CAT II Holding Sign
A red mandatory instruction sign with white text that marks the Category II holding position, located further from the runway to protect ILS critical areas during low‑visibility operations. Pilots and vehicles must stop and hold at this position before entering the runway.
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What Is It?
A CAT II Holding Sign is a red mandatory instruction sign with white text that identifies the holding position for Category II precision approach operations. This holding position is located further from the runway threshold than the standard (CAT I) holding position to protect the ILS critical area and ensure signal integrity during low‑visibility approaches. The sign typically displays "CAT II" or "CAT II/III" along with the runway designation. It is a critical component for runway safety and incursion prevention during low‑visibility operations, and it is used in conjunction with stop bar lights and holding position markings.
Purpose & Function
The CAT II holding sign exists because the standard CAT I holding position, adequate for ordinary operations, sits too close to the runway to guarantee ILS signal integrity once low-visibility procedures are active. Its purpose is to mark a second, more conservative stop line so that any aircraft or vehicle still waiting to enter the runway remains clear of the ILS critical area for the duration of a CAT II approach, protecting the accuracy of the guidance the arriving crew is relying on. Because the applicable holding position changes depending on which approach category is in use, the sign also functions as a procedural cue, reminding crews and vehicle drivers that low-visibility procedures are active and that the closer CAT I line no longer applies.
Where Is It Used?
Installed at airports that support CAT II and CAT III precision approach operations. The sign is placed at the CAT II holding position, which is typically located 250–300 metres from the runway threshold, or as defined by the ILS critical area boundary. It is positioned on the side of the taxiway, at an installed height of 0.7 to 1.1 metres above the pavement, on a frangible support. These signs are used at major international airports with low‑visibility capabilities and are essential for ensuring that aircraft and vehicles do not enter the ILS critical area during precision approaches.
Installation & Location
The sign's post foundation is set on the taxiway shoulder at the CAT II holding position distance defined by the ILS critical area survey for that runway, not a generic setback figure, since the exact distance shifts with localizer siting and runway category. The frangible mount is set back clear of aircraft wingtip clearance but close enough to the pavement edge to stay legible from the cockpit at the holding point. Illuminated panels are fed by cable from the same circuit as the co-located stop bar and holding-position lights so the sign, stop bar, and pavement marking can be commissioned and inspected as one holding-position installation; non-illuminated panels rely on retroreflective sheeting and need only a mechanical fixing.
How Does It Work?
The sign displays white text on a red background, following ICAO Annex 14 and FAA standards. The text typically reads "CAT II" (with or without the runway designator) to indicate the holding position category. The sign may be non‑illuminated (using retroreflective sheeting) or internally illuminated (LED) for nighttime visibility. The sign is positioned to be clearly visible to pilots and vehicle drivers as they approach the holding point. It works in conjunction with CAT II holding position markings on the pavement and stop bar lights to ensure compliance with ATC instructions during low‑visibility operations. The sign is mounted on a frangible support to minimise aircraft damage in case of collision.
System Architecture & Control Logic
An illuminated CAT II holding position sign shares its electrical circuit with the co-located stop bar, so the two are commanded, powered, and fault-monitored as a single loop rather than as separate systems. The Airfield Lighting Control and Monitoring System (ALCMS) selects the intensity step for that holding-position loop based on RVR and ATC instruction, and the CCR regulates the loop to the corresponding fixed current; the sign's isolating transformer taps that same current, so the sign's illumination rises and falls in step with the stop bar it sits beside rather than being switched independently. This matters operationally: an ATC command to illuminate the stop bar to bar traffic at the CAT II position also brings the sign to full intensity at the same time, reinforcing the same instruction visually in two ways at once. Fault detection is loop-based — a voltage rise at constant current flags an open-circuit lamp or failed transformer somewhere on the holding-position loop, without by itself distinguishing whether the fault lies in the sign, the stop bar, or another fixture on the same circuit, so field inspection follows to isolate the specific component. Where the sign is a non-illuminated, retroreflective panel instead, none of this applies — it carries no cable or transformer and depends entirely on aircraft and vehicle lights for visibility, with no loop membership or fault reporting of its own.
