Home/ Knowledge Hub/ Airfield Signage & Taxiway Guidance Signs
Safety-critical technical reference — read before use. Independent educational guidance for qualified AGL and aerodrome personnel. It is not an approved procedure, design, specification or regulatory instruction and does not replace your aerodrome’s permit-to-work, isolation/LOTO procedures, the approved design or the manufacturer’s instructions. Series-circuit AGL equipment can carry lethal voltages. Read the full disclaimer ↓
Precision AGL Academy · Airfield Signage & Taxiway Guidance Signs

Airfield Signage & Taxiway Guidance Signs

Airfield signs are an essential part of the airport visual guidance system. They communicate runway, taxiway, location, direction, routing and operational information to pilots while aircraft are moving on the aerodrome. A sign is therefore not simply a board installed beside a taxiway — it is a complete engineered system involving message, colour, character, visibility, location, structure, illumination, electrical supply, control, inspection and operational performance.

Start Learning Review AGL Fundamentals
Overview

What Is Airfield Signage?

Airfield signage consists of visual signs installed on an aerodrome to provide pilots with information needed for safe aircraft movement. Depending on their function, signs can communicate taxiway direction, taxiway location, runway identification, runway exit information, holding position information, runway approach information, mandatory instructions, operational information, geographic location and routing information.

The sign must present the correct information in a form that can be identified quickly from the aircraft. Message → Colour → Character → Visibility → Location → Structure → Illumination → Electrical Supply → Control → Inspection → Operational Performance.

Not Just a Board

A sign is a complete engineered system, not simply a board installed beside a taxiway.

Where, Where To, Which Way

Signs tell pilots where they are, where they need to go, which route they're following and where they must stop or hold.

Verified, Not Assumed

A sign reporting electrically healthy on ALCMS is not automatic proof of optical compliance.

Why It Matters

Why Airfield Signs Matter

Lighting systems show pilots where to move. Signs tell pilots where they are, where they need to go, which route they are following, and where they must stop or hold. A properly functioning taxiway sign reduces navigation uncertainty and ground movement risk — particularly in complex taxiway networks, intersections, runway exits and low-visibility operations.

Navigation Uncertainty

Reduces confusion about position and routing across the taxiway network.

Wrong Turns

Clear direction and location messaging reduces incorrect taxi routing.

Runway Entry Errors

Mandatory instruction signage supports correct holding behaviour at runway boundaries.

Taxiway ID Errors

Location signs help pilots correctly identify their position on the surface.

Ground Movement Confusion

Especially significant at intersections and complex layouts.

Low-Visibility Operations

Signage becomes particularly important during reduced-visibility ground movement.

The Complete Chain

Signage in the Visual Guidance System

Airfield signage works together with runway lighting, taxiway edge and centreline lights, stop bars, runway guard lights, approach lighting, PAPI, ALCMS, markings and aerodrome layout.

AIRPORT GEOMETRY
MARKINGS
LIGHTING
SIGNS
PILOT VISUAL INFORMATION
AIRCRAFT NAVIGATION
SAFE GROUND MOVEMENT
Sign Categories

Main Airfield Sign Categories

Airfield signs can broadly be understood by their operational function. The exact sign categories, colours, message formats and installation requirements must be determined from the applicable aerodrome standard and approved airport design.

Location Signs

Identify the taxiway or other location where the aircraft is positioned — particularly useful at intersections, runway exits and complex layouts.

Direction Signs

Provide directional information for taxi routes at intersections.

Destination Signs

Identify destinations such as terminals, aprons or other operational areas where applicable.

Runway Signs

Provide runway identification and associated information.

Mandatory Instruction Signs

Provide information that requires compliance, such as runway holding-related instructions.

Information Signs

Provide additional operational or routing information.

Classification note: The six cards above describe operational functions. In ICAO Annex 14 the formal grouping is two classes — mandatory instruction signs (including runway designation and holding-position signs) and information signs (including location, direction, destination and runway-exit/vacated signs). In common ICAO/FAA practice mandatory signs are white-on-red, location signs yellow-on-black and direction/destination signs black-on-yellow — always confirm against the current applicable standard and the approved project design.

Four sign types, four colour schemes.

FOUR SIGNS · SAME AIRFIELD, DIFFERENT MESSAGES MANDATORY SIGN the stop sign 27-09 WHITE ON RED marks a limit not to cross at the holding position hold until ATC clears you LOCATION SIGN where you are A YELLOW ON BLACK names taxiway or runway at or just past intersection confirms position only DIRECTION SIGN which way next B BLACK ON YELLOW names the next taxiway before the intersection arrow follows the turn DESTINATION SIGN which way to an area CARGO BLACK ON YELLOW names a functional area same format as direction sign points toward the area
Figure 1. Mandatory, location, direction and destination signs in schematic.
AGL Visual Encyclopedia: Different signs Sign selection Information signs
Location & Direction

Taxiway Location & Direction Signs

Location Signs

Identify the taxiway on which the aircraft is located — particularly useful at taxiway intersections, runway exits, complex taxiway layouts, junctions and changes in routing.

The pilot should be able to associate the sign message with the aircraft's actual position. Incorrect location information can create a navigation risk.

Direction Signs

Provide routing information at intersections and may indicate the taxiway direction an aircraft should follow. Engineering considerations include correct message, correct orientation, correct location, visibility from the intended movement direction, character legibility, sign illumination and structural stability.

A perfectly illuminated sign with the wrong message is still a failed operational asset.

A location sign confirms where you are on the taxiway.

TAXI DIRECTION TAXIWAY A A B LOCATION SIGN · NO ARROW DIRECTION SIGN (DIMMED, HAS AN ARROW) COLOURS INVERTED FROM A DIRECTION SIGN SO IT CAN NEVER BE MISREAD AS AN INSTRUCTION TO TURN
Figure 2. A black-and-yellow location sign at a taxiway intersection.
AGL Visual Encyclopedia: Location signs Combined signs

A direction sign before the junction shows which way each taxiway leads.

