AGL Component Library By Precision AGL Academy

Mandatory Instruction Sign

A red sign with white text, indicating mandatory instructions that must be followed by pilots and vehicle drivers on the airfield. Commonly used for runway holding positions, ILS critical areas, and no‑entry zones. Provides unambiguous regulatory guidance to ensure safe surface movements.

Mandatory Instruction Sign

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Overview

What Is It?

A Mandatory Instruction Sign is a red rectangular sign with white text and symbols that conveys a regulatory instruction that must be obeyed by pilots and vehicle operators. These signs are used at critical locations such as runway holding positions, ILS critical area boundaries, and no‑entry points. The red background and white text are internationally standardized to ensure immediate recognition. The signs may be illuminated (internally or externally) for nighttime visibility and are typically reflective for passive visibility. They are a crucial component of airfield surface movement guidance and control systems.

Purpose

Purpose & Function

The mandatory instruction sign is the top tier of the airfield's colour-coded signage hierarchy, reserved for the small set of locations where the instruction is not advisory but a regulatory requirement that must be followed regardless of clearance status: holding positions, ILS critical area limits, and no-entry points chief among them. Its purpose is to make that distinction unmistakable at a glance, so that a pilot or driver scanning ahead never has to read the full text of a sign to know whether it demands compliance; the red background alone carries that meaning. This universal colour coding is what allows the sign to function reliably across every airport that follows ICAO or FAA standards, regardless of the specific instruction printed on it.

Application

Where Is It Used?

Installed at runway holding positions (CAT I, II, III), ILS critical area boundaries, stop bars, no‑entry locations, and other points where a mandatory instruction must be conveyed. They are placed on the side of the taxiway or runway, typically at a height of 1.5 to 2 metres above the pavement. They are used at all civil and military airfields that operate under instrument flight rules (IFR) or have complex taxiway layouts.

Placement

Installation & Location

Posts are founded at the specific holding position, ILS boundary, stop bar, or no-entry point the sign protects, using the same setback and frangible-mount convention as the underlying location (the CAT I/II/III holding distance, the ILS critical area survey line, or the closed-taxiway boundary as applicable) rather than a single generic rule across the whole category. Because a mandatory instruction sign is paired with a physical control (a stop bar, a pavement marking, a barrier) at nearly every location it serves, its setout is checked against that companion control's position during installation and inspection rather than in isolation. Illuminated panels share cabling with the associated stop bar or holding-position lighting circuit where one exists; standalone locations are fed from the nearest taxiway distribution point.

Function

How Does It Work?

The sign displays a specific instruction using white text on a red background, following ICAO Annex 14 and FAA standards. Common instructions include "RUNWAY" (with a runway designator), "ILS" (critical area), "NO ENTRY", and "STOP" (at holding positions). The sign is either non‑illuminated (using retroreflective sheeting) or illuminated (with internal LED or fluorescent lamps) for nighttime operations. The text and symbols are designed for legibility at a distance of up to 200 metres. The sign is mounted on a frangible or breakaway support to minimize aircraft damage in case of collision.

Architecture

System Architecture & Control Logic

Mandatory instruction signs carry the same two-model split as other AGL signage, but with a stricter illumination requirement given their safety-critical messages (RUNWAY, ILS, NO ENTRY, STOP). Where illuminated, the sign is wired onto the constant current series circuit serving that holding position or taxiway: an ALCMS selects the required brightness step, the CCR regulates the whole loop to a fixed current, and the sign panel — fed through its own isolating transformer — burns at the same intensity as every other fixture on that loop, with a voltage rise at constant current flagging a loop-level fault rather than a sign-specific one. Because a mandatory instruction sign is often required to be illuminated at night or in low visibility precisely where retroreflective sheeting alone would not be reliable enough, many installations dispense with the passive-only option and default to the illuminated, ALCMS-controlled path even where a simpler information or exit sign nearby might still run on retroreflective sheeting with no electrical control at all.

Illuminated variant: ALCMS → CCR → Loop → Sign Panel. Retroreflective variant: no electrical path at all.