Where CAT II Sits on the Distance Ladder
Physical Construction
The panel is built on an extruded aluminium frame carrying a flat face of either engineering-grade retroreflective sheeting (Class 1 or 2) with the "CAT II" legend cut or screen-printed in white on red, or a translucent polycarbonate/acrylic panel with the same legend applied as a perforated film for internally illuminated versions, lit by an LED module run at low voltage from the adjacent circuit. The frame is sealed against the pavement environment and bolted to a frangible coupling — a breakaway post engineered to shear at a defined load if struck by an aircraft or vehicle — set on a base plate anchored below grade, keeping the sign rigid enough to hold its legend flat while still failing safely on impact.
Technical Parameters
| Parameter | Typical Value |
|---|---|
| Colour | Red background, white text (ICAO / FAA) |
| Dimensions | 1.2 m × 0.6 m (typical) or as per site requirements |
| Text Height | 200–400 mm (by runway code: 200/300/400 mm) |
| Text Content | "CAT II" or "CAT II/III" (with runway designator if required) |
| Illumination | Non‑illuminated (retroreflective) or internally illuminated (LED) |
| Retroreflective Class | Class 1 or Class 2 (ASTM D4956) |
| Mounting Height | 0.7–1.1 m above pavement (installed max., by runway code/legend size) |
| Frangible Support | Breakaway coupling or frangible post |
| Protection Class | IP65 (weatherproof) |
| Operating Temperature | -40°C to +60°C |
Electrical Requirements
| Requirement | Typical Value |
|---|---|
| Circuit Type | Series circuit, constant current (CCR fed) — illuminated variant only |
| Rated Series Current | 6.6A (3 to 5-step: 2.8A / 3.4A / 4.1A / 5.2A / 6.6A) |
| Isolation | Via dedicated isolating transformer, not directly connected to series loop |
| Transformer Secondary | Matched to sign wattage; sized to series circuit rating |
| Cable Type | L-824 single-conductor, direct-burial rated, 5kV or 600V per circuit design |
| Connector | L-823 primary connector, submersible-rated splice |
| Insulation Resistance | ≥ 50 MΩ (megger test, cable-to-ground) |
| Earthing/Grounding | Sign body and base can bonded to airfield earth grid |
| Fault Protection | CCR-level ground fault and open-circuit detection |
| Non-Illuminated Variant | No electrical supply required — passive retroreflective panel only |
Photometric Requirements
| Parameter | Typical Value |
|---|---|
| Background Luminance (Red) | ≥ 30 cd/m² average (illuminated variant), uniformity ratio ≤ 10:1 (max:min) |
| Legend Luminance (White) | ≥ 150 cd/m² average |
| Legend-to-Background Contrast Ratio | 5:1 to 10:1 per ICAO Annex 14 Attachment A |
| Retroreflective Coefficient (Non-Illuminated Variant) | RA ≥ 50 cd/lux/m² (white), ≥ 15 cd/lux/m² (red) per ASTM D4956 Class 1/2 |
| Chromaticity (Red Background) | Within ICAO Annex 14 aeronautical red boundary (CIE coordinates) |
| Chromaticity (White Legend) | Within ICAO Annex 14 white boundary (CIE coordinates) |
| Viewing Angle | ± 5° horizontal, legible from typical cockpit eye height at the holding position |
| Luminance Tolerance | ± 25% of rated value across the panel face |
| Maintenance Factor | ≥ 50% of new-panel luminance before lamp/LED or sheeting replacement is due |
| Test Method | Luminance meter/photometer at normal and oblique angles, or retroreflectometer for the non-illuminated variant |
Applicable Standards
ICAO Annex 14
CAT II holding sign colours, dimensions, and placement
FAA AC 150/5340-18
Standards for Airport Sign Systems — siting, design, and colour convention for this sign type
FAA AC 150/5345-44
Equipment/fixture specification for the sign unit itself (siting, design and colour are governed by AC 150/5340-18 above)
ASTM D4956
Retroreflective sheeting performance standards
IEC 60598
Luminaire requirements for illuminated signs
Installation Requirements
Setting-Out & Siting Survey
CivilSurvey and mark the sign position from the design/siting drawing before excavation, confirming clearance from the taxiway/runway edge safety area and any frangibility height limit.