TAXI DIRECTION TAXIWAY A B C A DIRECTION SIGN · BEFORE INTERSECTION LOCATION SIGN (DIMMED, OTHER CATEGORY) MULTIPLE DESIGNATIONS SHARE ONE PANEL, EACH WITH ITS OWN ARROW ANGLED TO THE ACTUAL TURN
Figure 3. A multi-designation direction sign mounted before a junction.
AGL Visual Encyclopedia: Direction signs Destination signs
Mandatory & Runway Signage

Mandatory Instruction & Runway Identification Signs

Mandatory Instruction Signs

Communicate information that requires particular pilot attention or compliance, and can be associated with runway holding positions, runway identification and other mandatory operational instructions.

Their colour, message format, location and visibility are highly controlled by applicable standards — these must not be treated as ordinary information boards.

Runway Identification Signage

Supports identification of runway environments and associated movement areas, and must be coordinated with runway designation, taxiway geometry, holding positions, runway entry points, runway lighting and markings.

The sign message should always correspond with the approved aerodrome layout and operational configuration.

Red-and-white mandatory signs sit beside the holding position marking.

TAXI DIRECTION TOWARD RUNWAY TAXIWAY HOLDING POSITION MARKING A 27-09 B MANDATORY INSTRUCTION SIGN INFORMATION SIGN (SEPARATE CATEGORY) RED = MANDATORY: MUST NOT PROCEED WITHOUT ATC CLEARANCE
Figure 4. A mandatory instruction sign beside a runway holding position marking.
AGL Visual Encyclopedia: Mandatory signs Holding position signs No-entry signs

The taxiway meeting a runway, seen from the taxiway.

09 27 RUNWAY HOLDING POSITION MARKING (PATTERN A) AIRCRAFT APPROACHING THE HOLDING POSITION B 09 – 27 LOCATION SIGN (OUTBOARD) 09 – 27 B RUNWAY DESIGNATION SIGN · ON EACH SIDE OF THE HOLDING POSITION NUMBERS ARE ORIENTED TO THE RUNWAY ENDS AS SEEN FROM THE APPROACHING TAXIWAY
Figure 5. Runway designation sign, holding position marking and the runway ahead.
AGL Visual Encyclopedia: Runway designation signs Holding position marking, signs and lights
Engineering the Message

Sign Message, Colour, Character & Arrow Engineering

Message design is one of the most important parts of airfield signage. A sign should never be fabricated directly from an informal instruction without checking the approved drawing or airport information.

Message Engineering

Before fabrication, verify correct designation, spelling, abbreviation, arrow direction, destination, runway/taxiway identification, character format, sign category and orientation.

Colour Engineering

Sign colours are function-specific — the combination communicates the type of information presented. Colour is part of the operational meaning of the sign, not an aesthetic choice.

Character & Arrow Engineering

Characters must be designed for rapid recognition from the intended viewing distance — height, width, stroke, spacing, message length and arrow geometry all matter. An incorrect arrow can be more dangerous than a missing sign.

Character Height Stroke Dimensions Spacing Viewing Distance Viewing Angle Sign Face Dimensions Arrow Direction Arrow Geometry Junction Geometry
Character Sizing & Viewing Distance

Character height, stroke width and spacing are engineered together against the intended viewing distance and angle from the cockpit — not chosen independently

12L CHAR. HEIGHT STROKE SPACING intended viewing distance & angle PILOT EYE POSITION Message must be recognized correctly at the required distance — not merely legible up close
Construction & Illumination

Sign Face Construction & Illumination

A sign face consists of engineered materials selected for structural strength, weather resistance, colour stability, character visibility, illumination compatibility and mechanical durability. Construction details depend on the approved sign type and manufacturer design.

Internally Illuminated Signs

Electrical Energy → Light Source → Internal Optical System → Sign Face → Visible Message. The engineering objective is uniform and compliant illumination of the required message.

Externally Illuminated Signs

Considerations include light-source positioning, illumination uniformity, glare, shadowing, maintenance access, weather protection, structural arrangement and electrical supply.

LED Airfield Signs

Potential advantages: lower energy consumption, long source life, reduced maintenance, fast switching, improved monitoring. LED does not automatically mean better performance — colour, luminance, uniformity and reliability must still be verified.

Housing, sealed face panel, gasket and light source in section.

SIGN HOUSING SIDE SECTION OF AN AIRFIELD SIGN HOUSING SHELL GASKET SEAL FACE PANEL LED MODULE LED DRIVER SEALED CABLE ENTRY MOUNTING LEGS FRANGIBLE POINT WHAT THE HOUSING HAS TO DO ProtectSeals keep water and dirt out HoldFrame holds the face and parts Break awayMounting fails safely if struck Open for serviceFace or cover opens for access The housing also sets the sign’s depth: an edge-lit sign can be thinner than a back-lit one. SCHEMATIC · CONSTRUCTION DIFFERS BETWEEN PRODUCTS · NOT TO SCALE
Figure 6. An airfield sign housing in side section.
AGL Visual Encyclopedia: Sign housing Sign selection

Edge-lit: LED strips along the top and bottom edges.

EDGE-LIT SIGN FRONT VIEW B LED STRIPS ALONG THE EDGES CROSS-SECTION VIEWER FACELIGHT GUIDE PLATEREFLECTORHOUSING LEDs HOW THE LIGHT TRAVELS 1 LEDs at the edgeLight is injected into the side of the plate 2 Plate spreads itLight travels inside by reflection 3 Pattern lets it outFine marks send light toward the face 4 Face shows legendEven glow, no hot spots The reflector behind the plate returns light that would escape backward. SCHEMATIC · LAYER THICKNESS AND LED POSITION ILLUSTRATIVE · NOT TO SCALE
Figure 7. Front view and cross-section of an edge-lit direction sign.
AGL Visual Encyclopedia: Edge-lit sign

Back-lit: the legend glows evenly from behind.

