CAT I, II AND III: WHAT THEY MEAN
CAT I, II and III are categories of precision instrument approach and landing operations in progressively poorer visibility. They involve aircraft systems, navigation, crew qualification, operator approval, aerodrome infrastructure, visual aids, RVR, decision height, power supply and maintenance. They are not a classification of lamps. In current ICAO Annex 6 (Part I) these categories sit under Type B instrument approach operations (DH below 75 m / 250 ft); Type A operations (MDH or DH at or above 75 m / 250 ft) sit outside CAT I to III.
| Category | Decision height (DH) | RVR / visibility |
|---|---|---|
| CAT I | DH ≥ 200 ft (60 m) | Visibility ≥ 800 m or RVR ≥ 550 m |
| CAT II | 100 ft ≤ DH < 200 ft | RVR ≥ 300 m |
| CAT III | DH < 100 ft, or no DH | Depends on sub-category: ≥ 175 m (IIIA), 50 m to < 175 m (IIIB), no limitation (IIIC) |
CAT IIIA, IIIB, IIIC
| Sub-category | DH (ICAO) | RVR (ICAO) |
|---|---|---|
| IIIA | < 100 ft (30 m) or no DH | ≥ 175 m |
| IIIB | < 50 ft (15 m) or no DH | < 175 m but ≥ 50 m |
| IIIC | No DH | No RVR limitation |
ICAO Annex 6 defines these sub-categories with the 175 m and 50 m RVR limits shown. Older documents and FAA practice use 200 m (700 ft) for IIIA, and some regulators no longer subdivide CAT III, so manuals, AIPs and LVPs may differ. Before using IIIA/B/C values as acceptance criteria, check: ICAO standard, State regulation, operator approval, AIP/LVP and the approved project design.
RVR
RVR (Runway Visual Range) is the distance over which a pilot on the centreline can see runway markings or lights of adequate intensity. It matters far more than ordinary visibility for CAT II/III. In India it may be reported at TDZ, MID and END positions.
- The lower the visibility/RVR, the more critical continuity, intensity, geometry, power, control, monitoring and maintenance become.
AGL BY CATEGORY
| Feature | CAT I | CAT II | CAT III |
|---|---|---|---|
| Approach lighting | Precision, ~900 m, 30 m crossbar at ~300 m | CAT II/III system: centreline ~900 m + two side rows ~270 m + crossbars at 150 m and 300 m | Same system, higher reliability |
| Edge, threshold, end | Required | Required | Required |
| Runway centreline (RCL) | Depends on conditions | Critical | Fundamental |
| Touchdown zone (TDZ) | Not an ICAO requirement for CAT I | Required (ICAO) | Required, critical |
| Taxiway centreline | May be provided | Required where RVR < 350 m | Required, very important |
| Stop bars | Required below RVR 550 m; ICAO also addresses RVR 550 to 1,200 m | Required at runway-holding positions where RVR < 550 m (limited exceptions) | Required, critical |
| Runway guard lights | RVR 550 to 1,200 m with medium or low traffic density | Where no stop bar and RVR < 550 m; also RVR 550 to 1,200 m with medium or low traffic density | Same as CAT II |
| Power / monitoring | Per system | Higher / critical | Highest / essential |
Exact provision depends on runway configuration, authority and approved design. ICAO allows the CAT II/III side rows to be 240 m instead of 270 m where the serviceability objectives in Section 5 can be demonstrated. The numbers above follow ICAO; check the applicable State and project standard. Note that several ICAO provisions (stop bars, runway guard lights, taxiway centreline lights) are triggered by RVR values such as 550 m and 350 m rather than by the category name, so confirm the exact clause in your edition.
Why each aid matters
- Side-row barrettes: lateral reference, roll guidance, orientation.
- RCL: longitudinal alignment when visibility is poor. ICAO: required on CAT II/III runways; about 15 m spacing (30 m allowed in specified conditions).
- TDZ: touchdown-zone information when the runway appears late.
- Stop bars: red, at holding positions; part of AGL + ATC control + interlocking + LVP + incursion prevention.
- Taxiway centreline: pilot may lose reference to surrounding pavement.
- Runway guard lights: yellow, extra conspicuity at holds.
