Extension Ring
A precision‑machined, cylindrical spacer that raises an elevated AGL fixture to the correct height above the pavement surface. Used to compensate for variations in pavement thickness, accommodate cable bends, and ensure that the fixture's light beam meets the required photometric specifications.
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What Is It?
An Extension Ring (also known as a spacer ring, riser ring, or height‑adjustment ring) is a cylindrical steel or aluminium spacer that is installed between the base can (or mounting plate) and the elevated light fixture. Its primary purpose is to raise the fixture to the correct height above the pavement surface, ensuring that the light beam is emitted at the proper angle and meets the photometric requirements specified in ICAO Annex 14 and FAA standards.
Key characteristics of extension rings include:
- Precision Height: Available in standard heights (typically 25mm, 50mm, 75mm, 100mm, 150mm) to accommodate different pavement build‑ups.
- Structural Integrity: Made from high‑strength steel or aluminium, capable of supporting the weight of the fixture and withstanding aircraft loads, vibration, and impact.
- Corrosion Protection: Hot‑dip galvanised, powder‑coated, or made from stainless steel to resist the harsh airfield environment.
- Compatibility: Designed to match the bolt pattern and diameter of the base can and the fixture base plate, ensuring a secure and leak‑proof connection.
Extension rings are used in the following scenarios:
- Pavement Resurfacing: When an existing pavement is overlaid with a new asphalt or concrete layer, the added thickness increases the height of the base can, requiring an extension ring to raise the fixture back to the correct height.
- New Installations: When the base can is set too deep during construction, an extension ring can compensate for the error.
- Cable Bend Radius: Providing additional space for the secondary cable to bend without being pinched or stressed.
- Fixture Type Change: When replacing an old fixture with a new one that has a different mounting height requirement.
The extension ring is installed between the top flange of the base can and the fixture's base plate, with gaskets or sealant used to maintain a watertight seal. The ring is secured with bolts, and the fixture is then mounted on top of the ring.
Purpose & Function
A base can is installed once, but the pavement around it does not stay fixed — resurfacing adds new layers of asphalt or concrete on top of the original surface, gradually burying the can deeper than it was designed to sit. Without correction, the fixture mounted on it would end up sitting lower than specified, changing its height above the runway and, with it, the angle at which its beam meets a pilot's eye — a change that can be enough to fail the photometric tolerances set out in ICAO Annex 14 or FAA standards. The extension ring's purpose is to restore the fixture to its correct design height by inserting a precise, known-height spacer between the can and the fixture, avoiding the alternative of excavating and resetting the entire base can — a far more disruptive and costly repair — every time the pavement is resurfaced.
Where Is It Used?
Extension rings are used in a wide range of AGL applications where elevated fixtures are installed on paved surfaces, including:
- Runway Edge Lighting: For elevated runway edge light fixtures, especially after pavement resurfacing or where the base can was set too deep.
- Taxiway Edge Lighting: For elevated taxiway edge lights on curves, intersections, or where pavement thickness varies.
- Approach Lighting: For elevated approach light fixtures, barrettes, and flashing light units.
- Obstruction Lighting: For elevated obstruction lights on poles or structures where height adjustments are needed.
- Airfield Signage: For elevated mandatory instruction signs and direction signs that require precise height positioning.
- Sensor Mounting: For anemometers, wind cones, and other instrumentation that requires a raised mounting.
The ring is typically used in conjunction with a base can and a base plate. The ring is placed between the two components, and the entire assembly is bolted together to form a rigid, stable mounting column. The ring is designed to be watertight, with a gasket at each mating surface to prevent moisture ingress.
Installation & Location
The ring fits between the base can's original rim and the fixture or cover plate above it, used to raise the effective height of the can after pavement resurfacing (overlay) has changed the grade above the existing can, or where the can was originally set slightly too deep. Ring height is selected to bring the fixture's finished mounting surface back to flush with the new pavement grade, checked with a straightedge across the surrounding surface before final fixing; the ring's gasket faces at both the base-can and fixture ends are inspected during fitting, since adding an extra jointed interface into the stack introduces two more seal points that must both hold.
How Does It Work?
The extension ring functions as a height‑adjustment spacer that raises the fixture to the correct elevation above the pavement. Its design ensures that the fixture is mounted securely, level, and at the proper height for optimal light output:
- Height Adjustment: By adding one or more extension rings of the required height, the fixture can be positioned so that the light source is at the precise height specified in the design standards (typically 150mm to 300mm above the pavement, depending on the fixture type).