ILLUMINATED VARIANT RETROREFLECTIVE VARIANT ALCMS Selects brightness step (RVR / ATC) AIRCRAFT / VEHICLE LANDING LIGHTS External light source, not airfield-commanded CONSTANT CURRENT REGULATOR Converts step command to fixed loop current RETROREFLECTIVE SHEETING Prismatic / glass-bead material on the sign face ISOLATING TRANSFORMER + HOLD-POSITION LOOP Often mandatory here — retroreflective alone may not suffice NO POWER, NO LOOP, NO ALCMS Passive material — nothing to command or monitor INTERNALLY-ILLUMINATED SIGN PANEL Red background lit to the same step as the loop BEAM REFLECTED BACK TO SOURCE Bright return along the incoming light path SAME MESSAGE READ BY THE PILOT Colour, symbol and text are identical either way ONLY THE ILLUMINATED VARIANT IS UNDER ALCMS / CCR CONTROL
System Logic

What Each Message Requires

Encounter
Sign Boundary Reached
Runway holding position, ILS area, or restricted zone
Read
Message Displayed
White text on red, standardized by ICAO / FAA
"RUNWAY [designator]"
Runway Holding Position
Stop; do not cross without ATC clearance
"ILS"
ILS Critical Area Boundary
Stop outside area whenever ILS is in use
"NO ENTRY"
Restricted / No-Entry Zone
Do not proceed under any circumstance
"STOP"
Stop Bar / Holding Point
Stop immediately, contact ATC before proceeding
All four messages carry the same weight — red background signage is always mandatory, never advisory, regardless of which instruction is posted. The sign's frangible mount handles the physical failure case; the message itself is what handles the procedural one, which is why legibility at up to 200 m matters as much as the mounting.
Construction

Physical Construction

The panel is an extruded aluminium frame around a translucent acrylic or polycarbonate face with the white-on-red legend applied as a perforated film, internally illuminated by an LED module (or an external floodlight on older installations) rather than left to retroreflective sheeting alone, since a mandatory instruction has to remain legible and unambiguous in every lighting condition rather than only under aircraft landing-light glare. The frame is gasket-sealed to IP65 and mounted on a frangible coupling engineered to a specified shear force, set on a below-grade base plate, so the sign fails safely if struck while still holding rigidly enough in normal service to keep the instruction legible. This construction is the base format that every specific mandatory sign in this library — holding position, no-entry, CAT II/III, ILS critical area — builds its own legend onto.

Specification

Technical Parameters

ParameterTypical Value
ColourRed background, white text (ICAO / FAA)
Dimensions1.2 m × 0.6 m (typical) or as per site requirements
Text Height150–200 mm (depending on viewing distance)
IlluminationInternally illuminated (LED) or externally illuminated (floodlight)
Retroreflective ClassClass 1 or Class 2 (ASTM D4956)
Mounting Height1.5–2 m above pavement
Frangible SupportBreakaway coupling or frangible post (shear force specified)
Protection ClassIP65 (weatherproof)
Operating Temperature-40°C to +60°C
Power Supply

Electrical Requirements

RequirementTypical Value
Circuit TypeSeries circuit, constant current (CCR fed) — illuminated variant only
Rated Series Current6.6A (3 to 5-step: 2.8A / 3.4A / 4.1A / 5.2A / 6.6A)
IsolationVia dedicated isolating transformer, not directly connected to series loop
Transformer SecondaryMatched to sign wattage; sized to series circuit rating
Cable TypeL-824 single-conductor, direct-burial rated, 5kV or 600V per circuit design
ConnectorL-823 primary connector, submersible-rated splice
Insulation Resistance≥ 50 MΩ (megger test, cable-to-ground)
Earthing/GroundingSign body and base can bonded to airfield earth grid
Fault ProtectionCCR-level ground fault and open-circuit detection
Non-Illuminated VariantNo electrical supply required — passive retroreflective panel only
Optical Performance

Photometric Requirements

ParameterTypical Value
Background Luminance (Red)≥ 30 cd/m² average, uniformity ratio ≤ 10:1 (max:min)
Legend Luminance (White)≥ 150 cd/m² average
Legend-to-Background Contrast Ratio5:1 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 on approach to the sign
Luminance Tolerance± 25% of rated value across the panel face
Maintenance Factor≥ 50% of new-panel luminance before lamp/LED or floodlight/sheeting replacement is due
Test MethodLuminance meter/photometer at normal and oblique angles, or retroreflectometer for the non-illuminated variant
Compliance

Applicable Standards

ICAO Annex 14

Mandatory instruction sign colours, dimensions, and placement requirements

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

Prerequisites

Installation Requirements

Setting-Out & Siting Survey
Civil

Survey 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
Civil

Excavate 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
Mechanical

Fit the frangible post (specified shear force) 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
Electrical

Where 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
Electrical

Where 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
Quality

Before the panel is fixed and the position closed out, confirm the regulatory text or symbol on the panel matches the airfield signage schedule for that location, since an incorrect mandatory message can misdirect pilots or vehicle operators.