BACK-LIT SIGN FRONT VIEW B EVEN GLOW, LIGHT FROM BEHIND CROSS-SECTION VIEWER FACEDIFFUSERCAVITYLED MODULESHOUSING HOW THE LIGHT TRAVELS 1 Sources behind the faceLED modules sit at the back of the housing 2 Light fills the cavityIt spreads out before reaching the front 3 Diffuser evens itNo bright spot behind each source 4 Face shows legendLight passes straight through The depth of the cavity is what lets separate sources blend into one even glow. SCHEMATIC · MODULE COUNT AND CAVITY DEPTH ILLUSTRATIVE · NOT TO SCALE
Figure 8. Front view and cross-section of a back-lit direction sign.
AGL Visual Encyclopedia: Back-lit sign Sign visibility
Electrician taking out new lighting for a broken airfield sign
Photo 1. An electrician brings out new lighting for a broken airfield sign. The team collects a list of broken lights and signs each morning.Credit: U.S. Air Force photo by Senior Airman Brittany A. Chase (VIRIN 170823-F-BM568-042). Public domain (U.S. Government work). Source page
Electrical Interface

Sign Electrical System & the AGL Series Circuit

Where a sign is designed for connection to an AGL series circuit, the complete interface must be considered. The transformer rating, sign load and electrical interface must be compatible with the approved system.

CCR
Primary Series Cable
Isolation Transformer
Secondary Connector
Sign Electrical System
Light Source

Isolation Transformers

Selection should consider sign load, primary circuit characteristics, secondary requirements, connector arrangement, environmental conditions and manufacturer instructions. Do not select a transformer only by physical size.

Sign Connectors

Engineering should consider correct mating, mechanical security, water ingress, insulation, contact condition and environmental exposure. A poor connector can produce intermittent operation, high resistance, heating, voltage drop, flickering or complete sign failure.

Sign Tapped Off the Series Circuit Loop

The sign is one more isolated load on the constant-current series loop — its isolation transformer and connectors must suit the sign's own electrical characteristics, not just its physical size

CCR constant current IT Fixture IT SIGN ... remainder of the loop ... Sign load, secondary voltage and connector rating must match the sign's own electrical requirements

A sign sits on the series circuit through its own isolating transformer.

SIGN TRANSFORMER 1 PRIMARY IN THE LOOP, SECONDARY TO ONE SIGN FROM LOOP TO NEXT LIGHT SIGN PRIMARY SECONDARY MAGNETIC COUPLING NO DIRECT PATH Commonly a 1:1 current ratio, for example 6.6 A : 6.6 A. 2 RATING MUST COVER THE SIGN LOAD TRANSFORMER RATING SIGN LOAD (LED + DRIVER) SPARE CAPACITY A heavier sign needs a larger rating from the standard sizes. Wattages are illustrative. Sizes and limits vary by standard. 3 WHAT THE SIGN TRANSFORMER DOES Isolates Keeps the sign apart from the loop Secondary has no direct link to primary Keeps the loop Primary stays in the series chain Loop stays closed if the sign is off Sized to load Rating covers the sign load CCR carries the sum of all loads Unplugging a primary connector opens the loop. Isolate the circuit and make it safe first. SCHEMATIC · WINDINGS AND WATTAGES ILLUSTRATIVE · NOT TO SCALE
Figure 9. The sign transformer: primary in the series loop, secondary to the sign.
AGL Visual Encyclopedia: Sign transformer Sign circuit
Interactive

Sign Circuit Load Estimator

Sign electrical load must be included in the overall AGL design and evaluated against the CCR and series-circuit design. Adjust the sliders to see how sign count, unit load and estimated losses combine into a total circuit requirement.

12 signs
35 VA
8%
Sign Load
0 kVA
Estimated Losses
0 kVA
Total Sign Circuit Requirement
0 kVA
Indicative estimate of the sign load only. CCR sizing must consider the complete circuit — all lighting loads, isolation-transformer losses and magnetising current, series cable losses, power factor and design margin — in accordance with the approved design and CCR manufacturer data.
Control

Sign Control & ALCMS

Signs may be integrated into the airport lighting control architecture. ALCMS may provide centralized control and monitoring, but ALCMS indication is not automatically proof of optical compliance — a sign may report electrically healthy while still having a dirty or damaged face, wrong colour, reduced illumination or mechanical misalignment.

Control Functions

ON/OFF Brightness Control Group Control Circuit Control Status Monitoring Fault Indication

Operator → ALCMS → Control Command → Lighting Control → Sign → Visual Output.

Sign → Status/Fault → ALCMS → Operator. This creates a complete control and monitoring loop.

Electrical operation ≠ visual performance.

The ALCMS commands the sign CCR on and sets its brightness.

ALCMS-TO-SIGNAGE · ON, BRIGHTNESS, STATUS ALCMS SIGN CONTROL ON / OFF BRIGHTNESS SIGN CCR SERIES CIRCUIT LOOP RETURNS TO THE CCR 09-27 MANDATORY A → B DIRECTION A LOCATION C ← DIRECTION EACH SIGN FED THROUGH ITS OWN SIGN TRANSFORMER CIRCUIT STATUS & FAULTS BACK SWITCH ON FOR LOW LIGHT DIM BRIGHTNESS FOR CONDITIONS MONITOR CIRCUIT STATUS KEY POINT FOR FIXED SIGNS THE ALCMS SETS ON / OFF AND BRIGHTNESS — THE MESSAGE IS ON THE PANEL SCHEMATIC · SIGN LAYOUT IS ILLUSTRATIVE · NOT TO SCALE
Figure 10. ALCMS-to-signage control.
AGL Visual Encyclopedia: ALCMS to signage Sign circuit
Installation Fundamentals

Placement, Orientation & Foundations

Sign Placement

Verify approved location, offset from taxiway, orientation, clearance, height, viewing direction and relationship with holding positions and taxiway geometry. Check placement against approved drawings and applicable standards.

