Adapter Plate
A versatile mounting plate that enables the installation of AGL fixtures, signs, or equipment onto existing base plates, base cans, or structures with different bolt patterns or mounting configurations. Designed to bridge compatibility gaps, simplify retrofits, and reduce installation costs while maintaining structural integrity and precise alignment.
Image disclaimer: For identification and illustration only. It may show a product by another manufacturer and may not depict the exact item described; it is not a statement of any product’s quality, approval or performance. Third-party image and trademark rights belong to their owners. See full notice.
What Is It?
An Adapter Plate (also known as a conversion plate, mounting adapter, or interface plate) is a precision‑machined steel or aluminium plate that provides a mechanical interface between AGL equipment (fixtures, signs, sensors) and existing mounting structures (base plates, base cans, poles) that have different bolt patterns, diameters, or mounting configurations. It allows new equipment to be installed on legacy infrastructure without the need for costly and disruptive pavement modifications.
Key characteristics of adapter plates include:
- Dual Bolt Patterns: The plate features two sets of mounting holes: one set matches the existing base (base plate or base can), and the other set matches the new fixture's mounting pattern.
- Precision Machining: The plate is machined to tight tolerances to ensure accurate alignment of the fixture's optical axis and correct beam orientation.
- High Strength: Made from high‑grade steel (S235JR or S355JR) or aluminium, designed to withstand the same loads as the original mounting structure.
- Corrosion Protection: Hot‑dip galvanised, powder‑coated, or stainless steel construction ensures long‑term durability in the harsh airfield environment.
Adapter plates are used in the following scenarios:
- Fixture Replacement: When replacing an old AGL fixture with a new model that has a different bolt pattern or mounting height.
- Retrofit Upgrades: When upgrading from halogen to LED fixtures, or from one manufacturer's fixture to another.
- Compatibility Bridging: When installing new equipment (signs, sensors, junction boxes) onto existing foundations.
- Height Adjustment: Some adapter plates also provide a small height increase (10‑50mm) to bring the fixture to the correct elevation.
The adapter plate is typically installed between the existing base plate (or base can top flange) and the new fixture's base plate. It is bolted to the existing structure using the original anchor bolts, and the new fixture is bolted to the adapter plate using new bolts. This provides a secure, level, and accurately aligned mounting solution without modifying the pavement.
Purpose & Function
The adapter plate exists to protect the airfield's investment in its lighting circuits and pavement while enabling technology upgrades and manufacturer changes. Rather than requiring an old base plate, base can, or foundation to be demolished and re-cast whenever a fixture with a different bolt pattern is fitted — work that would take a runway or taxiway out of service and disturb the surrounding pavement — the adapter plate lets the new fixture bolt straight onto the existing infrastructure. Its function is therefore twofold: it preserves structural continuity, carrying the same design loads (weight, wind, impact, vibration) as the original mounting, and it preserves optical accuracy, since precise hole location and orientation marking ensure the new fixture's beam is still aimed correctly. In this way the adapter plate supports lifecycle upgrades — most commonly halogen-to-LED retrofits — without compromising either the mechanical integrity or the photometric performance the airfield depends on.
Where Is It Used?
Adapter plates are used throughout the airfield in retrofit and upgrade projects, including:
- Runway Lighting Retrofits: Installing new LED runway edge lights, centreline lights, or threshold lights onto existing base plates with different bolt patterns.
- Taxiway Lighting Upgrades: Replacing old taxiway edge lights or centreline lights with new fixtures that have different mounting dimensions.
- Approach Lighting Conversions: Retrofitting approach lighting systems with new fixtures while reusing existing foundations.
- Signage Replacement: Installing new airfield signs (mandatory instruction, direction, information) onto existing sign bases.
- Obstruction Lighting Upgrades: Replacing old obstruction lights with new LED units on existing poles or structures.
- Sensor Mounting: Installing new weather sensors, RVR sensors, or other instrumentation on existing mounting structures.
- Manufacturer Changes: Converting from one equipment manufacturer's products to another when bolt patterns differ.
