CAT III Holding Sign
A red mandatory instruction sign with white text that marks the Category III holding position, located at the maximum distance from the runway to protect ILS critical areas during the lowest‑visibility CAT III operations. Pilots and vehicles must stop and hold at this position before entering the runway.
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What Is It?
A CAT III Holding Sign is a red mandatory instruction sign with white text that identifies the holding position for Category III precision approach operations. This holding position is located further from the runway threshold than both the CAT I and CAT II positions, to provide maximum protection for the ILS critical and sensitive areas during the lowest‑visibility conditions (CAT IIIA, IIIB, and IIIC). The sign typically displays "CAT III" or "CAT III/II" 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, holding position markings, and ILS critical area signage.
Purpose & Function
CAT III approaches leave the flight crew with the least visual margin of any approach category, sometimes touching down with virtually no outside visual reference until very late in the flare, which makes ILS signal integrity more critical here than at any other stage of operation. The CAT III holding sign marks the outermost of the tiered holding positions specifically to keep waiting traffic clear of both the critical and the wider sensitive area during these approaches, since even a vehicle or aircraft that would be harmless under CAT I minima can distort the signal enough to matter when the crew has no visual backup. Its position is deliberately conservative, reflecting the fact that CAT III protection has the least tolerance for compromise of any holding position on the airfield.
Where Is It Used?
Installed at airports that support CAT III precision approach operations (CAT IIIA, IIIB, and IIIC). The sign is placed at the CAT III holding position, which is typically located 300–400 metres or more from the runway threshold, depending on the ILS critical area and aerodrome layout. 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 and sensitive areas during the most demanding approach conditions.
Installation & Location
Foundation and setback follow the CAT III holding position distance set by the ILS critical/sensitive area survey for that runway, which sits further back than the CAT II position and can differ between CAT IIIA/B/C depending on decision-height and rollout-guidance requirements. The frangible post is set on the taxiway shoulder clear of wingtip clearance, with illuminated panels cabled from the same circuit as the co-located stop bar so sign, stop bar, and marking are installed and inspected as a single holding-position package; because the CAT III position protects the more sensitive of the two boundaries, its setout is re-checked against the ILS critical area survey whenever runway category or ILS siting changes.
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 III" (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 III 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 III holding position sign shares its electrical circuit with the co-located stop bar, the same arrangement used at the CAT II position, so sign and stop bar are commanded, powered, and fault-monitored as a single loop. 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 it brightens and dims in step with the stop bar rather than being switched independently. Because the CAT III holding position protects both the ILS critical and sensitive areas, its stop bar is typically among the most safety-critical fixtures on the airfield during low-visibility operations, and a loop fault here — detected the usual way, as a voltage rise against the CCR's commanded current — is treated as high priority for field verification before CAT III operations resume. As with the CAT II sign, a non-illuminated, retroreflective CAT III panel sits entirely outside this circuit, carrying no cable or transformer and relying solely on reflected aircraft and vehicle light for visibility.
Why CAT III Protects a Second Zone
Physical Construction
Construction mirrors the CAT II sign it sits alongside: an extruded aluminium frame holds either a Class 1/2 retroreflective face with the "CAT III" (or "CAT III/II") legend cut in white on red, or a translucent acrylic panel backlit by an LED module for internally illuminated installations. The frame is weather-sealed and mounted on a frangible coupling and base plate rather than a rigid post, so the sign shears cleanly if struck rather than transmitting the impact into the pavement structure. Because CAT III protects a second, deeper zone than CAT II, the two signs are typically manufactured and installed as a matched pair along the same taxiway centreline, sharing the identical panel construction and mounting detail.
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 III" or "CAT III/II" (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 III 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 III" (or "CAT III/II") with the correct runway designation, and that the sign is sited at the surveyed CAT III holding position, clear of the ILS critical and sensitive areas.
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 III" (or "CAT III/II") together with the correct runway designation, and that the sign is sited at the surveyed CAT III holding position, clear of the ILS critical and sensitive areas.
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, especially for signs used in CAT III operations where visibility is critical.
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 (winter, fog-prone periods).
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 Near-Zero 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.
Near-Zero-Visibility Critical Area Discipline
OperationalBecause this sign marks a CAT III ILS critical/sensitive area boundary, treat any outage or obscuration as operationally significant: notify ATC immediately, NOTAM the condition, and suspend CAT III approaches until the sign is restored or an equivalent procedural safeguard is in place.
Engineering & Expert Notes
Zero-Visibility Margin Leaves No Room for “Mostly Working”
InspectionFor a CAT III boundary, treat partial degradation — slightly reduced brightness, early-stage sheeting fade — as a fail condition rather than waiting for outright failure. Crews have the least visual reference margin of any operation type here.
Confirm Which Survey Applies Before Repositioning
ComplianceCAT III critical/sensitive areas are often larger than the equivalent CAT II boundary at the same holding position. After pavement work, verify you're re-surveying against the CAT III reference specifically — reusing an old CAT II figure can leave the sign inside the wrong boundary.
Log Every Outage, Even Momentary
ReliabilityBrief interruptions during switching or circuit tests should be logged and reviewed, not waved off as test artefacts. CAT III operational approval rests on a demonstrated reliability history, not just the sign's current status.
Seal Condition Matters Most in the Weather That Triggers CAT III Ops
MaintenanceCAT III approaches typically coincide with dense fog — exactly the conditions that stress housing seals. Time gasket and desiccant checks ahead of fog season rather than on a generic annual cycle.
Give This Sign's Repair Queue Priority Over Standard Taxiway Signage
OperationalWhen triaging a fault list, a CAT III boundary sign should jump ahead of ordinary taxiway or apron signage almost automatically — the operational consequence of an unresolved fault here is disproportionate to how it looks on a severity scale.
Report to ATC in Real Time, Not at End-of-Shift
OperationalA fault found on routine patrol needs to reach ATC immediately rather than in an end-of-shift summary — CAT III approaches may already be in progress while the fault sits unreported.
Frequently Asked Questions
Short answers about the CAT III holding sign: purpose, siting, power, standards, maintenance and common faults.
Q1What is a CAT III holding sign?
A CAT III holding sign is a red mandatory instruction sign with white text that identifies the holding position for Category III precision approach operations (CAT IIIA, IIIB and IIIC). The position is further from the runway threshold than both the CAT I and CAT II positions, giving maximum protection to the ILS critical and sensitive areas in the lowest-visibility conditions.
Q2What does a CAT III holding sign display?
White text on a red background reading CAT III, or CAT III/II, with or without the runway designator, following ICAO Annex 14 and FAA standards.
Q3Where is the CAT III holding position?
Typically 300 to 400 metres or more from the runway threshold, depending on the ILS critical area and aerodrome layout. 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 major airports with low-visibility capability.
Q4How does a CAT III holding sign differ from a CAT II holding sign?
The CAT III holding position is further from the runway (typically 300 to 400 metres or more from the threshold, against about 250 to 300 metres for CAT II), because the lowest-visibility CAT III operations need the widest protection for the ILS critical and sensitive areas.
Q5How does a CAT III holding sign work with markings and stop bars?
It works with the CAT III 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 III holding sign powered?
An illuminated CAT III 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 III 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 III 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 III 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 III 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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