Sign Orientation

A sign may be mechanically intact but operationally ineffective if it faces the wrong direction. Aircraft Direction of Travel → Pilot Viewing Position → Sign Face → Message Recognition. Intersections require particular attention.

Foundations & Frangibility

Foundations must provide stability, alignment, load resistance and drainage. Supports must be compatible with the required aerodrome safety philosophy — including frangibility where required. Do not replace approved supports with ordinary commercial sign posts.

Sign Placement & Orientation — Plan View

Correct offset and orientation put the sign face square to the approaching pilot's line of sight at the intended reading point — not merely near the taxiway

taxiway edge aircraft direction of travel SIGN offset from pavement edge reading point holding position RUNWAY BEYOND Sign face is square to the reading point — not merely aligned to the taxiway edge

Frangible legs on a concrete pad with template-set anchor bolts.

SIDE SECTION · A FOUNDATION FOR AN AIRFIELD SIGN B2 GROUND LEVEL CABLE IN CONDUIT GUIDANCE SIGN LEGS ABOVE BREAK POINT FRANGIBLE COUPLING + BOLTS CONCRETE PAD / FOOTING COMPACTED BASE CONDUIT + SLACK LEAD TO SIGN WHERE CIVIL MEETS AGL 1 POSITION & SETBACK From the signage layout 2 FACING & ALIGNMENT Readable from the right approach 3 FRANGIBLE FIT Coupling matches the mounting 4 LEVEL & ANCHORS Template-set bolts, level top 5 CABLE ENTRY & EARTH Entry, slack lead, earth provision
Figure 11. A sign foundation in side section.
AGL Visual Encyclopedia: Sign foundations Sign installation

Square to the approaching aircraft, not turned away.

SIGN ALIGNMENT 1 FACING THE APPROACH (PLAN VIEW) TAXIWAY APPROACH SQUARE APPROACH TURNED FACE FORESHORTENED 2 UPRIGHT AND LEVEL (FRONT VIEW) B PLUMB LEVEL LEANING OR TILTED UPRIGHT 3 A GROUP READS LEFT TO RIGHT, IN THE ORDER OF THE TURNS ← ABC → turn left you are here turn right ALL FACES IN LINE, SAME SETBACK, SAME HEIGHT Keep · each face square to the approach · the group in line and at one setback · arrows matching the turn they show · no sign hiding another SCHEMATIC · PROPORTIONS AND ANGLES EXAGGERATED FOR CLARITY · NOT TO SCALE
Figure 12. Sign alignment in plan, with a sign turned away from the approach.
AGL Visual Encyclopedia: Sign alignment Sign visibility
Environmental Protection

Environmental Exposure & Water Ingress

Airfield signs operate in harsh environments — rain, dust, sand, UV radiation, heat, cold, humidity, chemicals, jet blast, wind and flooding. The enclosure and electrical system must provide appropriate protection.

Water Ingress

Common entry points include housing joints, access covers, cable entries, connectors, damaged seals and condensation paths. Water can cause insulation deterioration, corrosion, driver failure, connector failure, short circuits and intermittent operation.

Sign Cleaning

Dirty sign faces reduce visual performance. Maintenance may require removal of dust, mud, oil, rubber contamination, insects and other surface deposits. Cleaning methods must be compatible with the sign face and manufacturer requirements.

Verification

Photometry & Electrical Testing

A sign can be electrically operational but visually non-compliant. Photometric verification confirms optical performance where applicable, addressing luminance, colour, uniformity, brightness and optical distribution.

Photometric Verification

Prepare → Identify Sign → Verify Test Configuration → Measure → Compare with Applicable Criteria → PASS/FAIL → Record.

Colour & Uniformity

Colour verification uses appropriate measurement equipment against applicable criteria. Uniformity means the illuminated area should not contain unacceptable dark or excessively bright regions.

Electrical Testing

Visual inspection, continuity, insulation resistance where applicable, supply verification, current measurement, driver and transformer verification, connector inspection, functional operation and control verification.

Four groups of tests, then the order they are run in.

SIGN TESTING1 WHAT IS TESTED1 FUNCTIONSign lights upWhole message lit evenlyEvery brightness step2 ELECTRICALContinuity of wiringInsulation to earthEarth bond sound3 LIGHTBrightness is adequateFace clean, colours correctLegible at distance4 POSITIONUpright and alignedFacing the right wayView not blocked2 THE TEST SEQUENCE1ISOLATEOff, locked out, proved dead2TESTFunction, electrical, light, position3COMPAREAgainst baseline and past records4RESTORE & RECORDRe-energise, log the resultIsolate and prove the circuit dead before any electrical test. The primary is in the series loop.SCHEMATIC · TEST LISTS ILLUSTRATIVE · VALUES AND INTERVALS SET BY THE SPECIFICATION AND SITE PROGRAMME
Figure 13. Sign tests: function, electrical, light and position.
AGL Visual Encyclopedia: Sign testing Sign commissioning
Diagnostics

Common Sign Failures

Tap a symptom to see possible causes. The actual cause must always be established through systematic testing rather than assumption.

Sign completely OFF

Possible Cause

  • Power, circuit, transformer, driver or source failure

Sign flickering

Possible Cause

  • Driver, connection or supply problem

Dim sign

Possible Cause

  • Driver, source, optical or control problem

Uneven illumination

Possible Cause

  • LED/source or optical problem

Wrong colour

Possible Cause

  • Source or optical degradation

Water inside housing

Possible Cause

  • Seal or enclosure failure

Sign tilted

Possible Cause

  • Foundation or support problem

Message difficult to read

Possible Cause

  • Face, illumination, contamination or optical problem

ALCMS alarm

Possible Cause

  • Control, monitoring or equipment fault

Sign ON but visually poor

Possible Cause

  • Optical degradation or contamination

Six numbered failure points on one sign.

