CAT I CAT II CAT III
Precision Approach Lighting Across Three Low-Visibility Categories — CAT I, CAT II and CAT III represent progressively more demanding precision approach operations. The approach lighting system is an important part of this progression, but it is not the sole factor that determines the approach category. CAT III is not simply "brighter CAT II" — it is a higher-integrity low-visibility operation in which visual references may be extremely limited.
At a Glance
| Parameter | CAT I | CAT II | CAT III |
|---|---|---|---|
| Approach type | Precision approach | Precision approach | Precision approach |
| Typical decision height | ≥ 200 ft | < 200 ft and ≥ 100 ft | Depends on subcategory & applicable criteria |
| Visibility | Higher | Lower | Very low |
| RVR | Typically ≥ 550 m | Typically ≥ 300 m | Typically < 175 m, per subcategory |
| Typical FAA ALS | ALSF-1 | ALSF-2 | ALSF-2 |
| Approach lighting | Precision approach lighting | Enhanced precision approach lighting | High-integrity low-visibility system |
| Inner visual references | Standard | Enhanced | Enhanced |
| Red side-row configuration | Not the defining CAT I arrangement | Yes, with ALSF-2 | Yes, with ALSF-2 |
| Runway centreline lights | Depends on runway/design | Required | Required |
| Touchdown zone lights | Depends on runway/design | Required | Required |
| Sequenced flashers | Applicable | Applicable | Applicable |
| Monitoring | Required as applicable | More demanding | Highly critical |
| Power continuity | Important | Critical | Extremely critical |
| System redundancy | Applicable | Higher | Very high |
| Dependence on visual references | Higher | Reduced | Potentially very limited |
*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.
What Changes from CAT I to CAT III?
Instrument approach → visual acquisition at a higher decision height → landing.
Instrument approach → visual acquisition at a lower decision height → enhanced runway visual guidance → landing.
Highly reliable instrument/landing system → extremely limited visual reference → highly controlled landing and rollout environment.
The CAT I → CAT II → CAT III progression means increasing requirements for low visibility capability, visual guidance, lighting availability, monitoring and overall system integrity.
CAT I Approach Lighting
CAT I precision approaches are generally associated with a decision height of 200 ft or higher. For FAA configurations, ALSF-1 is commonly associated with CAT I approaches. A typical CAT I system provides:
- Approach centreline
- Crossbars
- High-intensity lights
- Sequenced flashing lights where applicable
- Red inner approach references
- Threshold visual reference
The objective is to provide sufficient visual information for the pilot to transition from instrument guidance to the runway environment. Engineering memory: CAT I → ALSF-1 → standard precision visual transition.
CAT II Approach Lighting
CAT II allows operations at a lower decision height and lower visibility than CAT I. For FAA operations, ALSF-2 is the typical high-intensity approach lighting configuration. The inner approach area adds:
- White centreline bars
- Red side-row bars
- White crossbars, incl. 500-ft & 1,000-ft crossbars
- Sequenced flashing lights
CAT II also requires the appropriate runway lighting system, including runway centreline and touchdown zone lighting. Engineering memory: CAT II → ALSF-2 → enhanced inner approach + low-visibility runway lighting.
CAT III Approach Lighting
CAT III does not necessarily use a separate ALS configuration. In FAA practice, ALSF-2 provides visual guidance for both CAT II and CAT III instrument approaches. Therefore: CAT II ≠ ALSF-2 alone, and CAT III ≠ ALSF-3.
A better representation: CAT I → ALSF-1 · CAT II → ALSF-2 · CAT III → ALSF-2 + complete CAT III low-visibility system requirements. The approach lighting configuration alone does not establish CAT III capability.
CAT III Subcategories
CAT III operations are further divided according to the applicable regulatory system. A simplified engineering view:
CAT IIIA
Very low visibility with limited or no reliance on decision height, depending on applicable criteria.
CAT IIIB
Lower visibility than IIIA with greater system and operational requirements.
CAT IIIC
Conceptually no decision height and no RVR limitation, though operational implementation is highly constrained.
The exact values and operational criteria must be taken from the applicable current regulatory standard. For airport engineering, do not use a generic CAT III table to establish design requirements.