Foundation / Base Plate Installation
CivilExcavate and set the base plate or footing to design depth, keeping the finished mount level and within the frangible height limit for the sign's location.
Frangible Mount Fitting
MechanicalFit the frangible post or breakaway coupling and torque to the specified value, confirming it shears as designed under lateral load and does not bind.
Cable Route & Connector Termination
ElectricalWhere the sign is internally illuminated, lay the duct/cable route to the panel and terminate the L-823 connector per manufacturer procedure, keeping the splice fully submersible-rated before backfill.
Isolating Transformer Matching
ElectricalWhere the sign is internally illuminated, confirm the isolating transformer wattage and ratio match the panel and the series circuit's rated current step before connection.
Panel Legend & Content Setting
QualityBefore the panel is fixed and the position closed out, confirm the panel reads "CAT II" (or "CAT II/III") with the correct runway designation, and that the sign is sited at the surveyed CAT II holding position rather than the CAT I position.
Retroreflective Sheeting Application
MechanicalApply and inspect the retroreflective sheeting for correct grade, alignment, and freedom from bubbles or edge lifting before the panel is mounted.
Pre-Backfill Sign-off
DocumentationRecord base/footing position, frangible coupling torque, cable route, and panel content as-built before backfill, since these are not inspectable once the site is closed.
Testing & Commissioning
Insulation Resistance (Megger) Test
Pre-EnergisationWhere the sign is internally illuminated, megger-test the isolating transformer and internal wiring before the circuit is energised, confirming insulation resistance clears the acceptable minimum ahead of backfill.
Loop Continuity & Resistance Test
Pre-EnergisationFor an illuminated installation, verify series loop continuity through the sign and record loop resistance, ruling out open circuits or high-resistance splices before first energisation.
Primary/Secondary Voltage Verification
EnergisationWith the CCR energised, confirm secondary voltage at the illuminated sign's L-823 connector matches the transformer's rated ratio at the commissioned current step.
CCR Brightness Step Response
EnergisationCycle the CCR through every intensity step and confirm an illuminated panel responds correctly at each, ruling out a marginal transformer or a loose connection.
Retroreflectivity / Luminance Verification
Final AcceptanceMeasure sheeting retroreflectivity with a retroreflectometer against ICAO/FAA commissioning minimums; where the panel is internally illuminated, also verify luminance and colour uniformity with a luminance meter.
Legend & Content Verification
Final AcceptanceConfirm the panel reads "CAT II" (or "CAT II/III") together with the correct runway designation, and that the sign is sited at the surveyed CAT II holding position rather than the CAT I position.
Frangible Coupling & Mounting Verification
Pre-EnergisationConfirm the frangible coupling, base depth, and panel height meet as-built tolerances before the site is finally backfilled.
Documentation & As-Built Sign-off
Final AcceptanceRecord retroreflectivity/luminance readings, and (where illuminated) insulation, continuity, and voltage readings, alongside as-built position in the commissioning dossier for handover.
Inspection & Maintenance
Retroreflectivity Check
AnnualMeasure sheeting retroreflectivity with a retroreflectometer against ICAO / FAA minimums, and budget for sheeting replacement roughly every 5–7 years.
Illumination System Test
AnnualTest internal LED or fluorescent illumination for uniform, even lighting so the sign remains legible at night, especially ahead of low-visibility seasons.
Frangible Coupling Inspection
AnnualCheck the coupling or post for brittleness or corrosion and replace per manufacturer schedule to preserve breakaway performance on impact.
Seal & Cable Gland Check
AnnualInspect housing seals and cable glands for degradation, since moisture ingress through damaged seals is a leading cause of corrosion and electrical faults.
Sign Face Cleaning
QuarterlyClean dirt, de-icing fluid residue, and rubber deposits off the sign face to keep both reflectivity and legibility within spec.
Insulation Resistance Test
AnnualMegger-test internal wiring and any isolating transformer to confirm insulation resistance stays above minimum, catching moisture ingress early.
