What lighting does each approach category need?
CAT I, CAT II and CAT III compared: which lights must be provided, how closely they are spaced, how fast the power must switch over and how well they must be kept.
The approach category is set by the decision height and the runway visual range the approach is flown to. The lower the limits, the more lighting the aerodrome must provide, and the better it must be kept. CAT I needs the 900 m approach lights with one crossbar. CAT II adds red side rows, a 150 m crossbar, touchdown zone and centreline lights. CAT III adds the closest centreline spacing and the tightest availability.
Behind the lights sit the supporting requirements: secondary power that switches over fast enough, circuits laid out so one failure does not blank a pattern, monitoring, and a minimum serviceability for each light type.
Always check the current edition of Annex 14, your State's regulations and the category approved for your runway. This entry is a summary for learning, and the aerodrome's approved operations manual decides what applies.
| Category | Decision height | RVR | Lighting effect |
|---|---|---|---|
| CAT I | Not lower than 60 m (200 ft) | Not less than 550 m (or visibility 800 m) | Lead-in and alignment picture |
| CAT II | Lower than 60 m, not lower than 30 m (100 ft) | Not less than 300 m | Adds roll and distance-to-go cues |
| CAT IIIA | Lower than 30 m, or none | Not less than 175 m | Same approach pattern as CAT II |
| CAT IIIB | Lower than 15 m (50 ft), or none | Less than 175 m, not less than 50 m | Closest centreline spacing, tightest guidance |
| CAT IIIC | None | No limit | Needs the full system working |
| Light group | CAT I | CAT II | CAT III |
|---|---|---|---|
| Approach lighting | 900 m white centreline, 30 m spacing, crossbar at 300 m | As CAT I, plus red side rows to 270 m, a 150 m crossbar and barrettes in the inner 300 m | Same pattern as CAT II |
| Runway edge, threshold, end | Required, stepped to the visibility; edge lights yellow in the caution zone | Required, high-intensity, stepped by RVR; edge caution zone yellow | Required, high-intensity, stepped by RVR; edge caution zone yellow |
| Touchdown zone lights | Where the aerodrome needs them | Required | Required |
| Runway centreline lights | Where needed, and for low-visibility take-off | Required, about 30 m (about 15 m where used below RVR 350 m) | Required, about 15 m |
| Stop bars | Where the runway is used in low visibility | Required where the runway is used below RVR 550 m, unless aids and procedures prevent incursions or, from 350 m, movements are limited to one at a time | Same as CAT II |
| Taxiway guidance | As provided for the traffic | Continuous centreline guidance, switched by segment | Same, with monitoring |
| Secondary power switch-over | 15 seconds | 1 second for the most critical lights, 15 seconds for the rest | 1 second for the most critical lights, 15 seconds for the rest |
| Circuit design | Series circuits | Interleaving for key patterns | Interleaving for key patterns |
| Serviceability objective | 85% of approach, threshold, edge and end lights | 95% approach (inner 450 m), centreline, threshold, edge; 90% touchdown zone; 85% approach beyond 450 m; 75% end | Same as CAT II, with all lights serviceable as the aim |
Frequently Asked Questions
Q1What decides the approach category?
The category is set by the decision height and the runway visual range the approach is flown to, and it depends on three things together: the aircraft and crew approval, the ILS or other ground equipment, and the aerodrome's lighting, markings and procedures. The lighting is only one part. A runway can be lit for a high category but used at a lower one on a given day.
Q2What lighting does CAT I need?
A CAT I runway has the 900 m approach lighting with a white centreline and one crossbar at 300 m, plus runway edge, threshold and runway end lights. Touchdown zone and centreline lights are provided where the aerodrome needs them. The decision height is not lower than 60 m and the RVR is not less than 550 m, so the pilot sees the lead-in and alignment picture at that point.
Q3What does CAT II add?
The approach pattern gains red side rows to 270 m and a crossbar at 150 m, with barrettes in the inner 300 m, so the pilot keeps roll and distance cues when only a short length of pattern is visible. Touchdown zone and runway centreline lights are required, stop bars are provided at runway holding positions where the runway is used below RVR 550 m (unless aids and procedures prevent incursions, or, from 350 m, movements are limited to one at a time), and the most critical lights need a 1 second secondary power switch-over.
Q4What does CAT III add?
The approach pattern is the same as for CAT II, and the difference is in the runway and the system. The centreline lights are at the closest spacing, taxiway guidance and stop bars are relied on fully, and the serviceability and monitoring must hold up in the lowest visibility. IIIA, IIIB and IIIC differ in decision height and RVR limits, not in the approach light pattern.
Q5What if the lights fall below the serviceability objective?
ATC is told straight away so operations can be adjusted, which can mean a higher minimum, a lower category or a restriction, under the aerodrome's procedure. Serviceability is a percentage per light type and a rule against adjacent outages, and both must hold. The decision to restrict is for the aerodrome operator and ATS, not for the maintenance team alone.
Q6Does the intensity change with the category?
The category sets which lights must be provided and how well they must be maintained. The brightness step on the day follows the visibility, normally from the RVR step table, and the lights are high-intensity types for the lower categories. See the RVR-to-intensity-step entry for how the step is chosen.
Q7Is it safe to work on these lights?
Work on the approach pattern, centreline, stop bars, power changeover or monitoring can lower the available category, so it needs a permit-to-work, ATC agreement and a documented return-to-service test. Plan work outside low-visibility periods, and check the lighting is back to its full configuration before low-visibility procedures start.
See It. Understand It. Then Learn How to Engineer It.
Recognising an AGL system visually is the first step. The next step is knowing how to design, install, test, commission and maintain them correctly.
Build Practical AGL Engineering Skills
Go deeper with structured training from Precision AGL Academy, covering:
Learn how each light, sign and circuit works as part of a complete airport lighting system, not just as an isolated diagram.