The adapter plate is designed to match the specific bolt patterns of both the existing infrastructure and the new equipment. It is available in various sizes (150mm to 400mm diameter or square) and thicknesses (10mm to 25mm) to accommodate different load requirements and fixture weights.
Installation & Location
The plate is bolted directly onto the existing base can or foundation, using the original bolt circle, with the new fixture's mounting holes drilled or pre-machined into the plate's upper face to the new equipment's pattern. A gasket is fitted on both faces, below the plate against the base can and above it against the fixture, so the swap doesn't introduce a new leak path into an otherwise sealed foundation. Fitting is checked for flatness and full contact across the plate before final torqueing, since any rocking under wheel or foot load will fatigue the bolts over time, and the plate's thickness is chosen to keep the new fixture at the correct beam or optical height once stacked on top of the original foundation.
How Does It Work?
The adapter plate functions as a mechanical interface that translates between two different mounting systems. Its design ensures that the new fixture is securely and accurately mounted on the existing foundation:
- Bolt Pattern Translation: The plate has two sets of holes. The lower set matches the existing base plate or base can's bolt pattern (e.g., 4‑hole on a 150mm PCD, M12 bolts). The upper set matches the new fixture's bolt pattern (e.g., 6‑hole on a 200mm PCD, M16 bolts). This allows the new fixture to be bolted securely to the existing foundation.
- Load Transfer: The plate transfers all loads (vertical weight of the fixture, wind loads, vibration, impact) from the new fixture down through the existing base plate and into the pavement. The plate is designed to be at least as strong as the original mounting structure.
- Alignment: The plate is machined with precise hole locations and is marked with orientation indicators to ensure that the fixture is correctly aligned. This is critical for maintaining the required beam direction and photometric performance.
- Height Compensation: Some adapter plates are designed with an integrated height offset to compensate for differences in mounting height between the old and new fixtures, ensuring that the fixture is at the correct elevation above the pavement.
The installation process typically involves:
- Cleaning: The existing base plate or base can flange is cleaned and inspected for damage.
- Adapter Placement: The adapter plate is positioned on the existing base, aligning the lower bolt holes with the existing anchor bolts.
- Levelling: The adapter plate is levelled using shims or a levelling compound to ensure the top surface is perfectly horizontal.
- Securing: The adapter plate is bolted to the existing base using the original anchor bolts (or new bolts if the originals are damaged).
- Fixture Mounting: The new fixture is placed on the adapter plate, aligned, and secured with new bolts through the upper bolt holes.
- Alignment Check: The fixture's orientation is checked using a digital level or alignment jig to ensure the beam is correctly aimed.
The adapter plate is designed to provide a service life of 20+ years, with regular inspection for corrosion, bolt tightness, and damage.
Mounting Interface Stack & Control Logic
Weight, wind, vibration, and impact loads all pass straight down through this stack into the pavement — the plate must be at least as strong as the original mounting structure it bridges.
Physical Construction
The plate is CNC-machined from steel or aluminium billet or plate stock to hold flatness and bolt-hole tolerances that keep both mating flanges square once torqued together, since any warp or misalignment translates directly into fixture tilt. Steel plates are typically hot-dip galvanised or zinc-plated, and aluminium plates anodised, to resist corrosion in the buried or semi-buried environment of a base can. Two bolt-hole patterns are machined into the same plate — one matching the legacy base plate, base can, or pole, the other matching the new fixture's mounting flange — with a gasket or O-ring groove on one or both faces so the stack still meets the fixture's IP rating once assembled. Stainless-steel fasteners, torqued to the fixture manufacturer's spec, complete the joint on both sides of the plate.