- Structural Support: The ring transfers the weight of the fixture and any applied loads (wind, snow, maintenance) directly down through the base can to the pavement, providing a robust and stable mounting.
- Watertight Seal: O‑rings or flat gaskets are fitted between the ring and the base can, and between the ring and the fixture base plate. These seals prevent water, de‑icing fluids, and debris from entering the base can and damaging the transformer or cable connections.
- Cable Bend Radius: The additional height provides more space for the secondary cable to bend from the transformer to the fixture, reducing stress on the cable and preventing damage.
The installation process typically involves:
- Cleaning: The top flange of the base can and the bottom of the fixture base plate are cleaned and inspected for damage.
- Gasket Placement: A gasket is placed on the base can flange.
- Ring Placement: The extension ring is positioned on the base can, aligning the bolt holes.
- Additional Gaskets: A second gasket is placed on the top of the ring.
- Fixture Mounting: The fixture base plate is placed on top of the ring, and the bolts are inserted and torqued to the specified value.
- Levelling: The fixture is checked for level and adjusted if necessary by using shims or adjusting the ring.
Extension rings are typically used in stacks of up to three rings (e.g., 25mm + 50mm + 75mm) to achieve the required height. The total height should not exceed the manufacturer's recommended maximum to ensure stability.
System Architecture & Control Logic
The extension ring is a purely mechanical spacer with no control-logic role — it sits between the base can and the fixture or mounting ring above it, and stacking one or more rings is how the fixture's final height above the pavement is set. This matters because the fixture's beam geometry and photometric performance depend on it sitting at the specified elevation relative to the pavement surface, so the extension ring is effectively where that height tolerance is taken up during installation, rather than through any adjustment in the fixture itself. It carries the same load path as the base can beneath it and is sealed against the same base can, so its structural and sealing integrity matter even though it has no electrical function of its own.
Beam geometry depends on the fixture sitting at the specified elevation — the ring stack is where that tolerance is set.
Installation Sequence
Physical Construction
The extension ring is a short, hollow cylinder of structural steel, stainless steel, or aluminium, machined with matching bolt-hole patterns on both its top and bottom faces so it sandwiches between the base can and the fixture (or an adjoining ring) to raise the fixture's effective height when pavement resurfacing changes the finished grade. Wall thickness is sized to carry the same vertical load rating as the base can it extends, and the ring's internal diameter matches the base can bore so wiring and the isolating transformer below remain accessible and unobstructed. A gasket or O-ring seats in a machined groove at each mating face, maintaining the assembly's IP67/IP68 seal across the added joint. Multiple rings of different standard heights can be stacked to reach the required elevation, with each joint sealed and bolted independently, and the same corrosion-resistant finish (galvanising, powder-coating, or stainless) is applied as used on the base can and fixture it connects.
Technical Parameters
| Parameter | Typical Value |
|---|---|
| Material | Structural steel (S235JR or S355JR), hot‑dip galvanised (min 85µm) or stainless steel (316L); also available in aluminium |
| Height (standard sizes) | 25mm (1"), 50mm (2"), 75mm (3"), 100mm (4"), 150mm (6") |
| Diameter (internal) | 150mm to 300mm (6" to 12") – matches base can diameter |
| Wall Thickness | 4mm to 8mm |
| Bolt Hole Pattern | Matches base can and fixture base plate (typically 4 or 6 holes) |
| Gasket Type | Neoprene or silicone O‑ring or flat gasket (included or optional) |
| Load Capacity (vertical) | ≥ 50 kN (depending on size and material) |
| IP Rating (with gaskets) | IP67 / IP68 (when properly sealed) |
| 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, and Accessories (includes extension rings)
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
Flange Cleaning & Inspection
MechanicalClean and inspect the base can and fixture flanges before fitting, since the ring adds two more sealing joints into the stack that must both hold.
Ring Height Selection
CivilSelect ring height (or stack multiple standard-height rings) to bring the fixture's finished mounting surface back to flush with the new pavement grade after resurfacing.
Gasket Seating at Each Joint
MechanicalSeat a gasket or O-ring in the machined groove at each mating face, sealing every added joint independently to maintain the assembly's IP67/IP68 rating.
Bolt Alignment & Torque
MechanicalAlign bolt holes through the ring stack and torque the fixture down to spec once the rings and gaskets are in place.
Level Check & Shimming
MechanicalCheck the fixture is level against the surrounding pavement with a straightedge, shimming as needed before final fixing.
Fitment Sign-off
DocumentationRecord ring height(s) used, gasket condition, and torque values as-built before the fixture is accepted back into service.