Retroreflective Sheeting Application
Mechanical

Apply 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
Documentation

Record 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.

Verification

Testing & Commissioning

Insulation Resistance (Megger) Test
Pre-Energisation

Where 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-Energisation

For 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
Energisation

With 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
Energisation

Cycle 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 Acceptance

Measure 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 Acceptance

Confirm the regulatory text or symbol on the panel matches the airfield signage schedule for that location, since an incorrect mandatory message can misdirect pilots or vehicle operators.

Frangible Coupling & Mounting Verification
Pre-Energisation

Confirm the frangible coupling, base depth, and panel height meet as-built tolerances before the site is finally backfilled.

Documentation & As-Built Sign-off
Final Acceptance

Record retroreflectivity/luminance readings, and (where illuminated) insulation, continuity, and voltage readings, alongside as-built position in the commissioning dossier for handover.

Upkeep

Inspection & Maintenance

Retroreflectivity Check
Annual

Measure sheeting retroreflectivity with a retroreflectometer against ICAO / FAA minimums, and budget for sheeting replacement roughly every 5–7 years.

Illumination System Test
Annual

Test 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
Annual

Check the coupling or post for brittleness or corrosion and replace per manufacturer schedule to preserve breakaway performance on impact.

Seal & Cable Gland Check
Annual

Inspect 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
Quarterly

Clean dirt, de-icing fluid residue, and rubber deposits off the sign face to keep both reflectivity and legibility within spec.

Insulation Resistance Test
Annual

Megger-test internal wiring and any isolating transformer to confirm insulation resistance stays above minimum, catching moisture ingress early.

Field Notes

Common Field Issues

Retroreflective Sheeting Degradation

UV exposure, weathering, and abrasion from sand or de‑icing chemicals can reduce the reflectivity of the sign. Regular inspection and replacement of sheeting (typically every 5–7 years) is recommended to maintain night visibility.

Internal Illumination Failure

LED modules or fluorescent tubes inside illuminated signs can fail, reducing legibility at night. Regular testing of the illumination system and replacement of failed components is essential.

Frangible Support Fatigue

The frangible coupling or post can become brittle over time due to weather and vibration. Regular inspection and replacement as per manufacturer recommendations are required to ensure breakaway performance.

Moisture Ingress

Water can enter the sign housing through damaged seals or cable glands, causing corrosion and electrical faults. Routine seal checks and the use of desiccants are recommended.

Diagnostics

Troubleshooting Guide

Red Background or White Text Unreadable at Night
Likely Cause
Retroreflective sheeting has degraded past its useful reflectivity, or (on an illuminated sign) the internal LED module or fluorescent tube has failed or dimmed.
Diagnostic Steps
Take a night or low-light reflectivity/luminance reading against the applicable ICAO/FAA minimum; on illuminated signs, confirm supply voltage is present at the fixture before condemning the lamp or LED module.
Corrective Action
Replace the retroreflective face panel if sheeting is below minimum, or swap the failed LED module/tube; re-verify with a follow-up reading given this sign carries a mandatory instruction.
Illuminated Sign Dark or Flickering
Likely Cause
Failed LED module, fluorescent tube, or driver, a loose/corroded cable connection, or an upstream circuit fault feeding the sign.
Diagnostic Steps
Check for supply voltage at the sign's connector; inspect and re-torque cable terminations; if voltage is present but the sign stays dark, isolate the lamp/LED module or driver.
Corrective Action
Replace the failed lamp, LED module, or driver, or re-terminate corroded connections with fresh crimps and sealant; treat as high priority given the sign's mandatory-instruction function.
Moisture Ingress at Housing or Cable Gland
Likely Cause
Damaged seals or cable glands have allowed water into the sign housing, risking corrosion and electrical faults.
Diagnostic Steps
Inspect gaskets and cable glands for cracking or compression set; check for visible condensation or staining inside the housing; verify desiccant (if fitted) hasn't saturated.
Corrective Action
Replace failed gaskets and cable glands, dry out the housing, refresh or add desiccant, and re-check for recurrence at the next inspection interval.
Sign Leaning, Loose, or Misaligned
Likely Cause
Frangible coupling or post has fatigued or partially sheared, or the base plate/subgrade has settled, shifting the sign out of its correct position.
Diagnostic Steps
Visually and physically check the frangible coupling for cracking or play; confirm the sign face is still square and legible from the holding position or approach; inspect the base plate for settlement.
Corrective Action
Replace the frangible coupling on schedule if fatigued, re-level and re-orient the sign, and address any base plate or subgrade settlement to prevent recurrence.
Reliability