Common Field Issues
Retroreflective Sheeting Degradation
UV exposure, weathering, and abrasion can reduce the reflectivity of the red background and white text, making the sign difficult to read at night or in low visibility. Regular inspection and replacement (typically every 5–7 years) is essential.
Internal Illumination Failure
LED modules inside illuminated signs can fail, reducing visibility at night. Regular testing of the illumination system and replacement of failed components is required, especially before low‑visibility seasons.
Frangible Support Fatigue
Breakaway couplings can become brittle over time due to weather and vibration. Regular inspection and replacement as per manufacturer guidelines are required to ensure breakaway performance.
Moisture Ingress
Water ingress through seals can cause corrosion and electrical faults. Routine seal checks and use of desiccants are recommended, especially in wet climates.
Troubleshooting Guide
Red Background or White Text Unreadable at Night
Illuminated Sign Dark or Flickering
Moisture or Condensation Inside Housing
Sign Leaning, Loose, or Misaligned
Failure Modes & Root Causes
LED Panel / Internal Light Source Failure
Retroreflective Sheeting Degradation
Frangible Coupling Fatigue / Impact Damage
Power/Circuit Connector Degradation
Legibility Shortfall in Low-Visibility Conditions
Safety & Operational Notes
Circuit Isolation Before Access
Electrical SafetyDe-energise and lock out the feeder circuit at the CCR or panel, then verify zero energy at the base can before opening the housing or handling cable connectors.
Airside Work Coordination
OperationalAny work on or near the movement area requires ATC/airside coordination and a NOTAM where the sign will be dark, obscured, or out of tolerance, with vehicle and personnel movement kept clear of the taxiway or runway strip per the aerodrome's safety management procedures.
Frangibility Not to Be Defeated
MechanicalNever substitute the frangible coupling with a rigid fitting or over-torque it during installation or repair; the coupling must still shear cleanly on aircraft impact to limit damage under the frangibility requirement.
FOD Control at the Base
OperationalKeep tools, fasteners, and removed gaskets accounted for at all times when the base can or housing is open, and secure or remove any loose material before reinstating the sign, to prevent foreign object debris on the movement area.
Insulation Test Before Re-Energisation
Electrical SafetyAfter any cable, connector, or panel work, megger test the affected circuit before restoring power, since a residual fault at re-energisation can trip the feeder circuit or damage healthy units on the same loop.
PPE & Grounding
Personnel SafetyUse insulated tools and appropriate PPE when working on circuit connections, and confirm the sign housing and base can are bonded/grounded per the electrical design before reconnecting the circuit.
Precision-Approach Critical Area Discipline
OperationalBecause this sign marks a CAT II ILS critical/sensitive area boundary, treat any outage or obscuration as operationally significant: notify ATC immediately, NOTAM the condition, and restrict CAT II approaches until the sign is restored or an equivalent procedural safeguard is in place.
Engineering & Expert Notes
Hold This Sign to a Tighter Margin, Not Just a Standard Pass
InspectionBecause this sign protects a CAT II critical area where the visual recognition margin is already reduced, treat photometric and retroreflectivity readings that barely clear the standard minimum as due for replacement soon — not as a pass to file away until the next cycle.
Rule Out Sheeting Fade Before Condemning the LED Driver
TroubleshootingA housing that “reads dark” on a night patrol isn't automatically a driver or LED fault — take a reflectivity/luminance reading first, since degraded sheeting on a partially-illuminated design can look like an electrical failure when it isn't.
Time Seal Inspection to the Fog Season, Not the Calendar
MaintenanceGasket and desiccant condition matters most exactly when CAT II operations actually happen. Schedule the housing seal check ahead of the season when low-visibility approaches pick up, rather than on a generic annual date that might land mid-dry-season.
Don't Let Coupling Wear Wait for a Legibility Complaint
MechanicalA leaning or slightly misaligned frangible coupling on this sign should trigger priority replacement even if the face still reads fine, because the geometry it marks matters as much as its legibility — a tilted sign can misrepresent the boundary it's protecting.