SIGN FAILURE MODES1 WHERE A SIGN CAN FAILA B1234561Housing and sealsCracks let in water and dirt2Face and legendFaded, scratched, dim or patchy3Mount and legsLoose, tilted or broken leg4Base, joint and cableWater, open circuit, earth fault5Light source and driverDark or dim section, dark sign6View and positionBlocked, turned or out of place2 WHAT IT LOOKS LIKEDARK OR DIMWhole sign out, or one part dimPATCHY OR FADEDUneven light, hard to readDAMAGED OR MOVEDCracked, tilted, wet or blockedCIRCUIT FAULTOthers on the circuit out, or it tripsOpening a sign or touching its wiring needs the circuit isolated, locked out and proved dead first.SCHEMATIC · FAILURES SHOWN TOGETHER FOR TEACHING · CAUSES DEPEND ON THE DESIGN AND THE SITE
Figure 14. Sign failure modes: housing, face, light source and more.
AGL Visual Encyclopedia: Sign failure modes Sign troubleshooting
Troubleshooting

Sign Troubleshooting Philosophy & Fault Isolation

Observe → Identify Sign → Check Message → Check Physical Condition → Check Power → Check Transformer → Check Connectors → Check Driver/Source → Check Control → Check Illumination → Check Colour/Uniformity → Verify Operational Performance.

1

Supply Side

Is the required electrical supply available?

2

Circuit Side

Is the series circuit functioning correctly?

3

Transformer Side

Is the isolation transformer healthy and correctly connected?

4

Connector Side

Are the primary and secondary connections sound?

5

Sign Side

Is the internal electrical system functioning?

6

Optical Side

Is the sign producing the required visual output?

This prevents unnecessary replacement of healthy components. If multiple signs fail simultaneously, investigate the common system first — CCR, series-circuit fault, control-system failure or a common power supply issue.

ProblemFirst Engineering Checks
Sign OFFSupply → circuit → transformer → connector → driver
Sign dimControl → supply → driver → source → optical system
FlickeringConnection → driver → source → supply
Wrong colourSource → optical material → contamination
Uneven illuminationLED/source → optics → face
Water ingressHousing → seals → cable entry → connectors
Sign tiltedFoundation → support → mechanical alignment
Wrong messageDrawing → sign schedule → physical face
ALCMS alarmControl → communication → equipment → circuit
Multiple signs OFFCommon circuit → CCR → control → power supply

From the series loop to the sign: what to check at each point.

SIGN TROUBLESHOOTING · SYMPTOM, CAUSE, CHECKSERIES LOOPPRIM.CONN IN/OUTSIGNTRANSFORMERin base canSEC.CONNECTORTAXIB2→FACE · LED MODULES BEHINDFROSTED FACE, EVEN LIGHTISOLATED SECONDARY: ONE SIGN ONLY1SYMPTOMSIGN DARKCause: secondary joint,lead, modules or supplyCheck: secondary, then sign2SYMPTOMALL SIGNS OUTCause: sign CCR off oropen loop at a jointCheck: CCR, loop continuity3SYMPTOMDARK SEGMENTCause: failed LED moduleor lamp, loose wiringCheck: modules, wiring4SYMPTOMLIT BUT UNREADABLECause: dirty face, turned,leaning, view blockedCheck: view, alignment5SYMPTOMCCR EARTH FAULTCause: water in housing,base or connectorCheck: insulation to earthISOLATE · LOCK OFF · PROVE DEAD BEFORE A SIGN OR CONNECTOR IS OPENED
Figure 15. Sign troubleshooting map along the series loop, through the connectors and transformer to the sign.
AGL Visual Encyclopedia: Sign troubleshooting Series circuit troubleshooting

Make safe first, then isolate the fault step by step.

FAULT-ISOLATION FLOWCHART · SYMPTOM, TYPE, SECTIONFIRSTTELL ATC · SWITCH OFF AND ISOLATE THE CCR · LOCK OFF · AUTHORISED PERSONNEL ONLYALL LIGHTS OUTCCR open-circuit alarm, no currentOPEN CIRCUITa break in the loopTEST CONTINUITY BY SECTIONFault is where it failsCCR TRIPSEarth-fault or overcurrent alarmEARTH FAULTinsulation to earth is lowTEST INSULATION RESISTANCE BY SECTIONLow reading marks the sectionSOME LIGHTS OUTCCR normal, circuit runningLOCAL FAULTfixture, connector or transformerCHECK THE DEVICE ITSELFLight, secondary connector, transformerNARROW DOWN BY HALF-SPLITTEST: GOOD · SET ASIDEBAD · SPLIT AGAINfault sectionTHEN1 Repair the faulty part2 Retest section and circuit3 Restore, run, confirm status4 Record results, tell ATC
Figure 16. Fault isolation flowchart for a series circuit.
AGL Visual Encyclopedia: Fault-isolation flowchart Sign failure modes
Asset Records

The Sign Schedule

A sign schedule should provide a controlled record of installed signage. A good asset record allows engineers to understand a sign's complete lifecycle.

ParameterInformation
Sign IDUnique asset identification
Sign TypeLocation / Direction / Mandatory / Information etc.
MessageApproved sign message
LocationApproved installation location
OrientationViewing direction
CircuitAssociated lighting circuit
TransformerAssociated transformer
Electrical RatingApplicable rating
ControlLocal / Remote / ALCMS
StatusOperational condition
RemarksEngineering observations
Maintain corresponding records for installation date, inspection history, maintenance history, test results, photometric results where applicable, replacement history and fault history.