Testing & Commissioning
Height Verification
Pre-InstallationConfirm the ring (or stack of rings) brings the fixture to the exact height specified in the design standard before backfill.
Gasket Seal Check
Pre-InstallationConfirm O-rings or flat gaskets are correctly fitted between the ring and base can, and between the ring and fixture base plate.
Bolt Pattern & Torque Verification
Pre-InstallationConfirm the bolt-hole pattern aligns with the base can and fixture base plate, and torque fasteners to spec for a stable mounting.
Cable Bend Radius Check
Pre-EnergisationConfirm the added height provides adequate clearance for the secondary cable to bend without stress from transformer to fixture.
Ingress Protection Verification
Final AcceptanceConfirm the assembled stack meets its rated IP67/IP68 class before the base can is finally backfilled.
Level & Alignment Check
Final AcceptanceVerify the ring assembly is level so the fixture mounted on top sits at the correct aim and elevation for its beam pattern.
Documentation & As-Built Sign-off
Final AcceptanceRecord ring height, quantity, and seal verification results in the commissioning dossier for handover.
Inspection & Maintenance
Gasket / O-Ring Inspection
AnnualCheck the neoprene or silicone gasket between the ring and base can for compression set or cracking that could break the IP67/IP68 seal.
Bolt Torque Verification
AnnualRe-torque bolts joining the ring to the base can and fixture so the stack maintains its rated ≥50 kN vertical load capacity.
Alignment / Bolt Pattern Check
AnnualConfirm the ring's bolt hole pattern remains correctly aligned with both the base can and fixture base plate after any re-grading or resurfacing work.
Corrosion Check
AnnualInspect galvanised or stainless surfaces, especially the internal bore, for coating breakdown or pitting from trapped moisture.
Height / Grade Adjustment Review
AnnualVerify the ring height still brings the fixture flush with finished pavement grade after any resurfacing or settlement.
Wall Thickness / Deformation Check
AnnualLook for cracking, dents, or wall thinning that could reduce load capacity, particularly on rings under trafficked pavement.
Standing Water Check
AnnualConfirm the extended stack still drains properly with no water trapped between the ring and base can gaskets.
Stack Stability Check
AnnualVerify there is no lateral play or rocking in the ring-to-base-can-to-fixture stack, which can develop after repeated freeze-thaw or traffic loading cycles.
Common Field Issues
Incorrect Ring Height Selection
Using a ring that is too short or too tall will result in the fixture being at the wrong height, causing the light beam to be misaimed (either too high or too low). Always verify the required height from the design drawings and measure the actual pavement thickness before selecting the ring height.
Gasket Deterioration
The gaskets between the ring and the base can, and between the ring and the fixture base plate, can deteriorate over time due to UV exposure, temperature cycling, and chemical attack. This leads to water ingress and corrosion. Replace gaskets at every maintenance opportunity and use high‑quality neoprene or silicone gaskets.
Loose Bolts
Vibration from aircraft can cause the bolts securing the ring and fixture to loosen, leading to misalignment or instability. Use lock washers, thread‑locking compound, and schedule regular bolt torque checks as part of the maintenance program.
Corrosion Between Ring and Base Can
If the ring and base can are made of dissimilar metals (e.g., galvanised steel ring on a stainless steel base can), galvanic corrosion can occur at the interface. Use a corrosion‑resistant coating, apply anti‑seize compound, or use a gasket to isolate the dissimilar metals.
Using Too Many Rings
Stacking more than three extension rings can result in an unstable mounting column, as the increased height reduces the rigidity and resistance to lateral loads. If more than 150mm of height is needed, consider using a taller base can or a custom‑made extension ring.
Damaged Threads on Bolt Holes
The bolt holes on the ring, base can, or fixture base plate can become stripped or damaged, preventing secure bolting. Use thread repair kits or replace the damaged component. Apply anti‑seize compound to the bolts to prevent galling.
Inadequate Sealing of Cable Entry
If the secondary cable passes through the extension ring (in some designs), the cable entry must be properly sealed to prevent water ingress. Use a cable gland or sealant compound to seal the entry point.
Troubleshooting Guide
Fixture Beam Aimed Too High or Too Low
Water Ingress or Corrosion at the Ring Joints
Fixture Loose, Wobbling, or Misaligned
Corrosion Developing Between Ring and Base Can
Unstable Mounting Column / Fixture Wobble Under Load
Bolts Will Not Tighten or Strip Out During Assembly
Water Ingress at the Cable Entry Through the Ring
Failure Modes & Root Causes
Gasket Deterioration at Ring Joints
Galvanic Corrosion Between Ring and Base Can
Bolt Loosening from Vibration
Incorrect Ring Height Selection
Excessive Ring Stacking
Safety & Operational Notes
Verify Isolation Before Removal
Electrical SafetyDe-energise and lock out the circuit and verify zero energy at the fixture before adding, removing, or exchanging an extension ring, since the ring sits directly in the cable and connector path.
