CAT I CAT II
Precision Approach Lighting for Two Different Visual Environments — both categories support precision instrument approaches, but CAT II demands a more structured inner approach configuration because the pilot may acquire visual references at a lower decision height and lower visibility.
At a Glance
| Parameter | CAT I | CAT II |
|---|---|---|
| Approach type | Precision approach | Precision approach |
| Typical decision height | Not lower than 200 ft | Below 200 ft but not lower than 100 ft |
| Visibility requirement | Higher than CAT II | Lower than CAT I |
| RVR requirement | FAA: typically ≥ 1,800 ft · ICAO: ≥ 550 m | FAA: typically ≥ 1,200 ft (≈ 366 m) · ICAO: ≥ 300 m |
| Approach lighting | Precision approach lighting | Enhanced precision approach lighting |
| Typical FAA ALS association | ALSF-1 | ALSF-2 |
| Inner approach configuration | Less complex | More extensive visual references |
| Red side-row lighting | Not the defining CAT I feature | Important feature of ALSF-2 |
| 500-ft crossbar | Not the defining CAT I feature | Included in ALSF-2 configuration |
| Runway centreline lighting | Depends on runway category/design | Required for CAT II operations |
| Touchdown zone lighting | Depends on applicable runway system | Required for CAT II operations |
| System monitoring/reliability | Required per applicable system | More demanding operational requirements |
What Is a CAT I & CAT II Approach?
CAT I
A precision instrument approach category that allows the aircraft to descend to a decision height of not less than 200 ft under the applicable criteria. At the decision height, the pilot must have the required visual references to continue visually.
The lighting system must guide the pilot to identify
- Runway alignment
- Runway centreline
- Threshold location
- Runway environment
- Approach direction
- Distance/proximity to threshold
CAT II
A precision approach category designed for lower decision heights and lower visibility than CAT I. Under ICAO definitions, CAT II operations use a decision height below 200 ft but not lower than 100 ft, together with the applicable RVR requirements.
Why this matters
Because the pilot may acquire visual references much closer to the runway, the lighting system must provide strong and unambiguous visual information during the final stage of the approach — which is why CAT II approach lighting is generally more extensive in the inner approach area.
From Instrument Guidance to Landing
Both categories rely on the same underlying transition, but CAT II compresses it into a shorter, lower-visibility final segment.
Visual sequence: Instrument guidance → approach lighting → runway visual references → landing.
Decision Height, Visibility & Visual Transition
CAT I
Higher decision height + higher visibility + less demanding visual transition.
CAT II
Lower decision height + lower visibility + more demanding visual transition.
Therefore, CAT II requires a more comprehensive combination of approach lighting, runway centreline lighting, touchdown zone lighting, runway edge lighting, threshold lighting, centreline visual references, monitoring, reliability and backup arrangements.
Approach Lighting Configuration
| Feature | CAT I | CAT II |
|---|---|---|
| Approach centreline | Yes | Yes |
| Approach crossbars | Yes | Yes |
| Sequenced flashing | Where applicable | Where applicable |
| Inner approach visual references | Standard precision configuration | Enhanced configuration |
| Red side-row bars | Not the defining CAT I feature | Used in ALSF-2 |
| 500-ft crossbar | Not the defining CAT I feature | Used in ALSF-2 |
| Runway centreline lights | Design-dependent | Required |
| Touchdown zone lights | Design-dependent | Required |
| High-intensity approach lights | Yes | Yes |
| Low-visibility visual guidance | Moderate | Enhanced |
Typical FAA Configuration: ALSF-1
ALSF-1 provides a high-intensity approach lighting system with sequenced flashing lights. The inner approach area provides the pilot with visual references associated with the centreline, crossbars, threshold and the red terminating/wing-bar arrangement.
Inner approach references
- Centreline
- Crossbars
- Threshold
- Red terminating/wing-bar arrangement
The system supports the transition from the instrument approach to visual runway acquisition.
Typical FAA Configuration: ALSF-2
ALSF-2 provides additional visual references in the inner approach area, giving the aircraft a more structured visual environment as it approaches the runway under lower visibility conditions.
Configuration includes
- White centreline bars
- Red side-row bars
- 500-ft white crossbar
- 1,000-ft crossbar
- Sequenced flashing lights
- High-intensity approach lighting
Why Does CAT II Need More Inner Lighting?
Imagine two aircraft approaching the same runway.
CAT I
The pilot reaches the decision height at approximately 200 ft or higher — there is comparatively more opportunity to acquire the runway environment.
CAT II
The pilot may reach decision height at 100–199 ft — the available visual acquisition time is reduced.
Engineering point: Lower DH + lower visibility requires stronger and more structured visual references. This is why the inner approach configuration becomes particularly important.
Runway Lighting Difference
Approach lighting alone does not make a runway CAT II capable — the runway lighting system also becomes more comprehensive.
CAT I
- Runway edge lights
- Threshold lights
- Runway end lights
- Runway centreline lights where required
- Other applicable visual aids
CAT II
- High-intensity runway edge lighting
- Runway threshold lighting
- Runway centreline lighting
- Touchdown zone lighting
- Appropriate approach lighting
- Supporting monitoring and control systems
The approach and runway lighting systems must therefore be considered as one operational visual guidance system.
CAT I vs CAT II: Touchdown Zone Lighting
CAT I
Touchdown zone lighting is not universally required simply because an approach is CAT I. Its requirement depends on the applicable runway configuration and governing standard.
CAT II
Touchdown zone lighting is a critical part of the visual reference system, providing the pilot with a strong indication of:
- Runway touchdown area
- Longitudinal position
- Runway alignment
- Proximity to the threshold
Electrical Engineering Perspective
From an AGL engineering point of view, CAT II systems require careful coordination of multiple lighting circuits.