Technical Parameters
| Parameter | Typical Value |
|---|---|
| Material | Structural steel (S235JR or S355JR), hot‑dip galvanised (min 85µm) or stainless steel (316L); aluminium (6061‑T6) for lightweight applications |
| Thickness | 10mm to 25mm (depending on load rating) |
| Dimensions (L x W or Diameter) | 150mm to 400mm (6" to 16") – custom sizes available |
| Surface Finish | Hot‑dip galvanised, powder‑coated, or stainless steel (plain or polished) |
| Lower Bolt Pattern | Matches existing base plate (e.g., 4‑hole, 150mm PCD, M12) |
| Upper Bolt Pattern | Matches new fixture (e.g., 6‑hole, 200mm PCD, M16) |
| Height Offset (optional) | 10mm to 50mm (to compensate for fixture height differences) |
| Load Capacity (vertical) | ≥ 10 kN to 100 kN (depending on thickness and material) |
| Flatness Tolerance | ≤ 0.5mm over the plate area |
| Corrosion Resistance | ≥ 500 hours salt spray (galvanised); ≥ 1000 hours (stainless) |
| Operating Temperature | -40°C to +85°C |
| Standards | FAA AC 150/5345‑42, ICAO Annex 14, EN 10025, ASTM A123 |
Applicable Standards
FAA AC 150/5345‑42
Specification for airport light bases, transformer housings, junction boxes & accessories (covers adapter rings/flange hardware)
ICAO Annex 14
Physical characteristics of AGL mounting structures and fixtures
EN 10025
Hot‑rolled products of structural steels (material specification)
ASTM A123
Standard specification for zinc (hot‑dip galvanised) coatings on iron and steel products
Installation Requirements
Bolt Pattern Verification
Pre-InstallationConfirm the plate's hole pattern, bolt circle diameter, and thread size match both the new fixture and the existing base plate or can before offering it up.
Existing Surface Preparation
MechanicalClean and inspect the legacy base plate, base can, or pole mounting surface for corrosion or damage before the adapter plate is offered up.
Gasket/O-Ring Seating
MechanicalFit the gasket or O-ring into the retaining groove on one or both faces before assembly, so the completed stack meets the fixture's IP rating.
Levelling & Shimming
MechanicalSet the plate flush and level on the mounting surface using a spirit or laser level, shimming as required to remove any gap before final bolting.
Galvanic Isolation Fitting
MechanicalFit isolating washers or coatings between dissimilar metals at the joint, preventing galvanic corrosion over the assembly's service life.
Stainless Fastener Torquing
MechanicalTorque stainless-steel fasteners on both sides of the plate to the fixture manufacturer's spec, in the correct sequence, to seat the gasket evenly.
Pre-Energisation Sign-off
DocumentationRecord bolt torque values, shim thickness, and gasket condition as-built before the fixture on the adapter plate is energised.
Testing & Commissioning
Bolt Pattern & Fit Verification
Pre-InstallationConfirm hole pattern, bolt circle diameter, and thread size match both the new fixture and the existing base plate or can before installation.
Flatness & Levelling Check
InstallationVerify the adapter plate sits flush and level on the mounting surface using a spirit or laser level, shimming as required to remove any gap.
Mounting Bolt Torque Verification
InstallationTorque all mounting bolts to the specified value in the correct sequence, then re-check after initial thermal or load cycling.
Galvanic Isolation Test
Pre-EnergisationVerify isolating washers or coatings prevent direct contact between dissimilar metals, checking resistance across the joint where applicable.
Alignment & Height Verification
Final AcceptanceConfirm the mounted fixture achieves the correct height, orientation, and optical alignment relative to the design datum.
Documentation & As-Built Sign-off
Final AcceptanceRecord bolt torque values, shim thickness, and final alignment readings in the commissioning dossier for handover.
Inspection & Maintenance
Mounting Bolt Torque Check
AnnualRe-torque adapter plate mounting bolts to spec and inspect threads for corrosion or elongation from repeated cycling.
Interface Corrosion Inspection
AnnualCheck the contact face between the plate and housing or frame for corrosion, especially where dissimilar metals meet.
Levelling Verification
Semi-annualRecheck plate level with a digital level, since a beam misalignment fault often traces back to a settled or shifted adapter plate.
Galvanic Isolation Check
AnnualVerify isolating washers or coatings between dissimilar metals remain intact to prevent accelerated galvanic corrosion.
Common Field Issues
Bolt Pattern Mismatch
If the existing bolt pattern is not accurately measured, the adapter plate may not fit. Always verify the existing bolt pattern, hole diameter, and PCD (pitch circle diameter) before ordering the adapter plate. Use a bolt‑pattern gauge or accurate measurement tools.