Correct Height & Photometric Impact
ComplianceUse only the specified ring height and stack combination; an incorrect height changes the fixture's beam elevation angle and can put it out of photometric tolerance.
Frangibility Not to Be Compromised
MechanicalWhere the ring sits within a frangible assembly, use only rings and fasteners rated for that assembly; an incompatible or over-stiff ring can raise the effective breakaway force.
Verify Gasket Seal at Each Joint
Environmental SealingInspect and, if needed, replace the gasket between the ring and the base can or fixture at each joint to maintain the assembly's environmental seal.
Airside Work Coordination
OperationalCoordinate with ATC/airside and raise a NOTAM where the fixture will be dark or out of tolerance during the ring change.
FOD Control
OperationalAccount for fasteners, gaskets, and removed rings on the movement area, and clear the site before reinstating the fixture.
Engineering & Expert Notes
Stacking Multiple Rings Compounds Small Tolerance Errors
PrecisionUsing two or three extension rings to reach a required height, rather than the single correct-height ring, stacks each ring's small tolerance error into a cumulative height and level deviation that can affect photometric compliance. Use the correct single ring where available rather than stacking.
Check the Ring-to-Can Interface for Trapped Moisture
CorrosionThe gasket or seal between an extension ring and the base can below it is a common point for moisture to collect and corrode both surfaces from the inside; inspect this interface specifically, not just the ring's exterior.
Verify Ring Level Independently of Fixture Level
AlignmentA slightly out-of-level ring can be masked by fixture-side leveling adjustments, hiding a foundation or ring-seating problem that will resurface at the next fixture replacement. Check ring level on its own before assuming the fixture's level reading reflects the ring beneath it.
Confirm Bonding Continuity Is Maintained Through the Ring
ElectricalWhere the extension ring is part of the fixture's earthing/bonding path, corrosion or poor seating at the ring interface can quietly break continuity even though the ring is mechanically secure. Verify continuity with a meter, especially after any ring has been added or replaced.
Record Which Ring Height Was Installed at Each Location
DocumentationPavement build-up varies across a site, so different locations often use different ring heights; keeping this on record prevents an incorrect height being installed during a future fixture replacement based on an assumption that all locations match.
Re-Torque Ring Fasteners After the First Season
StructuralLike other threaded connections exposed to thermal cycling, extension ring fasteners can loosen slightly after their first seasonal cycle; schedule a re-torque check rather than assuming initial installation torque holds indefinitely.
Frequently Asked Questions
Short answers about extension rings: purpose, height, loading, standards and maintenance.
Q1What is an extension ring in airfield ground lighting?
An extension ring, also called a spacer, riser or height-adjustment ring, is a cylindrical steel or aluminium spacer installed between the base can (or mounting plate) and an elevated light fixture to set the correct fixture height.
Q2When is an extension ring needed?
Typically after pavement resurfacing, on curves and intersections where pavement thickness varies, or wherever a fixture needs precise height adjustment. It is used with elevated runway and taxiway edge lights, approach lights, obstruction lights and signs.
Q3What heights do extension rings come in?
Standard sizes are 25 mm, 50 mm, 75 mm, 100 mm and 150 mm, in internal diameters from 150 mm to 300 mm to match the base can.
Q4How many extension rings can be stacked?
The page warns that stacking more than three can give an unstable mounting column. Select the correct ring height for the job instead of stacking excessively.
Q5How does an extension ring stay watertight?
O-rings or flat gaskets sit between the ring and the base can and between the ring and the fixture base plate, keeping water, de-icing fluids and debris out of the base can (IP67/IP68 when properly sealed).
Q6Which standards apply to extension rings?
The main references are FAA AC 150/5345-42 (light bases, transformer housings, junction boxes and accessories), ICAO Annex 14, EN 10025 for the steel and ASTM A123 for galvanising.
Q7How often should extension rings be inspected?
Annually. Check the gaskets or O-rings, re-torque the bolts and confirm the bolt pattern still aligns with both the base can and the fixture base plate.
Q8What are the common problems with extension rings?
Selecting the wrong height, gasket deterioration, loose bolts from aircraft vibration, corrosion between dissimilar metals such as a galvanised ring on a different base can material, and using too many rings.
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