POWER SWITCH-OVER AND UPS
CAT I aids generally allow a switch-over of up to 15 s. CAT II/III is stricter for critical aids (ICAO Annex 14 Vol. I, Table 8-1):
| CAT II/III facility | Max switch-over |
|---|---|
| Inner 300 m approach lighting | 1 s |
| Other approach lighting | 15 s |
| Runway threshold, end, centreline, TDZ | 1 s |
| All stop bars | 1 s |
| Runway edge, essential taxiway, obstacle lighting | 15 s |
Normal power → UPS / fast source → critical loads (RCL, TDZ, threshold, end, stop bars)
+ DG / standby source for longer outages
Zero interruption vs 1-second switch-over: a UPS can give no visible interruption. A generator-only chain (start, ATS, restore) must still meet the approved changeover time. UPS-supported critical circuits with automatic generator takeover are a common way to meet the 1 s requirement, subject to the approved design.
Check the footnotes to Table 8-1: for example, runway edge lights also need 1 s where the runway is used for take-off in RVR below 800 m.
INTERLEAVING AND CONTROL
Interleaved: A B A B A B A B Circuit A fails → X B X B X B X B (pattern kept) Not: A A A A B B B B Circuit A fails → X X X X B B B B (dark section)
Interleaving preserves the basic visual pattern after a circuit failure. The control and monitoring system (ALCMS) should let ATC/operators select configuration and intensity, show circuit status, flag critical failures, coordinate with LVP and control stop bars. Aim: detect, identify, report, act.
CCR brightness steps (typically 3 or 5) depend on the approved design. Too dim means guidance cannot be identified; too bright causes glare. Correct output, correct step and correct aiming together make guidance usable.
PHOTOMETRY AND SERVICEABILITY
For CAT II/III, ICAO recommends preventive maintenance with visual inspection, in-field intensity, beam spread, orientation and electrical measurement, and verification of intensity settings, using mobile units accurate enough to assess individual lights. Frequency should reflect traffic, pollution and reliability; guidance indicates at least twice a year for in-pavement lights and once a year for others.
| Element | CAT II/III objective |
|---|---|
| Approach lighting, inner 450 m | 95% |
| Runway centreline, threshold, edge | 95% |
| Touchdown zone | 90% |
| Approach beyond 450 m | 85% |
| Runway end | 75% |
CAT I objective is about 85% for approach, threshold, edge and end lights, while keeping the pattern intact. These objectives are in ICAO Annex 14 Vol. I, Chapter 10 (paragraph 10.5.7 for CAT II/III and 10.5.10 for CAT I in recent editions).
Worked example
240 RCL lights (about 3.6 km at 15 m spacing) with 6 failed: 234 / 240 = 97.5%, above 95%. But ask: are failures adjacent? Concentrated in one section? Inside a critical element? Does the LVP restrict operations? Percentage alone never tells the full story; where the lights fail matters. ICAO Annex 14 Chapter 10 also protects continuity of guidance: the allowed unserviceable lights must not be adjacent (unless the spacing is much closer than standard), and stop bars used in RVR below 350 m have their own limit (no more than two unserviceable, and not adjacent). A high percentage can therefore still fail the pattern test. Check the exact clause in your edition.
Isolated: ● ● ● X ● ● ● ● Adjacent: ● ● X X ● ● ● ● ← pattern damaged Cluster: X X X X X ● ● ●
Electrical health is not photometric health: a circuit can be electrically sound yet low in intensity, mis-aimed, wrong in colour or contaminated. Record date, weather, RVR, runway status, lighting step, instrument and calibration. LED fixtures still need photometric maintenance (driver, optics, thermal, colour stability). Identify the actual fixture type and standard (for example AC 150/5345-46F), never just "CAT III light". See Note 15 AGL Photometry.
LVP, FAULTS AND COMMISSIONING
LVP (Low Visibility Procedures) are airport procedures, not an AGL setting. A typical LVP sequence (check your aerodrome AIP/LVP) covers: RVR/MET trigger, safeguarding, AGL inspection, movement-area restrictions, critical-area protection, CAT II/III lighting selection, ATC coordination, LVP active, then periodic AGL inspection and reporting. Routine maintenance and LVP inspection are different activities.
When a light fails, ask
- Which location and which circuit?
- Is an adjacent light out?
- What percentage has failed?
- Is the basic pattern affected?
- What does the LVP manual say, and is CAT II/III restricted?
- Has ATC been told?
Commissioning: design approval, installation, IR/continuity/CCR/circuit/fixture/control tests, failover test, photometry and alignment, then operational demonstration (power failure, automatic transfer, alarm, recovery). Verify every required intensity step, not only maximum. See Note 14.
ICAO, FAA AND LOCAL STANDARDS
ICAO is not FAA is not DGCA or State implementation. They differ in category definitions, RVR terms, approach configurations, fixture classes, photometric criteria and power requirements. Use the contractually and regulatorily applicable standard, never a mix of numbers.