Four decisions fix which sign goes where.

SELECTION FLOWCHART · FOUR DECISIONS IN ORDER 1FUNCTION What must it tell the pilot? Hold, must not cross → Mandatory: red, white text Where, which way → Information: yellow and black 2LEGEND How large must it read? Larger runway code → Larger legend height Smaller runway code → Smaller legend height 3LIGHTING How is the face lit? LEDs behind the face → Back-lit, deeper housing LEDs at the edges → Edge-lit, thin housing 4SITE What must it suit? The series circuit → Transformer and LED driver The airfield edge → Low, frangible, sealed CONFIRM BEFORE ORDERING colour and inscription · legend height · luminance · frangibility · transformer and driver · standard FLOWCHART · THE MANUFACTURER DATA SHEET AND STANDARD DECIDE THE FINAL CHOICE
Figure 17. Selecting an airfield sign: function and colour, then size, mounting and light source.
AGL Visual Encyclopedia: Sign selection Design requirement flow
Installation & Commissioning

Sign Installation, Commissioning & Acceptance

Approved Drawing
Foundation Verification
Receipt Inspection
Message Verification
Mechanical Install
Orientation Check
Electrical Connection
Transformer/Connector Check
Control Connection
Illumination Testing
Operational Verification
Documentation

Acceptance sequence: Visual Inspection → Mechanical Inspection → Electrical Testing → Control Testing → Illumination Verification → Optical/Photometric Verification Where Applicable → Operational Verification → Defect Rectification → Re-Test → Acceptance.

Setback, height and position of an installed sign.

SIGN INSTALLATION SIDE VIEW PAVEMENT EDGE FOUNDATION / BASE CAN VIEWER INSTALLED HEIGHT (MAX) SETBACK CABLE FROM SERIES CIRCUIT EARTH BOND SIGN FRANGIBLE COUPLING PLAN VIEW TAXIWAY PAVEMENT EDGE SIGN SETBACK TO NEAR SIDE LINE OF SIGHT FROM TAXIING AIRCRAFT INSTALLATION SEQUENCE 1 Set out Survey position and setback 2 Foundation Place and level the base 3 Mount Fit coupling and sign 4 Connect Cable, connectors, earth 5 Check Level, height, lit test The structure is fixed first, the cable second, and the lit check last. SCHEMATIC · PROPORTIONS ILLUSTRATIVE · NOT TO SCALE
Figure 18. An installed airfield sign: setback from the pavement edge and installed height.
AGL Visual Encyclopedia: Sign installation Sign installation construction

Five checks on each sign before it goes into service.

SIGN COMMISSIONING · IS THIS SIGN RIGHT, SAFE AND WORKING SIGN BY SIGN · BEFORE IT GOES INTO SERVICE 1 LEGEND Matches signage plan Colours & arrows right Correct face shown RECORD: legend 2 POSITION At the plan location Faces the right way Upright & unobstructed RECORD: position 3 ELECTRICAL Wiring & connectors Insulation to earth Earth bond RECORD: readings 4 LIGHT Lights evenly Every brightness step Colour & legibility RECORD: light check 5 CONTROL Circuit switched on Brightness follows Status indicated RECORD: control SIGN COMMISSIONED RIGHT · SAFE · WORKING 6 DEFECT ISOLATE · FIX · RE-TEST PASSED READY FOR ACCEPTANCE SCHEMATIC · CHECKS SHORTENED · NOT TO SCALE
Figure 19. Sign commissioning checks.
AGL Visual Encyclopedia: Sign commissioning Sign testing
Maintenance

Sign Maintenance

Preventive maintenance should follow applicable airport procedures and equipment requirements. Additional inspection may be appropriate after severe weather — a sign that looks physically normal may still have hidden electrical damage.

Cleaning & Visual Inspection

Face cleaning, visual and mechanical inspection, alignment and foundation condition.

Electrical Inspection

Connector, transformer and driver condition, protection and supply checks.

Illumination & Colour

Illumination, colour, uniformity and optical performance checks.

ALCMS Status

Confirm status, control response and monitoring/alarm behaviour.

Six inspection points on one sign.

SIGN INSPECTION1 WHAT IS LOOKED ATA B1234561Face and legendClean, undamaged, correct2HousingSeals, cracks, no water inside3MountFasteners, frangible legs sound4Ground and baseCover seated, no heave or standing water5View and alignmentUpright, facing right, not blocked6Lit at nightWhole message even and legible2 WHEN AND HOWDAYTIMESeen as a pilot or driver seesNIGHTWhole message lit and legibleCLOSE-UPSeals, fasteners and baseDEFECT FOUNDRecord, grade and report itLooking from outside needs no isolation. Opening the sign does: isolate, lock out and prove dead first.SCHEMATIC · POINTS ILLUSTRATIVE · INTERVALS SET BY THE SITE INSPECTION PROGRAMME
Figure 20. Sign inspection points: face and legend, housing, mount, ground and base, view and alignment.
AGL Visual Encyclopedia: Sign inspection Sign maintenance

What a cracked, dirty or part-dark sign looks like in front and ground view.

SIGN IN SERVICE · WHAT SIGN MAINTENANCE CHECKSGROUND LEVELBISOLATION TRANSFORMER PITFACE & LEGENDdirt, fading, cracks, legend legibleILLUMINATIONevery segment lit, even brightnessMOUNT & ORIENTATIONfrangible joints, height, facing, clearanceTO CCRELECTRICAL & EARTHINGconnector, transformer, bond, cable12346ENCLOSURE & SEALSgaskets, cracks, water ingress5RECORD & ACTdefect · action1 · Face & legend2 · Illumination3 · Mount & orientation4 · Electrical & earthing5 · Enclosure & seals6 · Record & restore
Figure 21. Sign maintenance: face, light source, frangible legs and base.
AGL Visual Encyclopedia: Sign maintenance Sign visibility
Electrician replacing a bulb in a runway sign at night
Photo 2. An electrician replaces a bulb in a runway sign. The airfield lighting team works every night to maintain the lights.Credit: U.S. Air Force photo by Stephenie Wade (VIRIN 130625-F-IW762-156). Public domain (U.S. Government work). Source page
Engineering Checklist

Airfield Sign Engineering Checklist

A practical way to track a sign project from message approval through acceptance. Click any item to mark it complete.