Failure Modes & Root Causes

LED Panel / Internal Light Source Failure
High Severity
Root Cause
Thermal cycling or driver-circuit fault in the LED panel of an illuminated unit, or a manufacturing defect in the light source.
Effect
Sign face goes dark or dim on the illuminated variant, removing the sign's night/low-visibility conspicuity while the panel itself remains structurally intact.
Detection Method
Night or low-light visual check against the panel's expected output, and driver-circuit voltage check at the housing.
Retroreflective Sheeting Degradation
Medium Severity
Root Cause
UV exposure, surface abrasion, or accumulated grime/rubber film reducing the sheeting's coefficient of retroreflection over time.
Effect
Reduced legibility under aircraft or vehicle landing-light illumination at night, most critical for non-illuminated units that depend entirely on retroreflectivity for visibility.
Detection Method
Scheduled retroreflectivity measurement against ICAO Annex 14 / FAA minimums, and visual inspection for sheeting delamination or contamination.
Frangible Coupling Fatigue / Impact Damage
High Severity
Root Cause
Cyclic wind loading fatiguing the frangible coupling over time, or a direct strike from a vehicle, aircraft component, or ground equipment.
Effect
Sign leans, misaligns, or separates from its post, degrading legibility and, if debris remains in the movement area, creating a foreign-object hazard.
Detection Method
Post alignment and coupling condition check during scheduled inspection, and immediate post-incident inspection following any reported strike.
Loss of Synchronization With Paired Stop Bar / Holding-Position Circuit
High Severity
Root Cause
Fault in the shared cabling or control logic linking the illuminated sign to the stop bar or holding-position lighting circuit it is paired with at most locations.
Effect
Sign illumination state no longer matches the state of its companion stop bar or holding-position lights, undermining the combined visual instruction at the exact point it protects.
Detection Method
Joint functional check of the sign against its paired stop bar or holding-position lighting during scheduled testing, not the sign in isolation.
Power/Circuit Connector Degradation
Medium Severity
Root Cause
Moisture ingress at the base can, gasket, or connector feeding the sign, whether standalone-fed from the nearest taxiway distribution point or sharing the paired control's circuit.
Effect
Intermittent illumination or a partial ground fault on the feeding circuit, affecting sign reliability independent of the panel or the paired control.
Detection Method
Insulation resistance (megger) testing of the feeder circuit, and visual inspection of the base can, gasket, and connector during maintenance.
Precautions

Safety & Operational Notes

Circuit Isolation Before Access
Electrical Safety

De-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
Operational

Any 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
Mechanical

Never 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
Operational

Keep 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 Safety

After 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 Safety

Use 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.

Do Not Leave De-Energised During Traffic
Operational

Because this sign marks a mandatory holding position and protects against runway incursion, never leave it dark or unreadable while traffic is active; if it cannot be restored immediately, close the associated taxiway or coordinate an alternative procedural safeguard with ATC first.

Field Insight

Engineering & Expert Notes

Never Walk Away From a Dark Sign During Active Traffic
Operational

If power can't be restored immediately, coordinate a procedural stop-bar instruction or ATC safeguard before leaving the site — a dark mandatory sign silently removes the “must stop” cue it exists to give.

Know Which Specific Instruction It's Protecting Before Deciding Urgency
Operational

Red/white signs cover holding positions, ILS critical areas, and no-entry zones alike; identify which one you're looking at first, since a dark no-entry sign and a dark CAT II holding sign follow different escalation paths.

Contrast Fails Before Full Illumination Loss
Troubleshooting

Partial LED degradation on a red background reduces white-on-red contrast well before the sign goes fully dark — a sign that's “still lit” can already be below the legibility margin this category depends on.

Treat Coupling Fatigue Here as a Priority Item, Not Routine Maintenance
Mechanical

A leaning or misaligned mandatory instruction sign undermines the unambiguous instruction this category exists to give — prioritise coupling replacement accordingly rather than folding it into general sign upkeep.