Typical architecture: CCR → primary series circuit → isolation transformers → approach/runway fixtures → monitoring & control.
Separate circuits may be provided for
- Approach lighting
- Sequenced flashers
- Runway edge lights
- Runway centreline lights
- Touchdown zone lights
- Threshold lights
- Other runway visual aids
The exact circuit architecture must follow the approved project design.
Constant Current Regulator Sizing
The Constant Current Regulator (CCR) supplies the required constant current to the airport series circuit. For CAT II systems, the lighting system may require multiple selectable intensity steps and reliable control of the different circuits.
Engineering checks should include
- CCR capacity
- Circuit load
- Isolation transformer ratings
- Cable length
- Voltage drop
- Series circuit resistance
- Insulation resistance
- Earth leakage
- Circuit monitoring
- Control-system interface
Reliability Becomes More Important as Visibility Decreases
The difference between CAT I and CAT II is not only the physical lighting configuration — system reliability becomes increasingly important as operating visibility decreases.
Monitoring may include
- CCR status
- Circuit status
- Approach lighting status
- Runway centreline status
- TDZ lighting status
- Flasher status
- Control-system faults
- Power supply status
The actual monitoring requirements must be established from the governing operational and equipment standards.
Verification Is Not Just "Lights On"
For both CAT I and CAT II, simply switching on every fixture is not enough. The system should be verified for photometry, alignment and functional performance.
Photometry
- Intensity
- Distribution
- Colour
- Uniformity
- Applicable photometric performance
Alignment
- Centreline alignment
- Crossbar alignment
- Vertical aiming
- Horizontal aiming
- Red-light aiming
- Inner approach configuration
For CAT II, accurate alignment becomes particularly important because the pilot has less visual information available during the final approach.
Full Engineering Parameters
| Area | CAT I | CAT II |
|---|---|---|
| Decision height | ≥ 200 ft | < 200 ft and ≥ 100 ft |
| Visibility | Higher | Lower |
| RVR | FAA ≥ 1,800 ft · ICAO ≥ 550 m | FAA ≥ 1,200 ft (≈ 366 m) · ICAO ≥ 300 m |
| Approach lighting | Precision | Enhanced precision |
| Typical FAA ALS | ALSF-1 | ALSF-2 |
| Inner visual references | Standard | Enhanced |
| Runway centreline | Depends on design | Required |
| TDZ lighting | Depends on design | Required |
| Monitoring | Applicable | More demanding |
| Operational reliability | High | Higher |
| Maintenance discipline | High | Very high |
| Photometry | Required | Required |
| Alignment | Required | Critical |
| Low-visibility capability | Lower | Higher |
Myths vs Facts
CAT II is a complete operational approach category. The lighting system is only one part of the overall CAT II system.
No. CAT II capability depends on the complete approved system, including navigation, runway lighting, approach lighting, equipment reliability, procedures and operational requirements.
The important change is the visual information and configuration, particularly in the inner approach area.
Requirements depend on the specific runway, approach type and governing standard. It should not be generalized.
Operational readiness involves much more than fixture operation. Monitoring, redundancy, reliability, photometry, alignment, navigation equipment and procedures must all satisfy the applicable requirements.
CAT II Approach-Lighting Project Workflow
Sequence: Define approach category → determine applicable standard → establish approach lighting configuration → design approach series circuits → design runway centreline & TDZ lighting → select CCRs → calculate circuit loads → select isolation transformers → design primary & secondary cabling → design control & monitoring → installation → electrical testing → photometric testing → alignment verification → integrated commissioning → operational approval.
CAT I vs CAT II Approach Lighting
CAT II is not simply a more powerful version of CAT I approach lighting. The fundamental difference is the operational environment in which the visual system must perform.
CAT II allows approach operations at lower decision heights and lower visibility than CAT I. Consequently, the approach and runway lighting system must provide stronger, more structured and reliable visual references.
ALSF-1 is typically associated with CAT I; ALSF-2 is typically associated with CAT II. But the final CAT classification must always be determined from the complete approved approach system and applicable regulatory requirements, not from the approach-lighting system alone.
Engineering & Expert Notes
Lighting Is One Part of the Category
ContextCategory depends on the whole system, not the lights alone. Check the complete requirement.
Review Reliability and Secondary Power
ReliabilityLower-visibility operation raises availability expectations. Include power and monitoring.
Test Inner Approach Elements Carefully
Inner ApproachInner elements carry extra weight at low visibility. Verify them with care.
Document the Basis for the Category
RecordsKeep the design basis and verification evidence together.
Frequently Asked Questions
The decision height and visibility minima — CAT II allows operations down to a lower decision height and lower visibility, which requires a more structured inner approach lighting configuration.
ALSF-1 is typically associated with CAT I and ALSF-2 with CAT II, but the actual approach lighting configuration depends on the approved airport design and applicable standard.
Yes. Touchdown zone lighting is a required, critical part of the CAT II visual reference system.
No. CAT II capability depends on the complete approved system — navigation, runway lighting, approach lighting, reliability, monitoring, procedures and operational approval.
Because the pilot may reach decision height much closer to the runway, leaving less time to acquire visual references — so the inner approach zone must provide stronger, more structured visual information.
CAT II generally carries more demanding monitoring and reliability requirements, since system failures have a greater operational impact at lower visibility minima.
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-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 design a CAT I or CAT II system from a generic web diagram. The final installation drawing should always be developed from the current governing standard, approved airport design, approach category and project-specific requirements.
CAT II is not a brighter CAT I — it is a lower-DH, lower-visibility operational category that demands a more comprehensive, more reliable visual guidance system.
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