Approach Lighting Feature Comparison
| Feature | CAT I | CAT II | CAT III |
|---|---|---|---|
| High-intensity ALS | Yes | Yes | Yes |
| Approach centreline | Yes | Yes | Yes |
| Crossbars | Yes | Yes | Yes |
| Sequenced flashing | Applicable | Applicable | Applicable |
| Red inner references | ALSF-1 configuration | ALSF-2 configuration | ALSF-2 configuration |
| Red side rows | Not the defining CAT I configuration | Yes | Yes |
| 500-ft crossbar | Not the defining CAT I feature | Yes | Yes |
| Visual transition | Strongly dependent on pilot acquisition | Reduced visual acquisition window | Visual reference may be extremely limited |
Runway Lighting Comparison
The approach lighting system cannot be considered separately from the runway lighting.
| Runway Lighting | CAT I | CAT II | CAT III |
|---|---|---|---|
| Runway edge lights | Yes, as applicable | Yes | Yes |
| Threshold lights | Yes | Yes | Yes |
| Runway centreline | Design-dependent | Required | Required |
| Touchdown zone lighting | Design-dependent | Required | Required |
| Runway end lights | Applicable | Applicable | Applicable |
| High-intensity runway lighting | As required | Yes | Yes |
| Lighting monitoring | Applicable | Enhanced | Highly critical |
Touchdown Zone Lighting (TDZL)
TDZL becomes particularly important as approach visibility decreases, providing visual information about the touchdown area and helping the pilot identify longitudinal position relative to the runway.
CAT I
TDZL requirement depends on the applicable runway and approach configuration.
CAT II
TDZL is an important component of the low-visibility runway lighting system.
CAT III
TDZL remains part of the runway visual system, but CAT III cannot depend solely on visual identification of the touchdown zone.
Runway Centreline Lighting
Runway centreline lighting provides a strong longitudinal visual reference during final approach, flare, touchdown and rollout — its importance increases significantly with reduced visibility.
CAT I
Requirement depends on the runway and applicable design criteria.
CAT II
Required as part of the applicable CAT II runway lighting system.
CAT III
Critical component of the low-visibility runway lighting system, especially during rollout.
Electrical Engineering Progression
Simplified architecture: Main power supply → alternate/emergency power → CCR → primary series circuit → isolation transformers → AGL fixtures → monitoring & control.
For CAT II and CAT III, additional attention is required for
- CCR redundancy
- Power transfer
- Circuit monitoring
- Critical lighting circuits
- Control-system reliability
- Fault detection
- Emergency power
- System availability
CCR and Series Circuit
The Constant Current Regulator (CCR) maintains the required current in the airport series circuit. Typical engineering checks include CCR rating, number of fixtures, transformer wattage, circuit load, cable length and resistance, voltage drop, power factor where applicable, insulation resistance, earth leakage and circuit monitoring.
For CAT II/III systems, the design must also account for the applicable power-transfer and serviceability requirements.
Power Supply Reliability
CAT I
Reliable power is required for the applicable lighting system.
CAT II
Rapid transfer and reliable availability of critical lighting systems become much more important.
CAT III
Power continuity, redundancy and monitoring become fundamental to the operational approval of the system.
Simplified concept: Normal power → automatic transfer → alternate/emergency source → critical AGL loads. The exact transfer time, redundancy and backup architecture must be established from the governing standard and airport operational approval.
Monitoring Requirements
CAT I
- CCR status
- Circuit status
- Control status
- Major equipment faults
CAT II
- Approach circuits
- Runway centreline & TDZ lighting
- CCRs & flashers
- Power supply & control systems
Monitoring becomes even more critical because failures of essential equipment can directly affect low-visibility operational capability. The system should provide appropriate fault indication and support rapid maintenance response per the applicable requirements.
Photometric Verification
All three categories require appropriate verification of lighting performance.
CAT I
- Intensity
- Colour
- Alignment
- Beam distribution
- Crossbar performance
CAT II
- Inner approach configuration
- Red side-row performance
- Centreline alignment
- Runway centreline & TDZ lighting
CAT III
Same photometric principles apply, but consequences of degradation are more significant. Verification should include approach lighting, runway centreline, TDZ, edge and threshold lighting, colour, alignment, intensity and control functions.
Alignment Is Critical
A fixture can be electrically healthy but photometrically incorrect. Common causes include incorrect vertical/horizontal aiming, fixture movement, damaged optical assembly, prism contamination, improper installation and structural movement. For low-visibility systems, correct alignment of the approach and runway lighting system is essential.
Maintenance Comparison
| Maintenance Activity | CAT I | CAT II | CAT III |
|---|---|---|---|
| Routine visual / fixture inspection | Required | Required | Required |
| Cable inspection / insulation resistance | Required | Required | Required |
| CCR testing | Required | Required | Required |
| Photometry | Required | Required | Required |
| Alignment | Required | Critical | Critical |
| Flasher testing | Applicable | Required | Required |
| Power-transfer testing | Applicable | Critical | Extremely critical |
| Monitoring verification | Required | Critical | Extremely critical |
| Fault response | Normal | Fast | Very fast / operationally critical |
What Does CAT III Really Add?