Any Reposition Needs the ILS Survey, Not a Field Estimate
ComplianceAfter pavement work disturbs the foundation, re-survey the sign's position against the ILS critical area protection survey specifically — a position that looks “close enough” by eye can still sit outside the boundary the survey defines.
Log Marginal Readings, Not Just Fails
ReliabilityKeep a running record of readings that pass but trend downward, not only outright failures. A sign heading toward the minimum ahead of the low-visibility season is worth pre-emptive replacement rather than waiting for it to actually fail.
Frequently Asked Questions
Short answers about the CAT II holding sign: purpose, siting, power, standards, maintenance and common faults.
Q1What is a CAT II holding sign?
A CAT II holding sign is a red mandatory instruction sign with white text that identifies the holding position for Category II precision approach operations. The position is further from the runway than the standard CAT I holding position, to protect the ILS critical area and signal integrity during low-visibility approaches.
Q2What does a CAT II holding sign display?
White text on a red background reading CAT II, or CAT II/III, with or without the runway designator, following ICAO Annex 14 and FAA standards.
Q3Where is the CAT II holding position?
Typically 250 to 300 metres from the runway threshold, or as defined by the ILS critical area boundary. The sign stands at the side of the taxiway at an installed height of 0.7 to 1.1 metres above the pavement on a frangible support, at airports that support CAT II and CAT III precision approaches.
Q4How does a CAT II holding sign differ from a CAT III holding sign?
The CAT III holding position is further from the runway than both CAT I and CAT II, typically 300 to 400 metres or more from the threshold, to give maximum protection to the ILS critical and sensitive areas in the lowest-visibility (CAT IIIA, IIIB and IIIC) operations. The CAT II position typically sits 250 to 300 metres from the threshold.
Q5How does a CAT II holding sign work with markings and stop bars?
It works with the CAT II holding position markings on the pavement and stop bar lights to ensure compliance with ATC instructions during low-visibility operations. Together they mark the point where pilots and vehicles must stop and hold before entering the runway.
Q6How is an illuminated CAT II holding sign powered?
An illuminated CAT II holding sign is wired onto the constant current series circuit for its section of the airfield and fed through its own isolating transformer rather than directly from the loop. The Constant Current Regulator (CCR) holds the loop at a fixed current (up to 6.6 A, with steps of 2.8, 3.4, 4.1, 5.2 and 6.6 A) and the ALCMS selects the brightness step, so the sign burns at the same intensity as the other fixtures on that loop. The non-illuminated variant is a passive retroreflective panel with no electrical supply.
Q7Which standards apply to CAT II holding signs?
ICAO Annex 14 covers colours, dimensions and placement, and FAA AC 150/5340-18 covers siting, design and colour convention. FAA AC 150/5345-44 is the equipment specification for the sign unit, ASTM D4956 sets retroreflective sheeting performance, and IEC 60598 covers luminaires for illuminated signs.
Q8What are the photometric requirements for a CAT II holding sign?
On the illuminated variant the red background should average at least 30 cd/m² (uniformity no worse than 10:1) and the white legend at least 150 cd/m², giving a 5:1 to 10:1 contrast ratio. The non-illuminated variant needs a retroreflective coefficient of at least 50 cd/lux/m² for white and 15 cd/lux/m² for red per ASTM D4956 Class 1 or 2.
Q9How often should CAT II holding signs be inspected and maintained?
Typical intervals are quarterly for cleaning dirt, de-icing fluid residue and rubber deposits off the sign face, and annual for retroreflectivity measurement, illumination testing, frangible coupling inspection, seal and cable gland checks, and insulation resistance (megger) testing. Retroreflective sheeting is usually budgeted for replacement every 5 to 7 years.
Q10What are the common field issues with CAT II holding signs?
Four issues recur: retroreflective sheeting degradation from UV, weathering and abrasion (replacement is typically due every 5 to 7 years); failure of the internal LED illumination, which cuts night visibility; frangible support fatigue, as breakaway couplings turn brittle with weather and vibration; and moisture ingress through damaged seals or cable glands, which causes corrosion and electrical faults. Routine seal checks and desiccants are recommended.
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Please read before relying on any content on this page.
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