Message & Location

Correct sign category & message
Correct characters, arrow & colour
Approved location & offset
Correct orientation, clearance & height
0 / 4 complete

Mechanical & Electrical

Housing, face & support condition
Foundation & alignment verified
Frangible arrangement where applicable
Supply, transformer & connectors verified
Driver & protection verified
0 / 5 complete

Optical & Control

Illumination verified
Colour & uniformity verified where applicable
Brightness & visibility verified
Local, remote & ALCMS control verified
Monitoring & alarm verified
0 / 5 complete

Documentation

Sign schedule updated
Test report completed
Photometric report where applicable
As-built & maintenance records updated
0 / 4 complete
Lessons Learned

Common Engineering Mistakes

Treating a sign as an ordinary road sign Fabricating without verifying the approved message Ignoring viewing direction Using an incompatible transformer Ignoring connector condition Checking only whether the sign is ON Ignoring colour performance Ignoring illumination uniformity Ignoring foundation settlement Replacing a whole sign for a connector/transformer/driver fault Ignoring ALCMS monitoring Not updating the asset record after replacement
Standards & References

Applicable Standards

Exact sign colours, dimensions, character requirements, luminance/photometric criteria, mounting arrangements and installation clearances must be taken from the applicable current standard and approved project requirements.

ICAO

Annex 14, Volume I (visual aids — signs) & applicable ICAO aerodrome guidance

FAA

Airport sign-system standards (AC 150/5340 series) & sign equipment specifications (AC 150/5345 series)

DGCA

Applicable national aviation & DGCA requirements where applicable

PROJECT

Airport-specific standards, approved specifications, drawings & manufacturer instructions

Test Yourself

Quick Knowledge Check

Q1. Which sign type identifies the taxiway an aircraft is currently on?

A. Direction sign
B. Mandatory instruction sign
C. Location sign
D. Destination sign

Q2. Why is an incorrect arrow on a sign potentially more dangerous than a missing sign?

A. Because arrows are more expensive to fabricate
B. Because it can actively provide misleading guidance rather than simply providing no information
C. Because arrows must always be illuminated externally
D. Because arrows are not covered by aerodrome standards

Q3. A sign shows healthy status on ALCMS. What does this confirm?

A. Electrical/control status only — not optical compliance such as colour, uniformity or a clean face
B. That the sign message is correct
C. That the sign foundation is stable
D. That photometric verification is no longer required for any sign
Explore More

Explore More AGL Engineering

GUIDANCE

Taxiway Lighting Systems

Edge, centreline, lead-on/lead-off lighting, stop bars and runway guard lights.

Open Taxiway Lighting →
ENGINEERING

AGL Design & Electrical Engineering

Layout, series circuits, CCR sizing, transformers, cable design and control.

Open AGL Design →
PERFORMANCE

AGL Photometry

Verify the optical performance of the installed system.

Open AGL Photometry →
INTEGRATION

Airfield Markings & Signage Integration

How signage coordinates with markings and taxiway lighting.

Open Markings & Signage Integration →
SAFETY

Runway Incursion Prevention & SMGCS

How guidance signage supports incursion prevention.

Open Incursion Prevention →
SAFEGUARD

Runway Guard Lights & Stop Bars

The lighting counterpart to mandatory instruction signs.

Open Guard Lights & Stop Bars →
PROCESS

AGL Installation & Commissioning

Field installation of signage and guidance systems.

Open Installation & Commissioning →

Identify. Guide. Illuminate. Verify. Maintain.

From message engineering to energized circuit, from photometric verification to ALCMS integration — explore the complete AGL knowledge base for airfield signage engineering.

Explore the Knowledge Hub AGL Design Services
Keyword Reference

Mandatory Instruction Signs at an Airport

Mandatory instruction signs airport

Mandatory instruction signs at an airport tell pilots and drivers where they must stop or may not enter, such as at runway holding positions and no-entry areas. They are placed beside the holding position and must be readable day and night, so they are internally lit or retro-reflective as the standard requires. Sign colours, sizes and positions follow the applicable standard and the approved sign schedule.

AGL Engineering Hub

Explore the AGL Engineering Knowledge Hub

Fundamentals

All About AGL How Airfield Lighting Works Physics of AGL AGL Components Airfield Lighting Colour Codes LED Airfield Ground Lighting LED Benefits, Lifespan & Energy Savings Solar AGL (SAGL) Portable AGL (PAGL) Advantages & Disadvantages by System Type Sustainability in AGL

Standards & Compliance

ICAO Annex 14 AGL Requirements ICAO Aerodrome Lighting Requirements FAA AC 150/5340 Series EASA CS-ADR-DSN Compliance NOTAMs

Runway, Taxiway & Approach Lighting

Runway Lighting Systems Taxiway Lighting Systems Airport Approach Lighting Systems Approach Lighting Systems Apron Lighting & Stand Guidance Apron Floodlighting Design & Lux Levels VDGS: Visual Docking Guidance Obstruction Lighting & Marking Airfield Signage & Guidance Signs Markings & Signage Integration

Surface Movement & Safety

Follow the Greens Runway Guard Lights & Stop Bars Runway Incursion Prevention & SMGCS A-SMGCS ALCMS