Corrosion at Interface
Moisture and de‑icing fluids can become trapped between the adapter plate and the existing base, causing corrosion. Apply anti‑seize compound or a corrosion‑inhibiting coating between the mating surfaces, and use stainless steel bolts.
Inadequate Bolt Length
The existing anchor bolts may not be long enough to secure both the adapter plate and the fixture. Use longer bolts or bolt extensions. Ensure that there is sufficient thread engagement (at least 1.5 times the bolt diameter).
Misalignment of Fixture Beam
If the adapter plate is not oriented correctly, the fixture's beam will be misaimed. Mark the orientation (e.g., "forward", "towards runway") on the adapter plate and the existing base. Use an alignment jig to verify the beam direction after installation.
Insufficient Plate Thickness
Using a plate that is too thin for the fixture weight or wind loads can result in bending or deformation, causing misalignment or failure. Select a plate with adequate thickness and strength for the specific application.
Galvanic Corrosion with Dissimilar Metals
If the adapter plate and the existing base plate are made of dissimilar metals (e.g., galvanised steel adapter on an aluminium base), galvanic corrosion can occur. Use isolation gaskets, anti‑seize compound, or ensure both components are of the same material.
Inadequate Levelling
If the adapter plate is not level, the fixture will be tilted, affecting the beam pattern. Use shims and a precision level during installation. Grout under the plate if necessary to ensure full support.
Troubleshooting Guide
Fixture Beam Misaimed After Installation
Plate Won't Seat Flush / Bolt Holes Don't Line Up
Corrosion or Staining at the Mating Interface
Plate Shows Bending or Deformation
Failure Modes & Root Causes
Incorrect Installation Orientation
Bolt Pattern / PCD Mismatch
Mating Interface Corrosion
Plate Bending or Deformation
Safety & Operational Notes
Assume the Equipment Below Is Live
Electrical SafetyTreat the base can, base plate, or pole beneath the adapter plate as energised until the associated circuit has been de-energised, locked out, and verified zero energy, even though the plate itself is a passive mechanical part.
Manual Handling
Personnel SafetyAdapter plates can be heavy for their size, particularly steel variants; use correct lifting technique and avoid working alone on larger plates.
Bonding and Earthing Continuity
Electrical SafetyWhere the plate forms part of the equipment's earthing/bonding path, verify continuity across the plate after installation, since a poorly seated or painted-over interface can silently break an earth path that looks mechanically sound.
Correct Torque and Load Rating
MechanicalTorque mounting fasteners to the manufacturer's specification and confirm the plate's load rating matches the fixture and traffic it will carry; an under-torqued or under-rated plate can work loose or fail under aircraft/vehicle loading.
FOD Control
OperationalKeep fasteners and hardware accounted for at all times during fitting, and confirm nothing is left loose on the pavement before reinstating the fixture.
Restore to Compliant State
ComplianceConfirm the adapted mounting is flush, correctly torqued, and bonded before closing out the work and returning the fixture to service.
Engineering & Expert Notes
Match the Plate Material to the Base, Not Just the Bolt Pattern
Material SelectionA plate can share bolt spacing with the base can and still be the wrong choice if its material sets up galvanic corrosion against a dissimilar-metal base; specify plate material and any required isolating gasket together, not the hole pattern alone.
Shim Thickness Adds Up Faster Than It Looks
InstallationStacking several thin shims to correct fixture height under an adapter plate can quietly push the fastener grip length outside spec; where more than one or two shims are needed, re-check bolt length and thread engagement rather than assuming the original hardware still applies.
Re-Check Torque After the First Thermal Cycle
MechanicalFreshly installed adapter plates on aluminium or composite bases often lose a measurable amount of fastener torque after their first heat-cool cycle as the interface beds in; a short-interval re-torque check shortly after commissioning catches this before it becomes a loose-fixture complaint.
Paint and Coatings Can Hide a Broken Bond Path
ElectricalA repainted or recoated plate can look perfectly seated while the coating itself has broken electrical continuity across the mating faces; strip contact surfaces back to bare metal at the bonding points rather than relying on a visual fit-up check alone.