Annex 6 (CAT definition) → Annex 14 Vol I (visual aids, power, maintenance) → Annex 10 (ILS) → Doc 9157 → State regulation (DGCA) → AIP / LVP → approved project specification
FAA treats ALSF-2 as the CAT II/III system giving alignment, height perception, roll guidance and horizontal reference.
QUICK REVISION
CAT I = approach + edge + threshold + end (PAPI where provided) CAT II = CAT I + RCL + TDZ + low-vis taxi guidance + stop bars (RVR < 550 m) + 1 s critical power + interleaving + stronger control CAT III = same aids and power as CAT II, with the most stringent reliability, monitoring, photometry and LVP control
- CAT I: see the runway earlier. CAT II: later. CAT III: rely heavily on the system and approved procedures.
- CAT III is not just more lights: it is reliable guidance, power, control, monitoring, photometry and procedure.
- 50% (ICAO, per light) ≠ 70% (FAA target) ≠ 95% (system objective).
- Percentage and failure distribution both matter.
EXAM QUESTIONS
What is CAT I?
DH ≥ 200 ft and visibility ≥ 800 m or RVR ≥ 550 m (ICAO Annex 6).
What is CAT II?
DH 100 to below 200 ft and RVR ≥ 300 m.
What is CAT III?
DH below 100 ft or no DH. RVR is not less than 175 m for IIIA, 50 m to below 175 m for IIIB, and no limitation for IIIC.
Main AGL difference between CAT I and CAT II/III?
More comprehensive, reliable guidance: RCL, TDZ, low-visibility taxi guidance, stop bars, critical power, control and monitoring.
Why is RCL important in CAT II/III?
It gives longitudinal alignment guidance in poor visibility.
Why TDZ lighting?
Extra touchdown-phase information when external visibility is limited.
Why are stop bars critical?
Conspicuous red stop instruction supporting runway-incursion prevention, tied to ATC, interlocking and LVP.
Why UPS for CAT II/III?
Critical aids such as RCL, TDZ, threshold, end and stop bars need very fast restoration (1 s).
What is interleaving?
Arranging circuits so one circuit failure does not destroy the basic visual pattern.
What is the ICAO individual-light criterion?
Unserviceable when main-beam average intensity is below 50% of the specified value.
Is 95% serviceability enough?
No. Failure distribution and preservation of the pattern also matter.
FAA 70% vs ICAO 50%?
70% is an FAA maintenance targeting criterion for in-pavement lights; 50% is ICAO's per-light unserviceability criterion. Do not interchange.
Is electrical testing enough?
No. Photometry, visual and functional checks are also needed.
At what RVR must stop bars be provided?
ICAO requires a stop bar at every runway-holding position serving a runway intended for use in RVR below 550 m, with limited exceptions where aids and procedures already limit the risk of incursion.
FINAL STUDY TAKEAWAY
CAT I, II and III are progressively more demanding operations, and AGL must provide progressively more reliable, continuous and accurately controlled visual guidance as external visual reference decreases.
ENGINEERING & EXPERT NOTES
Treat Each Category as a Whole-System Change
ConceptLighting, power, monitoring and procedures all become more demanding at lower visibility. Do not think of it as only brighter lights.
Switch-Over and Standby Power Are Tested, Not Assumed
PowerVerify behaviour under load against the applicable requirement and record the results.
Monitoring and Alarm Response Carry More Weight
ReliabilityAgree what happens when a fault is reported and who acts.
Cite the Current Edition and Local Rules
ComplianceCheck ICAO, FAA and national documents before quoting a requirement.
REFERENCES & FURTHER READING
A study aid. Verify against current primary sources. Approach categories and minima are set by State regulation and operator approval. These notes are educational and must not be used to determine operating minima, approvals or compliance for a specific aerodrome.
| Source | Use it for |
|---|---|
| ICAO Annex 6, Part I | Instrument approach classification (Type A/B, CAT I to III) and definitions. |
| ICAO Doc 9365 Manual of All-Weather Operations | Background on categories, RVR and all-weather operations. |
| ICAO Annex 14 Vol. I (check current edition and amendments) | Visual aids, secondary power, switch-over, maintenance. |
| ICAO Doc 9157 Aerodrome Design Manual (Part 4 Visual Aids, Part 5 Electrical Systems) | Configuration, interleaving. |
| ICAO Doc 9137 Airport Services Manual, Part 9 (Airport Maintenance Practices) | Preventive maintenance, photometric measurement and serviceability. |
| FAA AC 150/5340-26C | Maintenance, 70% target. |
| FAA AC 150/5345-46F | Fixture classes and photometry. |
| FAA AC 150/5340-30J | Visual aid design and installation. |
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