It is tempting to say "CAT III adds more lights." That is not the correct engineering interpretation — CAT III adds system capability and integrity requirements.
The complete system may require greater attention to
- Navigation/landing system performance
- Lighting availability
- Power continuity
- Equipment redundancy
- Monitoring & fault detection
- Maintenance response
- Operational procedures & aircraft/crew capability
- Runway condition & low-visibility procedures
CAT III is a complete low-visibility operational system, not merely an approach-lighting upgrade.
CAT I vs CAT II vs CAT III: Engineering Matrix
| Engineering Area | CAT I | CAT II | CAT III |
|---|---|---|---|
| Visibility requirement | Higher | Lower | Very low |
| Decision height | Higher | Lower | Very low / potentially none |
| ALS | Typically ALSF-1 | Typically ALSF-2 | ALSF-2 in FAA configuration |
| Inner ALS complexity | Standard | Enhanced | Enhanced |
| Runway centreline | As applicable | Required | Required |
| TDZ lighting | As applicable | Required | Required |
| Power reliability | High | Very high | Extremely high |
| Monitoring | Required | Enhanced | Critical |
| Redundancy | Applicable | High | Very high |
| Photometry | Required | Required | Required |
| Alignment | Required | Critical | Critical |
| Dependency on visual reference | Higher | Reduced | Very limited |
| Overall system integrity | High | Very high | Highest |
Myths vs Facts
There is no simple FAA "ALSF-3 = CAT III" relationship. FAA uses ALSF-2 for CAT II/III visual guidance.
CAT III is primarily about complete system integrity and low-visibility operational capability, not simply light intensity.
Approach category depends on the complete approved operational system.
No. Navigation systems, runway lighting, power, monitoring, aircraft capability, procedures and regulatory approval are also involved.
CAT III runways use extensive lighting systems. However, the pilot's ability to obtain external visual reference can be extremely limited.
Engineering Workflow
Sequence: Determine approach category (CAT I/II/III) → identify governing standard → determine ALS configuration → determine runway lighting requirements → design approach series circuits → design runway centreline & TDZ circuits → select CCRs → calculate circuit loading → select isolation transformers → design primary & secondary cabling → design control & monitoring → design emergency/alternate power → installation → electrical testing → photometric testing → alignment verification → power transfer testing → integrated system testing → operational approval.
CAT I vs CAT II vs CAT III
The lighting system is one component of the overall approach and landing capability — CAT I, CAT II and CAT III are operational categories, not lighting-hardware tiers.
The key engineering lesson: CAT I, CAT II and CAT III are operational categories. ALSF-1 and ALSF-2 are approach-lighting configurations. They should not be treated as interchangeable classifications.
Engineering & Expert Notes
Treat Each Step as a Change of System, Not Just Intensity
PrincipleHigher categories bring more demanding lighting, monitoring and power. Check each part.
Check Secondary Power and Switch-Over Times
PowerVerify behaviour against the applicable requirement by test.
Look at Monitoring and Control Requirements
ControlStatus feedback and fault response matter more at lower visibility.
Verify With Current Official Documents
ComplianceConfirm requirements in the current edition before deciding.
Frequently Asked Questions
Decreasing decision height and visibility minima, paired with increasing requirements for runway lighting completeness, power reliability, monitoring and redundancy — CAT III is defined by system integrity, not just lighting hardware.
Not necessarily. In FAA practice, ALSF-2 provides visual guidance for both CAT II and CAT III approaches — CAT III is not automatically "ALSF-3."
It depends on the runway and design for CAT I, but is required for CAT II and CAT III, becoming increasingly critical to overall landing capability as visibility decreases.
No. Approach category depends on the complete approved system — navigation/landing equipment, runway lighting, power reliability, monitoring, redundancy, aircraft/crew capability and operational approval.
They are CAT III subcategories with progressively lower visibility and decision-height requirements, defined by the applicable regulatory framework — always verify exact values against the current governing standard rather than a generic table.
Because the pilot has progressively less external visual reference to fall back on, so the lighting and landing system itself must remain available and accurate through automatic transfer, redundancy and continuous monitoring.
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 — ALSF-1/ALSF-2 configurations, installation, photometric requirements and electrical systems.
ICAO Annex 14, Volume I
Aerodrome Design and Operations — precision approach categories, decision height and RVR criteria, 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 classify an airport as CAT I, CAT II or CAT III based only on its approach lighting system. Verify approach category, decision height, RVR, navigation/landing system, lighting configuration, power-transfer requirements, monitoring, redundancy, aircraft/crew requirements, low-visibility procedures and current governing standards.
CAT I → visual transition at a higher decision height. CAT II → enhanced visual guidance at lower visibility. CAT III → maximum low-visibility system integrity with very limited visual reference.
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