Retest Sightlines After Any Nearby Pavement Work
Inspection

Overlay or reconstruction work near a mandatory sign can change the sightline geometry aircraft use to spot it. Retest visibility from the approach direction, not just the sign's own structural integrity.

Agree the Escalation Path Before the Fault Happens, Not During
Operational

Know in advance who to call — ATC, duty ops — for an out-of-service mandatory sign. Working that out while the fault is live costs response time this sign category can't spare.

Quick Answers

Frequently Asked Questions

Short answers about mandatory instruction signs: colour, messages, siting, power, standards, maintenance and common faults.

Q1What is a mandatory instruction sign?

A mandatory instruction sign is a red rectangular airfield sign with white text and symbols that conveys a regulatory instruction pilots and vehicle operators must obey. It is used at runway holding positions, ILS critical area boundaries and no-entry points, and may be internally illuminated or externally lit for night use, or made passively visible with retroreflective sheeting.

Q2What colour is a mandatory instruction sign and why?

Mandatory instruction signs use white text on a red background. The colour combination is internationally standardised so that a pilot or driver recognises a regulatory, non-advisory instruction at a glance, without needing to read the full legend. Red-background signage is always mandatory, never advisory.

Q3What messages appear on mandatory instruction signs?

The common messages are RUNWAY with a runway designator (holding position: stop and do not cross without ATC clearance), ILS (critical area boundary: stop outside the area whenever the ILS is in use), NO ENTRY (restricted zone: do not proceed under any circumstance) and STOP (stop bar or holding point: stop immediately and contact ATC before proceeding). All four carry the same weight.

Q4Where are mandatory instruction signs installed?

They are placed at runway holding positions (CAT I, II and III), ILS critical area boundaries, stop bars and no-entry locations, on the side of the taxiway or runway at a typical mounting height of 1.5 to 2 metres above the pavement. Setback follows the convention of the location being protected, and setout is checked against the companion stop bar or pavement marking during installation.

Q5Are all mandatory instruction signs illuminated?

No. There are two variants. The illuminated variant uses an internal LED module (or external floodlight on older installations) fed from the airfield lighting circuit. The non-illuminated variant is a passive retroreflective panel with no electrical supply. Illuminated construction is preferred for mandatory messages because the sign must stay legible in every lighting condition, not only under aircraft landing-light glare.

Q6How is an illuminated mandatory instruction sign powered?

It is wired to the constant current series circuit serving that holding position or taxiway, through its own isolating transformer rather than directly on 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 other fixtures on that loop.

Q7Which standards apply to mandatory instruction signs?

ICAO Annex 14 covers colours, dimensions and placement. FAA AC 150/5340-18 covers siting, design and colour convention for airport sign systems. FAA AC 150/5345-44 is the equipment specification for the sign unit itself. ASTM D4956 sets retroreflective sheeting performance, and IEC 60598 covers luminaire requirements for illuminated signs.

Q8What are the photometric requirements for a mandatory instruction sign?

Typical values are a red background luminance of at least 30 cd/m2 average (uniformity no worse than 10:1) and a white legend luminance of at least 150 cd/m2 average, giving a legend-to-background contrast of 5:1 to 10:1 per ICAO Annex 14 Attachment A. For the non-illuminated variant, the retroreflective coefficient should be at least 50 cd/lux/m2 for white and 15 cd/lux/m2 for red per ASTM D4956 Class 1 or 2. The luminance tolerance is plus or minus 25% across the panel face.

Q9Why is a mandatory instruction sign mounted on a frangible support?

The panel sits on a frangible post or breakaway coupling with a specified shear force so that it shears cleanly if struck by an aircraft, limiting damage. The coupling must never be replaced with a rigid fitting or over-torqued during installation or repair, because that defeats the frangibility requirement.

Q10How often should mandatory instruction signs be inspected and maintained?

Typical intervals are quarterly for cleaning dirt, de-icing 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.

Q11What should be done if a mandatory instruction sign is dark or unreadable at night?

Check for supply voltage at the sign connector first, then isolate the LED module, tube or driver, and inspect cable terminations, seals and cable glands for moisture ingress. On a non-illuminated sign, take a reflectivity reading against the ICAO/FAA minimum and replace degraded sheeting. Treat it as high priority: never leave a mandatory sign dark or unreadable while traffic is active. If it cannot be restored immediately, coordinate with ATC on an alternative procedural safeguard or close the associated taxiway.

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