PAPI

How Does PAPI Work? PAPI Engineering Guide ICAO PAPI Requirements PAPI Installation & Aiming PAPI Calibration & Alignment PAPI Alignment & Maintenance PAPI Maintenance Checklist

Electrical Engineering & Power

AGL Design & Electrical Engineering Diagrams, Schematics & Wiring CCR Working Principle CCR Deep Dive AGL Isolating Transformers Airport Isolating Transformer Junction Box & Transformer Housing Cables & Connector Engineering Electrical Architecture & Redundancy Emergency Backup Power Emergency Power & Lighting Systems Surge Protection & Lightning Mitigation Lightning & Surge Protection for Circuits

Photometry & Testing

AGL Photometry Photometric Testing Guide Photometric Testing Procedure Runway Lighting Photometry Taxiway Lighting Photometry AGL Testing & Inspection Installation & Commissioning Testing & Commissioning Guide

Maintenance, Inspection & Safety

Preventive & Corrective Maintenance Maintenance Schedule & Checklist Inspection, Maintenance & Audit Checklists How to Inspect & Replace Airfield Lights Common Failure Causes & Fixes Outage Response & Runway Closure Maintenance Work Safety: LOTO, Permits, PPE

Resources, Documentation & Careers

Datasheets, Catalogues & PDF Guides Project Documentation: ITP, As-Built, O&M Engineer & Technician Job Description / Salary
FAQ

Frequently Asked Questions

Q1What is airfield signage?

Airfield signage consists of visual signs installed on an aerodrome to give pilots the information needed for safe aircraft movement, such as taxiway direction and location, runway identification, runway exit, holding position, mandatory instructions and routing information. A sign is a complete engineered system involving message, colour, character, visibility, location, structure, illumination, electrical supply, control and inspection, not simply a board beside a taxiway.

Q2Why do airfield signs matter?

Lighting shows pilots where to move, while signs tell them where they are, where they need to go, which route they are following and where they must stop or hold. A properly functioning taxiway sign reduces navigation uncertainty and ground movement risk, especially in complex networks, intersections, runway exits and low-visibility operations.

Q3What are the main categories of airfield signs?

Operationally they are location signs, direction signs, destination signs, runway signs, mandatory instruction signs and information signs. In ICAO Annex 14 the formal grouping is two classes: mandatory instruction signs and information signs, which include location, direction, destination and runway-exit or vacated signs.

Q4What colours are airfield signs?

In common ICAO and FAA practice, mandatory signs are white on red, location signs are yellow on black, and direction and destination signs are black on yellow. Always confirm against the current applicable standard and approved project design.

Q5What is the difference between location signs and direction signs?

Location signs identify the taxiway or other location where the aircraft is positioned, which is particularly useful at intersections and runway exits. Direction signs provide routing information at intersections. The pilot should be able to associate a location message with the aircraft's actual position, since incorrect location information can create a navigation risk.

Q6What do mandatory instruction signs do?

They communicate information requiring particular pilot attention or compliance, and can be associated with runway holding positions, runway identification and other mandatory operational instructions. Their colour, message format, location and visibility are highly controlled by applicable standards, so they must not be treated as ordinary information boards.

Q7Does a healthy ALCMS status mean an airfield sign is compliant?

No. A sign reporting electrically healthy on ALCMS is not automatic proof of optical compliance. Signs are verified, not assumed, and a perfectly illuminated sign with the wrong message is still a failed operational asset.

Q8How do signs fit into the visual guidance system?

Signage works together with runway lighting, taxiway edge and centreline lights, stop bars, runway guard lights, approach lighting, PAPI, ALCMS, markings and aerodrome layout. The chain runs from airport geometry through markings, lighting and signs to pilot visual information, aircraft navigation and safe ground movement.

Q9Can a sign be made from an informal instruction?

No. A sign should never be fabricated directly from an informal instruction without checking the approved drawing or airport information. The sign message should always correspond with the approved aerodrome layout and operational configuration.

Q10What should be considered when engineering a direction sign?

Engineering considerations include the correct message, orientation and location, visibility from the intended movement direction, character legibility, sign illumination and structural stability.

Field Insight

Engineering & Expert Notes

RS
By Rahul Sharma
Airfield Ground Lighting Specialist
Message Accuracy Is a Safety Item
Content

Check legends against the approved layout before installation. A wrong character or arrow can mislead crews.

Sign Circuits Share the Series Circuit Constraints
Electrical

Include sign transformers and loads in circuit calculations. Verify supply arrangements and monitoring.

Seals and Drainage Protect the Internals
Water

Check gaskets, entries and drainage during routine inspection to prevent moisture damage.

Foundations and Frangibility Matter
Mounting

Confirm mounting arrangements and breakaway features after impacts or work near the sign.

Evaluate Brightness and Contrast
Photometry

Check luminance and legibility at each brightness setting and from the viewing distance.

Keep Faces Clean and Undamaged
Maintenance

Dirt and scratches reduce legibility long before an electrical fault occurs.

Precision AGL Academy

Understand AGL. Build Your Engineering Skills.

You now understand how airfield signage and taxiway guidance signs direct aircraft safely — from mandatory instruction signs to location and direction signs.

Take the next step with Precision AGL Academy.

Learn AGL Step by Step

Build your knowledge through structured technical learning covering:

AGL Fundamentals Components Electrical Systems Design Photometry Testing Troubleshooting Maintenance

Move from understanding how AGL works to developing the practical knowledge needed to analyse, operate and maintain complete AGL systems.

Explore AGL Academy Start Your Learning Journey
Understand the system. Build the knowledge. Apply the engineering.

Talk to Precision AGL Academy

Call+91 98214 96412
Emailprecisionaglacademy@gmail.com

Follow the Academy

YouTube@precisionaglacademy LinkedInPrecision AGL Academy Instagram@PrecisionAGLAcademy Facebook@PrecisionAGLAcademy