Keep a Small Stock of Off-Standard Plates
Field PracticeOlder bases and newer fixtures don't always share a common bolt circle; keeping a handful of adapter plates for known legacy-to-current mismatches on the truck avoids a return trip when a straight swap doesn't line up in the field.
Standing Water Under the Plate Is the Usual Corrosion Starting Point
DurabilityMost premature corrosion at the plate-to-base interface starts from trapped moisture rather than atmospheric exposure alone; confirm drainage paths and gasket seating at every overhaul, not just when a corrosion complaint is already raised.
Frequently Asked Questions
Short answers about the adapter plate: what it is, how it works, specifications, standards, maintenance and common faults.
Q1What is an adapter plate?
An adapter plate (also called a conversion plate, mounting adapter or interface plate) is a precision-machined steel or aluminium plate that provides a mechanical interface between AGL equipment such as fixtures, signs and sensors and existing mounting structures such as base plates, base cans and poles with different bolt patterns or diameters.
Q2Why use an adapter plate?
It lets new equipment be installed on legacy infrastructure without costly and disruptive pavement modifications, which is why it is common in retrofit and upgrade projects.
Q3How does an adapter plate work?
It has two sets of holes. The lower set matches the existing base bolt pattern (for example 4 holes on a 150 mm PCD with M12 bolts) and the upper set matches the new fixture (for example 6 holes on a 200 mm PCD with M16 bolts). The plate transfers vertical, wind, vibration and impact loads from the fixture down into the existing foundation.
Q4Where are adapter plates used?
In runway lighting retrofits (LED edge, centreline and threshold lights onto existing base plates), taxiway lighting upgrades and approach lighting conversions, wherever new fixtures have different mounting dimensions from the old ones.
Q5What are the typical materials and dimensions?
Structural steel (S235JR or S355JR), hot-dip galvanised to at least 85 µm, or 316L stainless steel, with 6061-T6 aluminium for lightweight applications. Thickness is 10 mm to 25 mm and size 150 mm to 400 mm, with custom sizes available.
Q6Can an adapter plate change the fixture height?
Yes. An optional height offset of 10 mm to 50 mm compensates for fixture height differences.
Q7How flat and how strong is an adapter plate?
Flatness tolerance is 0.5 mm or better over the plate area, and vertical load capacity is typically 10 kN to 100 kN depending on thickness and material. Accurate machining preserves correct beam orientation.
Q8Which standards apply to adapter plates?
The main references are FAA AC 150/5345-42 (light bases and accessories, covering adapter rings and flange hardware), ICAO Annex 14, EN 10025 (structural steel) and ASTM A123 (galvanised coatings). Always verify the current edition and your aerodrome's approved requirements.
Q9How often should an adapter plate be inspected?
Typical intervals are semi-annual for levelling verification, and annual for mounting bolt torque, interface corrosion and galvanic isolation checks. Your airport's approved maintenance programme takes precedence.
Q10What are the common problems with adapter plates?
Common problems are bolt pattern mismatch, corrosion at the interface, inadequate bolt length, misaligned beams, insufficient plate thickness, galvanic corrosion with dissimilar metals and poor levelling. A plate that will not seat flush or bolt holes that do not line up usually indicates a pattern or orientation error.
Related Components
Go Beyond the Component. Understand the Complete AGL System.
You have explored the component. Now understand how it works within the complete Airfield Ground Lighting system.
Learn AGL from fundamentals to design, installation, testing, photometry, commissioning, maintenance and troubleshooting through structured technical training at Precision AGL Academy.
Build practical knowledge. Understand the circuit. Read the standards. Troubleshoot with confidence.
Your next step in AGL starts here.
Need expert guidance on adapter plate selection and installation for your AGL retrofit project?
Precision AGL Academy provides advice on bolt pattern verification, material selection, corrosion prevention, and alignment techniques for successful fixture upgrades.
Follow the Academy
Important Notice, Disclaimer & Limitation of Liability
Please read before relying on any content on this page.
