Bottom Housing
The foundational structural component of an AGL fixture, housing the electrical connections, isolating transformer interface, and mounting hardware. Designed for durability, corrosion resistance, and secure installation in the airfield pavement or on frangible poles.
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What Is It?
The Bottom Housing is the lower structural component of an AGL fixture that provides the foundation for the entire assembly. It houses the electrical termination points, cable entry glands, and—in many cases—the isolating transformer interface. The bottom housing is designed to be securely mounted to the base plate, base can, or frangible pole, and is sealed to prevent moisture ingress. It is typically made from die-cast aluminium, stainless steel, or high-strength engineering plastics, with corrosion-resistant coatings to withstand the harsh airfield environment. The bottom housing also serves as the primary mechanical interface between the fixture and its mounting infrastructure, ensuring stable positioning and accurate optical alignment.
Purpose & Function
The bottom housing's purpose is to give an AGL fixture a stable, watertight foundation so that everything mounted above it — the light source, optics, and top cover — stays correctly positioned and electrically safe over years of exposure to weather, vibration, and aircraft-generated loads. By combining structural support, sealed cable termination, and the mechanical interface to the base plate, base can, or frangible pole in a single component, it ensures the fixture's optical axis stays aligned to the beam pattern the airfield's photometric design calls for, and that moisture cannot reach the electrical connections that would otherwise create a safety hazard or an outage. Because it is the point where the fixture actually meets the airfield's fixed infrastructure, its integrity underpins both the reliability of the light and, in elevated installations, the frangible breakaway behaviour that protects aircraft on impact.
Where Is It Used?
Bottom housings are used in every AGL fixture type, with design variations for specific mounting configurations:
- Elevated Runway & Taxiway Lights: Housings designed for frangible pole mounting, with cable entry from below.
- Inset Runway & Taxiway Lights: Housings designed for flush mounting in pavement, with cable entry through the base.
- Threshold & End Lights: Housings for inset or elevated mounting with cable glands for secondary connections.
- Approach Lighting: Housings for barrettes and flashing light units, often with multiple cable entries.
- PAPI & VASI: Precision housings with integrated adjustment mechanisms for optical alignment.
- Obstruction Lights: Housings for pole-mounted beacon systems.
- Connector Enclosures: Housings for L-823 connectors and junction boxes.
Installation & Location
The housing is bolted to the base can or foundation using its integral flange, with a gasket fitted between the two mating faces to seal the joint against moisture ingress. Cable entry points are fitted with glands or grommets sized to the incoming armoured or unarmoured cable, and the housing is orientated so the entry points face the direction the duct or trench approaches from, keeping the run inside as short and straight as practical. Elevated versions incorporate (or bolt directly onto) the frangible coupling, so the housing's base flange must align precisely with the coupling's mating face; inset versions are set so the housing sits fully recessed with the top cover finishing flush with the surrounding pavement or shoulder once fitted.
How Does It Work?
The bottom housing performs several critical functions in an AGL fixture:
- Structural Support: Provides a rigid foundation for the entire fixture assembly, supporting the top cover, optics, and light source.
- Electrical Termination: Houses the cable glands and terminal blocks for connecting the secondary cable from the isolating transformer to the light source (LED driver or lamp holder).
- Sealing: Forms a watertight and dust-tight seal when mated with the top cover, using gaskets or O-rings to achieve IP67/IP68 ratings.
- Mounting Interface: Includes mounting holes, flanges, or threaded inserts for securing the fixture to the base plate, base can, or frangible pole.
- Thermal Management: Acts as a heat sink or provides thermal paths to dissipate heat from the internal components to the mounting structure.
- Access for Maintenance: Often includes removable access panels or covers for servicing cable connections and components.
In inset fixtures, the bottom housing is designed to be embedded in the pavement with a load-bearing top cover that is flush with the surface. In elevated fixtures, the housing is mounted on a frangible pole or pedestal, with a breakaway coupling to protect aircraft in case of impact.
