071 Operational Procedures topic guide
Low Visibility Operations: CAT II and III
Low visibility operations exist because an approach flown in poor visibility asks more of the crew, the aeroplane, and the aerodrome than a normal one does, and each of the three has to be upgraded together. CAT I, II and III describe successively lower decision heights and runway visual ranges, and each step down brings its own package of crew training, aircraft equipment, and aerodrome protection.
The aerodrome side of that package is called low visibility procedures, and its purpose is to protect the localiser-sensitive area, the ground zone around the localiser aerial that reflecting vehicles or aircraft would otherwise disturb. Understanding why that protection matters, rather than only memorising the numbers, is what lets a student answer a question that changes the category or the equipment rather than the exact figures learned by rote.
What actually changes between the categories
CAT I is the baseline instrument approach: a decision height is flown, and if the required visual reference is not in sight at that height, the crew goes around. CAT II lowers the decision height and the required runway visual range further, which in turn demands a more accurate autopilot or flight director coupling, a higher standard of ground lighting, and specific crew training and currency.
CAT III removes the decision height concept altogether, or reduces it to a very low value, and instead relies on the aircraft's automatic landing and, for the more advanced variants, automatic rollout systems to bring the aeroplane down and keep it on the centreline with little or no external visual reference. The commonly quoted bands are CAT I at a decision height of 200 ft or more with a runway visual range of 550 m or more, CAT II between 100 ft and 200 ft decision height with a runway visual range of 300 m or more, and CAT III below 100 ft or with no decision height at all, with runway visual range values quoted as low as 75 m for the most capable systems.
- CAT I: decision height 200 ft or above, RVR 550 m or above (commonly quoted)
- CAT II: decision height 100 ft to 200 ft, RVR 300 m or above (commonly quoted)
- CAT III: decision height below 100 ft or none, RVR down to values as low as 75 m depending on the system
Why the aerodrome has to change too
An ILS localiser signal can be distorted by a vehicle or aircraft sitting inside the area close to the localiser and glideslope aerials, an effect that matters far more when the crew and the automation are relying on that signal down to a very low height with little or no outside visual cross-check. Low visibility procedures protect that sensitive area by holding traffic further back, restricting vehicle movements, and applying stricter separation once the reported visibility drops below a defined trigger.
Low visibility take-off works on a related idea from the opposite direction: with visibility too poor for the crew to see and react to a rejected take-off in the usual way, or to maintain the aeroplane on the runway centreline visually, the aerodrome and the aeroplane both need the equivalent protection, so a minimum runway visual range value is set for take-off even though there is no decision height involved at all in a take-off.
Worked example
Worked example: identifying the approach category
Not quite. The correct answer is B.
Correct answer: B
- A. CAT I decision heights are commonly quoted at 200 ft or above. A 150 ft decision height is below that band.
- B. Correct: a decision height between 100 ft and 200 ft, with the runway visual range at or above the commonly quoted 300 m minimum, fits the CAT II band.
- C. CAT III operations are commonly associated with a decision height below 100 ft, or no decision height at all. A defined 150 ft decision height rules this out.
- D. CAT IIIB is the lowest common band, with runway visual range values far below 400 m and no decision height. Neither figure in the stem matches it.
Step by step
- Read the decision height: 150 ft. This falls inside the commonly quoted CAT II band of 100 ft to 200 ft, ruling out CAT I above it and both CAT III bands below it.
- Check the runway visual range: 400 m is at or above the commonly quoted CAT II minimum of 300 m, which is consistent with the decision height reading.
- Confirm both figures point to the same category rather than conflicting, since a mismatch between decision height and runway visual range would be the more usual trap.
- Conclude CAT II, since both the decision height and the runway visual range for this approach sit inside that band.
Common mistakes
Assuming a lower decision height always means a lower category number automatically applies without checking the runway visual range
The exam can quote a decision height from one band alongside a runway visual range from another to test whether the student checks both figures rather than reading only one.
Confusing low visibility procedures with low visibility take-off
Low visibility procedures protect the localiser-sensitive area for an arriving aircraft using instrument guidance; low visibility take-off protects a departing aircraft that has no decision height involved at all. Treating them as the same concept loses marks on take-off-specific questions.
Forgetting that CAT II and III also require specific crew and aircraft approval, not just the published minima
A crew or aeroplane without the relevant approval cannot use the lower minima even though the published category exists at that aerodrome, and questions that mention approval status are testing exactly this point.
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Last reviewed July 2026