- Purpose and intended users. This page on Adapter Plate is an independent educational and informational reference for competent, suitably trained and authorised airfield ground lighting (AGL) and aerodrome electrical personnel. It is not professional engineering, legal or regulatory advice, and using it does not create any professional or client relationship.
- Safety. Work on series circuits, constant current regulators (CCRs), isolating transformers, cables, connectors and lighting equipment can be fatal. Always apply the aerodrome’s permit-to-work and isolation/LOTO procedures, prove dead, use correctly rated instruments and PPE, and follow the manufacturer’s instructions. Nothing here authorises work on live equipment or the bypassing of any safety device or interlock.
- Aviation operations. Nothing here authorises installation, modification, energisation, adjustment, return to service, a change of operating category (CAT I/II/III), a NOTAM or any aerodrome certification or approval decision. These rest with the aerodrome operator, the air navigation service provider and the competent regulator.
- Specifications, ratings and typical values. Descriptions, parameters, ratings, dimensions, photometric values, wattages, service lives, maintenance tasks and intervals, failure modes, certifications (for example CE, FCC, UL or IEC marks) and part or type designations are general, typical and indicative. They may not match any particular product, make, model or version, and they are not a datasheet, design, specification, selection, sizing, procurement or acceptance criterion. The manufacturer’s current documents, the approved design and the project specification prevail.
- Standards and regulations. References to ICAO, FAA, IEC, IEEE, ISO, BIS, DGCA and other standards are paraphrased summaries for orientation only. They may be incomplete or out of date, and standards are revised from time to time. Consult the current official editions. Following this content does not guarantee compliance with any law, standard or regulatory approval, and the regulator’s requirements always prevail.
- No warranty. Content is provided “as is” and “as available”, without warranty of any kind, express or implied, including accuracy, completeness, currency or fitness for a particular purpose. Real installations differ in design, equipment make and model, age and condition.
- Limitation of liability. To the fullest extent permitted by applicable law, Precision AGL Academy, Precision AGL Solutions and their owners, authors and contributors are not liable for any injury, death, loss, damage, downtime, penalty or regulatory consequence, whether direct or indirect, arising from the use of, reliance on, or inability to use this content. Nothing in this notice excludes or limits any liability that cannot lawfully be excluded or limited.
- Independence, brands and no endorsement. This is an independent publication. It is not affiliated with, sponsored by or endorsed by ICAO, the FAA, the IEC, DGCA, AAI, any airport, regulator, supplier or equipment manufacturer. Third-party names, product designations, standard designations and trademarks (including Megger®, Siemens, Eaton, Crouse-Hinds, ADB SAFEGATE, Fluke, ABB and Honeywell) belong to their respective owners and are used only for identification and description. Mention of a brand, product or supplier is not a recommendation, and we do not sell, supply or quote for any component described.
- Images and third-party content. Images and illustrations are shown to help identify the type of component. They may show products of various manufacturers, may not depict the exact item described, and are not a statement about any product’s quality, approval or performance. Copyright and trademarks in third-party images, logos and photographs belong to their respective owners and are not claimed as ours. If you are a rights holder and want an image removed, replaced or credited, email precisionaglsolutions@gmail.com with the page address and we will act promptly.
- Training and services. Mentions of training, courses, consultation, advice, selection, installation, calibration or other support, including the “Learn…Certify” pathway, describe learning stages and are not an offer, quotation, tender or promise of any result. Contacting us or our replying creates no engagement or duty of care; an engagement exists only under a written agreement. We are not a regulator or certification body, and completing any course or reading this content is not a certification, accreditation or authorisation to work on any aerodrome.
- Copyright, privacy and corrections. © 2026 Precision AGL Academy. All rights reserved. Text, structure, compilations and diagrams may be viewed for personal reference and internal training only; any other copying, republication or commercial use needs our written permission. If you email, telephone or message us, we use your details only to respond and handle your matter, in line with applicable Indian law including the Digital Personal Data Protection Act, 2023; please do not send confidential or security-sensitive information. To display this page your browser requests fonts, icons and images from third-party servers, which receive your IP address and browser details under their own policies. Content is reviewed periodically with no obligation to update. To report an error, safety concern or rights issue, write to precisionaglsolutions@gmail.com.