Inset vs. Elevated Mounting Path & Control Logic
Bottom Housing
Structural, electrical termination, and sealing foundation for the fixture
Housing Embedded in the Pavement
Set into the runway or taxiway surface itself
Load-Bearing Top Cover, Flush With Surface
Rated for direct aircraft wheel loading
Housing Mounted on Frangible Pole / Pedestal
Raised above the surface for line-of-sight visibility
Breakaway Coupling Protects Aircraft
Pole shears on impact rather than damaging the airframe
The Same Sealing, Termination, and Thermal Functions Are Adapted to Two Very Different Mechanical Environments
Physical Construction
The housing is die-cast or machined as a single lower shell with an integral mounting flange that bolts directly to the base plate, base can, or frangible pole base, so mechanical load and electrical termination share the same structural part. Cable entry points are formed as threaded glands sized for the primary and secondary leads, each fitted with a compression seal or grommet to maintain the assembly's IP rating at the point of highest water-ingress risk. The upper rim carries a machined groove or flat face for the top cover's gasket or O-ring, with threaded bosses or a bayonet/threaded-ring feature for a repeatable, tool-free or low-tool reseal during lamp changes. Internal standoffs or brackets position the isolating transformer interface, lamp holder, or LED driver at a fixed height relative to the mounting flange, so optical alignment is set by the housing geometry rather than by field adjustment.
Technical Parameters
| Parameter | Typical Value |
|---|---|
| Material | Die-cast aluminium (A380, A413), stainless steel (316L), or engineering plastic (glass-filled PA) |
| Surface Finish | Powder-coated (RAL colours), anodized, or electroless nickel-plated |
| Mounting Options | Base plate, base can, frangible pole, or direct to pavement (inset) |
| Cable Entry | 1-2 cable glands (PG13.5, PG16, or metric), or factory-fitted cable |
| Sealing | Gasket or O-ring seal between housing and top cover; IP67/IP68 rated |
| Temperature Range | -40°C to +80°C (continuous) |
| Corrosion Resistance | Salt spray tested (500+ hours, ASTM B117) |
| Impact Resistance | IK10 (20J) for elevated fixtures; inset housings load-tested to heavy-aircraft wheel loads (on the order of 25,000–30,000 kg per FAA in-pavement fixture test data) plus a 4,990 kg (11,000 lb) shear-load test per the current FAA AC 150/5345-46F requirement |
| Frangible Coupling Interface | Threaded flange for breakaway coupling (elevated fixtures) |
| Weight | 0.5 – 5 kg (depending on size and material) |
Applicable Standards
FAA AC 150/5345-46
Specification for Runway, Taxiway, Heliport, and Vertiport Light Fixtures (includes housing/enclosure requirements)
IEC 60529
Degrees of protection provided by enclosures (IP code)
IEC 62262
Degrees of protection against mechanical impacts (IK code)
ASTM B117
Salt spray (fog) testing for corrosion resistance
Installation Requirements
Mounting Flange Fixing
MechanicalBolt the housing's integral mounting flange to the base plate, base can, or frangible pole base, torquing fasteners evenly so mechanical load and electrical termination share the same structural part correctly.
Cable Gland Fitting
ElectricalFit compression seals or grommets sized to the primary and secondary lead diameters at each threaded gland, maintaining the assembly's IP rating at the highest water-ingress risk point.
Internal Component Positioning
MechanicalFit the isolating transformer interface, lamp holder, or LED driver onto the internal standoffs or brackets at the fixed height set by the housing geometry.
Top Cover Gasket Seating
MechanicalConfirm the top cover's gasket or O-ring seats correctly in the machined groove or flat face on the upper rim before the cover is fitted.
Earthing & Bonding
ElectricalBond the housing to earth per the drawing, confirming continuity resistance meets the design requirement before the site is closed up.
