CAT II CAT III
Precision Approach Lighting for Progressively Lower-Visibility Operations — CAT III is not simply a brighter or longer version of CAT II. The approach lighting system is one part of a much broader low-visibility landing system that also includes runway lighting, navigation/landing equipment, monitoring, power supply, aircraft equipment and operational procedures.
At a Glance
| Parameter | CAT II | CAT III |
|---|---|---|
| Approach type | Precision approach | Precision approach |
| Operating visibility | Lower than CAT I | Very low visibility |
| Decision height (FAA) | Down to 100 ft | May have no decision-height limit |
| Approach lighting | Precision CAT II/III system | Precision CAT II/III system |
| Typical FAA ALS | ALSF-2 | ALSF-2 |
| Approach-lighting concept | High-intensity visual guidance | High-intensity visual guidance |
| Inner visual references | Enhanced | Enhanced |
| Red side-row lights | Yes in ALSF-2 | Yes in ALSF-2 |
| Centreline lighting | Required with applicable low-vis runway system | Required |
| Touchdown zone lighting | Required | Required |
| Monitoring | High | Very high |
| Power reliability | High | Very high |
| Low-visibility capability | High | Very high |
*CAT IIIA/IIIB permit a DH below 100 ft (or no DH) depending on subcategory; CAT IIIC has no DH and no RVR limit. Always confirm exact values against the current governing standard for the specific project.
Important: In the FAA system, ALSF-2 is used for both CAT II and CAT III visual guidance. CAT II vs CAT III should not be explained simply as "ALSF-2 vs another ALS."
What Is CAT II?
Category II is a precision approach category intended for operations in conditions where the pilot may not obtain the required visual references until significantly closer to the runway. The FAA defines CAT II as a facility providing operation down to a 100 ft decision height, with RVR not less than 1,200 ft / 366 m.
The visual system needs to provide strong references for
- Runway alignment
- Centreline
- Height perception
- Roll guidance
- Horizontal position
- Threshold identification
- Touchdown area
What Is CAT III?
Category III is designed for operations in substantially lower visibility than CAT II. It is divided into different operational subcategories, and the exact requirements depend on the applicable regulatory framework and approval. The FAA identifies CAT III as a facility supporting operations with no decision-height limit and very low RVR conditions, subject to the applicable CAT III criteria.
This changes the role of the visual system: the pilot may have little or no external visual reference during the final stage of landing. CAT III therefore depends much more heavily on the integrity, monitoring and availability of the complete landing system.
Decision Height, Visibility & System Dependence
CAT II
Low visibility + decision height + enhanced visual reference.
CAT III
Very low visibility + highly reduced visual reference + highly reliable landing system.
As visibility decreases across CAT I → CAT II → CAT III, the dependence on instrument guidance, runway centreline lighting, touchdown zone lighting, approach lighting, monitoring, redundancy, power supply and system availability all increase.
CAT II/III Approach Lighting Configuration
Under ICAO terminology, a precision CAT II/III approach lighting system uses an extended centreline with side rows and crossbars, typically comprising:
- White centreline lighting
- Red side-row lighting
- White crossbars
- Sequenced flashing lights where applicable
- Threshold lighting
ICAO material describes CAT II/III approach lighting as extending approximately 900 m from the threshold. For FAA ALSF-2, the approach lighting area typically spans 2,400–3,000 ft, depending on installation conditions.
Does CAT III Use a Different ALS From CAT II?
Not necessarily. For the FAA configuration, both CAT II and CAT III visual guidance are typically provided by ALSF-2 — the FAA specifically describes ALSF-2 as serving Category II/III instrument approaches.
It is incorrect to build a simple comparison such as "CAT II = ALSF-2, CAT III = ALSF-3" — that is not correct FAA terminology. For an ICAO-based project, terminology and configuration should be taken from the applicable ICAO and national requirements.
