CAT I - This category permits pilots to land with a decision height of 200 ft (61 m) and a forward visibility or Runway Visual Range (RVR) of 2400 ft (730 m). Simplex autopilots are sufficient.
CAT II - This category permits pilots to land with a decision height between 200 ft and 100 ft (≈ 30 m) and a RVR of 1000 ft (305 m). Autopilots have a fail passive requirement.
CAT IIIa -This category permits pilots to land with a decision height as low as 50 ft (15 m) and a RVR of 700 ft (213 m). It needs a fail-passive autopilot. There must be only a 10-6 probability of landing outside the prescribed area.
CAT IIIb - As IIIa but with the addition of automatic roll out after touchdown incorporated with the pilot taking control some distance along the runway. This category permits pilots to land with a decision height less than 50 feet or no decision height and a forward visibility of 250 ft (76 m, compare this to aircraft size, some of which are now over 70 m long) or 300 ft (91 m) in the United States. For a landing-without-decision aid, a fail-operational autopilot is needed. For this category some form of runway guidance system is needed: at least fail-passive but it needs to be fail-operational for landing without decision height or for RVR below 375 feet (114 m).
CAT IIIc - As IIIb but without decision height or visibility minimums, also known as "zero-zero". No aircraft is approved for this category. It would necessitate a reliable way for aircraft and ground vehicles to maneuver on the ground without any visual reference.
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