Bayonet/Threaded-Ring Function Check
MechanicalConfirm the bayonet or threaded-ring feature operates smoothly for repeatable, tool-free or low-tool reseal, so future lamp changes don't compromise the seal.
Pre-Energisation Sign-off
DocumentationRecord fastener torque, gland sealing, and bonding values as-built before the housing assembly is closed up and energised.
Testing & Commissioning
Cable Gland Sealing Check
InstallationVerify cable glands are correctly sized, seated, and torqued so the cable jacket is gripped and sealed without damage.
Insulation Resistance Test
Pre-EnergisationMegger-test terminations inside the housing to confirm insulation resistance clears the acceptable minimum before backfill.
Ingress Protection Verification
Final AcceptancePressure or water-immersion test the assembled housing to confirm the IP67/IP68 rating is achieved before the site is finally closed up.
Fastener Torque Check
InstallationTorque base and cover fasteners to the specified value in the correct sequence to ensure even gasket compression.
Earthing & Bonding Continuity Test
Pre-EnergisationConfirm the housing is bonded to earth per the drawing and that continuity resistance meets the design requirement.
Documentation & As-Built Sign-off
Final AcceptanceRecord insulation, bonding, and IP test results alongside as-built position in the commissioning dossier.
Inspection & Maintenance
Cable Gland Inspection
AnnualCheck cable glands and entry points for corrosion or degraded sealing compound that could let moisture into the housing.
Moisture Ingress Check
Semi-annualInspect for condensation or water staining inside the housing, a sign the cable entry seal is beginning to fail.
Fastener Torque Check
AnnualRe-torque mounting fasteners to spec, since loosened hardware is a common precursor to housing vibration damage.
Housing Crack Inspection
AnnualExamine the housing body for impact cracks from mowing equipment or vehicle contact, particularly around the base.
Galvanic Corrosion Check
AnnualInspect contact points between dissimilar metals for early signs of galvanic corrosion.
Common Field Issues
Corrosion of Cable Glands and Entries
Cable gland threads and entry points are common corrosion sites, especially in coastal environments. Use stainless steel glands and apply anti-seize compound to threads during installation. Regular inspection of cable glands is essential.
Moisture Ingress via Cable Entry
If the cable gland is not properly tightened or the seal is damaged, moisture can enter the housing through the cable entry. This can cause corrosion of electrical connections and short circuits. Ensure glands are tightened to the correct torque and seals are intact.
Loose Mounting Fasteners
Vibration from aircraft and ground vehicles can cause mounting bolts to loosen over time. This can affect optical alignment and lead to fixture damage. Regular torque checks and use of thread-locking compound are recommended.
Cracked Housing from Impact
In elevated fixtures, impact from snowploughs or debris can crack the housing. While frangible couplings are designed to break away, the housing itself can be damaged in side impacts. Inspect housings for cracks and replace if necessary.
Galvanic Corrosion
When different metals are in contact (e.g., aluminium housing with stainless steel bolts), galvanic corrosion can occur. Use insulating washers or compatible materials to prevent this.
Troubleshooting Guide
Water or Corrosion Found Inside the Housing
Fixture Loose or Optical Alignment Drifting
Visible Crack in the Housing
White or Powdery Corrosion Around Fasteners
Failure Modes & Root Causes
Cable Gland Seal Failure
Mounting Fastener Loosening
Housing Crack from Impact
Galvanic Corrosion at Fasteners
Safety & Operational Notes
Electrical Isolation Before Opening
Electrical SafetyDe-energise and lock out the series circuit at the CCR and verify zero energy before opening the bottom housing, since it contains the fixture's cable termination points and, on many designs, the isolating transformer interface.
Hot Surfaces Near the Transformer Interface
Personnel SafetyAllow the housing to cool before handling after an active period, particularly where the isolating transformer sits close to or within the housing.
FOD Control at the Base Can
OperationalKeep gaskets, fasteners, and cable glands accounted for at all times when the housing is open, and secure or remove loose material before reinstating the fixture.
