I was the program manager for Cat II/III at a recently failed airline. This document speaks only to the FAA definitions and viewpoint; however I believe you will find the concepts mostly consistent with other authorities. I can say that, as a developmental program manager and required document author, an in depth understanding of this material is remarkably elusive, even amongst the authorities. That is probably because of the use of identical terms with some different definitions between JAA, ICAO and FAA.
Most of the material below is extracted from FAA Advisory Circular 120-28D, which was referenced in a post above. A Google search will find it immediately. The material also relies on FAA Operations Specification C060, which specifies operator requirements for Cat III. The generic C060 is tailored by the operator in his program application, and yields the minima which can then be published by Jeppesen in tailored approach charts.
It is important to keep separate the aircraft certification, which relies heavily on AC 120-29A and 120-28D, and operator certification. The latter, in the States, relies heavily on AC 120-29A and 120-28D bust is specified in the Operations Specifications.
Anyway, I hope this is of some use.
United States Lower Landing Minimums Approval Demystified
Category II Operations
CAT II operations are precision approach and landing operations conducted with a DH of less than 200 feet (60 meters) but not less than 100 feet (30 meters), and a RVR of not less than 1200 feet (350 meters).
Certain foreign authorities authorize and publish CAT II approaches to minimums as low as RVR 300 meters (1000 feet). The FAA considers these approaches to require CAT III equipment and procedures.
Category III Operations
CAT III operations are separated into three separate subcategories:
• CAT IIIa
• CAT IIIb
• CAT IIIc
CAT IIIa is a precision approach and landing operation with an RVR of not less than 700 feet (200 meters) without a DH, or with a DH of less than 100 feet (30 meters), or an alert height (AH) of 100 feet (30 meters) or less.
CAT IIIb is a precision approach and landing operation with an RVR of less than 700 feet (200 meters) but not less than 150 feet (50 meters) and a DH of 50 feet (15 meters) or less, or an AH of 50 feet (15 meters) or less.
CAT IIIc is a precision approach and operation landing without a DH and without RVR limitations (zero-zero). CAT IIIc operations are not currently authorized.
Fail-Passive Operations
The term “fail-passive” means that any single failure of an airborne component will not result in a significant deviation in flight path or altitude. However, a fail-passive system is not capable of completing the approach, flare, landing and rollout in the event of a single component failure. Flight crew annunciation of such a failure is required, and a missed approach is the most likely best course of action.
Fail-Operational Operations
The term “fail-operational” means that in the event of a single failure of an airborne component, the system remains capable of completing the approach, flare, landing and rollout with the remaining components. A missed approach is not required below a specified point if such a single failure should occur.
The Use of CAT IIIA and IIIB
Fail-passive systems used to be restricted to an RVR of 700 feet, which corresponded to CAT IIIA. This is no longer true. Due to harmonization with the JAA, the FAA now permits operators to use fail-passive systems to an RVR of 600 feet. Therefore, there is no longer any correspondence between CAT IIIA and fail-passive systems. This has reduced the significance of the terminology. US runways are certified as either CAT I, CAT II or CAT III. The A and B terms are not used in the FAA runway certification. They still appear in generic Jeppesen charts.
What really matters is whether the aircraft system is fail-passive or fail-operational.
Category II and III Decision Heights
Category II approaches utilize a decision height in the same manner as a Category I approach. The visual references required at DH are intended to provide the pilot with adequate information to manually maneuver the aircraft to a normal landing.
CAT III approaches flown with fail-passive systems utilize a DH in a different manner than CAT I or CAT II approaches. Unless a HUD system is being utilized, this type of approach is always flown to an auto-landing. However, suitable redundancy is not present in the auto-flight system when it is functioning in a fail-passive capacity. Therefore, visual references are required for the pilot to verify that the auto-flight system is in fact guiding the aircraft to a safe landing.
Alert Height
Category III approaches to minimums less than RVR 600 are always conducted under the fail-operational concept. Fail-operational systems provide suitable redundancy to remove the requirement for the acquisition of visual references prior to landing. Instead, an Alert Height (AH) is specified. An Alert Height is the height above a runway, based on characteristics of the airplane and its fail operational system, above which the approach must be discontinued and a missed approach initiated if a failure has occurred in one of the redundant parts of the flight control or related aircraft systems, or if a failure has occurred in any one of the relevant ground systems.
Some authorities, however, may specify a decision height even for certain fail-operational approaches.
RVR Requirements
CAT III approaches flown using a fail-operational system and an alert height do not need visual references for the landing or the rollout. Therefore, any one of the three required transmissometers may be temporarily inoperative.
CAT III approaches flown using a fail-passive system, with either fail passive rollout guidance or no rollout guidance, require visual references for both touchdown and rollout. Therefore, a TDZ RVR, MID RVR and R/O RVR are required, with the R/O being advisory.
System Degradation
There is more to system requirements than just the number of autopilots. For example, the 757/767 will display LAND 2 when one of the three radio altimeters fails. In another case, the deferral, under the MEL, of the rudder pedal nose wheel steering will downgrade the aircraft because without it, there is no means for automatic rollout control. However, LAND 3 will still be displayed.
It is important to be familiar with all type specific requirements for fail-passive and fail-operational systems.