CAT II vs CAT III Approach Lighting
| Feature | CAT II | CAT III |
|---|---|---|
| Centreline lighting | Yes | Yes |
| Side-row lighting | Yes | Yes |
| Red side-row references | Yes | Yes |
| Crossbars | Yes | Yes |
| Sequenced flashing lights | Applicable | Applicable |
| High-intensity approach lighting | Yes | Yes |
| Threshold lighting | Yes | Yes |
| Visual runway references | Important | Extremely important where available |
| Low-visibility operation | Yes | Very low visibility |
| Dependence on instrument systems | High | Very high |
Runway Lighting Becomes Critical
Approach lighting alone cannot make an airport CAT II or CAT III capable — the runway lighting system also becomes critical. FAA guidance identifies runway centreline lights and touchdown zone lights as required for CAT II and CAT III runways.
Approach Area
- High-intensity approach lighting
- Centreline lights
- Side-row lights
- Crossbars
- Sequenced flashers
Runway & Control
- High-intensity runway edge lights
- Runway centreline lights
- Touchdown zone lights, threshold & runway end lights
- CCRs, ALCMS & monitoring
- Fault indication, automatic control & emergency power
CAT II vs CAT III: Touchdown Zone Lighting
The FAA describes touchdown zone lighting as two rows of transverse light bars positioned symmetrically about the runway centreline, extending from the threshold into the touchdown zone, to give the pilot a strong visual reference for the touchdown area.
CAT II
TDZ lighting provides an important visual reference during the final approach and landing.
CAT III
TDZ lighting remains important, but the overall CAT III system cannot depend solely on the pilot visually identifying the touchdown zone.
Runway Centreline Lighting
Runway centreline lighting provides a longitudinal reference during final approach, flare, touchdown and rollout. The FAA states that runway centreline and touchdown zone lights are required for CAT II and CAT III runways.
For CAT III operations, centreline lighting becomes particularly important during rollout because external visual references can remain extremely limited.
Electrical Engineering Perspective
From an AGL engineering perspective, the difference becomes particularly important in system reliability and continuity.
Simplified architecture: Main power → emergency/alternate power → CCR → primary series circuit → isolation transformers → approach/runway fixtures → monitoring & control.
The system should be designed so that critical lighting functions remain available within the applicable transfer and serviceability requirements.
Power Supply Reliability
CAT II and CAT III operations require much greater attention to power continuity, which may involve main power, alternate power, emergency power, automatic transfer, separate critical circuits, CCR redundancy, monitoring and control-system redundancy.
FAA AC 150/5340-30J identifies CAT II lighting facilities requiring rapid transfer arrangements — including CAT II high-intensity runway lighting, runway centreline lighting and touchdown zone lighting — among systems requiring particularly fast transfer. For CAT III, applicable operational approval and national requirements must be checked carefully, since availability and redundancy become even more critical.
Monitoring Requirements
CAT II
Monitoring should provide adequate information about critical system status and faults, typically including:
- CCR status
- Approach & runway centreline circuit status
- TDZ circuit status
- Flasher & power status
- Control-system faults
CAT III
Monitoring becomes even more critical — the system may need to identify failures rapidly enough to support the required operational decision-making and maintenance response. Exact serviceability and monitoring criteria should be taken from the applicable CAT III approval and governing standard.
Photometric & Functional Performance
Both CAT II and CAT III systems require appropriate photometric performance across approach lighting (intensity, beam distribution, colour, alignment, crossbar and centreline performance), runway lighting (edge, centreline, TDZ and threshold intensity), and functional performance (intensity steps, sequenced flashers, circuit switching, monitoring, control response).
Engineering point: A CAT II/III lighting system can be electrically healthy but still fail to provide the required visual performance because of incorrect aiming, damaged optics, contamination or photometric degradation.
CAT II vs CAT III: Maintenance
| Maintenance Area | CAT II | CAT III |
|---|---|---|
| Routine / fixture inspection | Required | Required |
| Cable testing / insulation resistance | Required | Required |
| CCR testing | Required | Required |
| Photometry / alignment verification | Required | Required |
| Circuit monitoring | Required | Critical |
| Power transfer testing | Critical | Extremely critical |
| Fault response | High priority | Very high priority |
| Documentation | Detailed | Very detailed |
The Key Issue: Visual Reference Availability
CAT II
The pilot may still obtain the required visual reference before landing — instrument guidance + approach lighting + runway lighting + pilot visual reference.
CAT III
Depending on the operation, the pilot may have extremely limited external visual reference — highly reliable instrument/landing system + highly reliable runway lighting + monitoring + redundancy + very limited external visual reference.
