Base Plate

A Base Plate (also known as a mounting plate, fixture plate, or foundation plate) is a robust metal plate that serves as the primary structural interface between an elevated AGL fixture (or other airfield equipment) and the pavement or foundation. It is designed to be securely anchored into the concrete or asphalt, providing a stable, level, and accurately positioned mounting surface for the fixture.

Key characteristics of base plates include:

• Precision Machining: The plate is machined to tight tolerances to ensure that the fixture is perfectly aligned (vertical and horizontal) and that the beam pattern is correctly oriented.
• High Strength: Made from high‑grade steel (often galvanised or stainless) to withstand heavy aircraft loads, impact, and vibration without deformation.
• Corrosion Protection: Hot‑dip galvanised, stainless steel, or coated with anti‑corrosion paint to resist the harsh airfield environment (moisture, de‑icing fluids, fuels).
• Mounting Holes: Pre‑drilled or tapped holes that match the fixture's bolt pattern, allowing for quick and secure attachment.

Base plates are used for a variety of AGL equipment, including:

• Elevated light fixtures: Runway edge lights, taxiway edge lights, approach lights, and obstruction lights.
• Junction boxes and distribution panels: Providing a mounting surface for underground or above‑ground enclosures.
• Signage: For airfield signs and guidance markers.
• Sensor housings: For weather sensors, RVR sensors, and other instrumentation.

The plate is typically embedded in a concrete foundation or directly into the asphalt pavement, with anchor bolts or expansion bolts securing it in place. The top surface of the plate is machined flat and level, and often includes a central hole or conduit entry for cable routing.

An elevated fixture's photometric performance depends on it being installed at a specific height, tilt, and orientation — figures decided on a drawing long before the equipment reaches site. The base plate's purpose is to make that specification physically enforceable: a level, precisely machined interface that fixes the fixture's angle and position at installation and keeps it there under the wind loading, vibration, and thermal cycling the fixture experiences afterward. Because the plate carries the full mechanical load from the fixture down into the foundation, it also protects the fixture itself — its housing, optics, and internal wiring are never expected to bear structural loads they were not designed for. In practical terms, the base plate is what turns an engineering drawing's height and angle tolerances into something a technician can actually deliver and trust on site.

Base plates are used throughout the airfield wherever elevated fixtures or equipment require a secure, level, and durable mounting foundation, including:

• Runway Edge Lighting: Providing the foundation for elevated runway edge light fixtures mounted on frangible poles.
• Taxiway Edge Lighting: Mounting elevated taxiway edge light fixtures, especially on curves or where flush mounting is not possible.
• Approach Lighting: Supporting elevated approach light fixtures, barrettes, and flashing light units.
• Obstruction Lighting: Mounting obstruction lights on poles, towers, or building rooftops.
• Airfield Signage: Providing the base for mandatory instruction signs, direction signs, and information signs.
• Junction Boxes and Distribution Panels: Supporting above‑ground enclosures that house cable terminations, connectors, or control equipment.
• Sensor Mounting: For anemometers, wind cones, visibility sensors, and other airfield instrumentation.

The plate is installed during the pavement construction or as a retro‑fit by cutting a recess into the existing pavement. It is leveled using grout or shims, and anchor bolts are torqued to the specified value. The fixture is then bolted to the plate using the pre‑drilled holes. The plate's design ensures that the fixture can be easily removed for maintenance or replacement without disturbing the foundation.

The base plate sits on top of the foundation or base can, bolted down onto anchor bolts embedded in the concrete, with a levelling check performed before final torque so the pole or fixture mounted on it stands plumb rather than leaning. Plate position and orientation are set out from the fixture's survey point, with any directional fixture's plate keyed or marked so the pole can only be bolted on facing the correct direction. Grouting or shimming under the plate is used where the foundation surface isn't perfectly flat, since an unsupported corner of the plate becomes a stress concentration point under wind or impact loading.

The base plate functions as the critical load‑bearing and alignment interface between the fixture and the pavement. Its design ensures that the fixture remains stable, level, and correctly oriented throughout its service life:

• Load Distribution: The plate distributes the weight of the fixture (and any additional loads, such as wind or ice) evenly over a larger area of the pavement, preventing localised stress or cracking.
• Precise Alignment: The machined top surface provides a true reference plane, ensuring that the fixture is perfectly vertical and that the light beam is aimed correctly (e.g., towards the runway centreline or approach path).
• Secure Fixing: Anchor bolts embedded in the foundation secure the plate, while the fixture bolts into the plate's threaded holes. This provides a robust connection that can withstand aircraft jet blast, vibration, and maintenance activities.
• Frangible Feature (if applicable): In some designs, the base plate includes a frangible coupling or shear‑off feature that allows the fixture to break away cleanly on impact, minimizing damage to the aircraft.

The installation process typically involves:

• Setting the plate: The plate is positioned in the wet concrete or asphalt, or bolted into a pre‑formed recess using epoxy anchors.
• Levelling: The plate is adjusted to the correct height and inclination using a spirit level or digital level.
• Grouting: A non‑shrink grout is injected under the plate to fill any voids and provide a solid, level bearing surface.
• Torquing: The anchor bolts are torqued to the manufacturer's specified value to ensure proper clamping force.
• Fixture mounting: The fixture is placed on the plate and secured using the fixture mounting bolts.

The plate is designed to withstand vertical loads (aircraft weight) and horizontal loads (braking, turning, and jet blast) without shifting or deforming. It is typically designed for a minimum service life of 20 years, with regular inspection for corrosion, loose bolts, or damage.

The base plate is a purely mechanical interface with no electrical or control role — it sits between the fixture or pole above and the anchor bolts and foundation below, and its job is to spread the loads from above (dead weight, wind, and any impact) evenly into the bolt pattern rather than concentrating them on one or two fasteners. Its thickness, footprint, and bolt-hole pattern are matched to the specific fixture it carries and to the load case at that installation, which is why it is not a generic, interchangeable part between fixture families even where the anchor bolt spacing happens to match.

A base plate is cut from structural steel, aluminium, or stainless steel plate stock to the thickness required for its rated load, then machined or laser-cut with a bolt hole pattern matched to both the fixture's mounting footprint on top and the anchor bolt layout set into the pavement below. The plate's top and bottom surfaces are finished flat within a tight tolerance so the fixture sits level and true once installed, since even a small deviation can throw off the fixture's beam alignment. Corrosion protection is applied as a hot-dip galvanised coating or powder coat finish over the full plate surface, with threaded holes or captive nuts often protected separately to prevent the coating from interfering with bolt engagement. Where the plate incorporates a frangible coupling interface for elevated fixtures, a weakened section or shear-designed connection point is machined into the plate itself, engineered to fail predictably under an aircraft impact load while retaining full strength under normal wind and vibration loading. The plate is bolted down onto the anchor bolts protruding from the pavement, with levelling nuts or shims used to fine-tune its final position before grouting or backfilling around the base.