Therefore, CAT III relies heavily on the integrity of the complete landing system, not just the pilot's own eyes.
Myths vs Facts
Not necessarily — in FAA practice, ALSF-2 provides visual guidance for both CAT II and CAT III approaches.
This is not correct FAA terminology — FAA identifies ALSF-2 as serving CAT II/III.
CAT III is fundamentally an operational and system-integrity requirement, not simply a lighting-intensity upgrade.
The approach category depends on the complete approved system — navigation/landing equipment, runway lighting, monitoring, power, procedures and operational criteria.
CAT III runways still have extensive lighting systems — the issue is that the pilot may have very limited visual reference during critical phases of the operation.
CAT II/III Approach-Lighting Project Workflow
Sequence: Define CAT II/CAT III operational requirement → identify governing standard → determine approach lighting configuration → design ALS circuits → design runway centreline lighting → design TDZ lighting → design runway edge & threshold lighting → CCR & series circuit design → emergency/alternate power design → control & monitoring design → installation → electrical testing → photometric testing → alignment verification → power transfer testing → integrated system testing → operational approval.
CAT II vs CAT III Approach Lighting
CAT III is not simply a different approach-lighting fixture configuration from CAT II. In FAA practice, ALSF-2 serves CAT II/III approach visual guidance.
The major difference between CAT II and CAT III is the overall operational and system-integrity requirement, particularly as visibility becomes extremely low.
Simple AGL memory rule — CAT II: strong visual guidance + low-visibility runway lighting. CAT III: maximum system integrity + very low-visibility operation + minimal dependence on external visual references.
Engineering & Expert Notes
CAT III Is Not Just a Brighter CAT II
ConceptLanding system, procedures and lighting reliability all change together. Review them as a package.
Plan for Higher Availability
ReliabilityRedundancy, monitoring and power arrangements need deeper checking.
Include Visual Aid Maintenance in the Plan
MaintenanceFrequent checks and quick repair of lights matter more at low visibility.
Keep Evidence for Audits
RecordsRetain test and inspection records that show how the category is supported.
Frequently Asked Questions
CAT II still depends significantly on the pilot acquiring visual references at around 100 ft. CAT III is designed for operations where visual reference may be extremely limited or absent, so system integrity, monitoring, power continuity and redundancy become far more critical than the approach-lighting hardware itself.
Not necessarily. In FAA practice, ALSF-2 serves both CAT II and CAT III visual guidance — it should not be assumed that CAT III automatically means "ALSF-3."
Yes, TDZ lighting remains required and important, though the CAT III system as a whole cannot rely solely on the pilot visually identifying the touchdown zone.
No. CAT III capability depends on the complete approved low-visibility landing system — navigation/landing equipment, runway lighting, monitoring, power reliability, procedures and operational approval.
Because the pilot may have extremely limited or no external visual reference during the final phase of landing, so the electrical, monitoring and control systems must be far more reliable to compensate.
Depending on the CAT III subcategory and applicable operational approval, CAT III operations may have no decision-height limit — this must be confirmed against the governing regulatory framework for the specific operation.
Governing Documents
For an actual project design, always verify the current edition of the applicable documents.
FAA AC 150/5340-30
Design and Installation Details for Airport Visual Aids — approach lighting systems, ALSF-2 configuration, installation, photometric requirements and electrical systems.
ICAO Annex 14, Volume I
Aerodrome Design and Operations — precision approach categories, decision height and RVR criteria, CAT II/III approach lighting configuration and operational requirements.
DGCA / AAI Requirements
For Indian airport projects, also verify applicable DGCA requirements, AAI requirements, airport-specific technical specifications, project drawings and contract specifications.
Final Engineering Note
Do not design a CAT II or CAT III system from a generic web diagram. The final installation drawing should always be developed from the current governing standard, approved aerodrome design, approach category and project-specific requirements.
CAT II and CAT III should be compared as complete low-visibility landing systems, not only as approach-lighting layouts. CAT II still depends significantly on the pilot acquiring visual references; CAT III is designed for operations where visual reference may be extremely limited, so system integrity, monitoring, power continuity and redundancy become increasingly critical.
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