Reseal to Maintain IP Rating
ComplianceReinstate gaskets, glands, and fasteners correctly and torque them evenly on closing, since a poorly resealed bottom housing allows moisture ingress into the termination points and transformer interface, which can go unnoticed until a fault develops.
Manual Handling
Personnel SafetyDie-cast or stainless-steel bottom housings can be heavy for their size; use correct lifting technique when removing or refitting the assembly.
Restore to Compliant State
ComplianceVerify a clean insulation test on the reassembled termination before re-energising the circuit, and confirm the housing is properly seated and sealed before closing out the work.
Engineering & Expert Notes
Gland Torque is a Recurring Check, Not a One-Time Install Step
SealingCable gland compression relaxes over time as the gasket material takes a permanent set under load, especially through repeated thermal cycling. A gland that sealed correctly at installation can loosen enough to admit moisture within a few seasons without any visible sign from outside; re-torque checks belong in the routine maintenance cycle, not just the commissioning checklist.
Isolate Dissimilar Metals at Every Fastener
CorrosionGalvanic corrosion at the housing-to-fastener interface is often blamed on "bad material" when the actual cause is a missing insulating washer between an aluminium housing and stainless steel bolt. Specify the isolation method explicitly in the installation procedure rather than assuming the fastener supplier has accounted for it.
In-Pavement Housings See a Different Failure Pattern Than Elevated Ones
DesignThe same bottom housing design serves flush in-pavement fixtures under direct wheel loading and elevated frangible-mounted fixtures under impact loading, but the dominant failure mode differs: flush housings tend to fail from repeated mechanical fatigue and water pooling around the top cover seal, while elevated housings tend to fail from single-event impact or vibration loosening. Tailor the inspection focus to which mounting configuration is actually in the field.
A Hairline Crack Rarely Stays Hairline
MechanicalA crack that looks cosmetic on a die-cast or composite housing is a stress concentration point that will propagate under continued vibration and thermal cycling, even without another impact. Replace, don't monitor, a cracked housing once identified — deferring replacement to "watch it next cycle" is a common way a minor defect becomes a moisture-ingress failure.
Loose Mounting Bolts Show Up as Optical Faults First
TroubleshootingBecause the bottom housing establishes the fixture's mechanical reference for optical alignment, a loosening mounting bolt often surfaces first as a photometric aim complaint rather than an obvious mechanical symptom. When a fixture's aim has drifted with no record of impact, check housing mounting torque before assuming the optical assembly itself has failed.
Trapped Water Does More Damage Than Water That Passes Through
DurabilityA housing that leaks slightly but drains is often less damaging long-term than one that seals well everywhere except a single low point, because standing water at a trapped low point accelerates corrosion at exactly the location hardest to inspect. Check for drainage provision or trapped-water pockets as part of the design review, not only the overall IP rating.
Frequently Asked Questions
Short answers about the Bottom Housing: what it is, how it works, typical specifications, standards, maintenance and common faults.
Q1What is a Bottom Housing?
The Bottom Housing is the lower structural component of an AGL fixture that provides the foundation for the entire assembly. It houses the electrical termination points, cable entry glands, and—in many cases—the isolating transformer interface. The bottom housing is designed to be securely mounted to the base plate, base can, or frangible pole, and is sealed to prevent moisture ingress. It is typically made from die-cast aluminium, stainless steel, or high-strength engineering plastics, with corrosion-resistant coatings to withstand the harsh airfield environment.
Q2What is the purpose of a Bottom Housing?
The bottom housing's purpose is to give an AGL fixture a stable, watertight foundation so that everything mounted above it — the light source, optics, and top cover — stays correctly positioned and electrically safe over years of exposure to weather, vibration, and aircraft-generated loads.
Q3Where is a Bottom Housing used and installed?
Bottom housings are used in every AGL fixture type, with design variations for specific mounting configurations: Elevated Runway & Taxiway Lights: Housings designed for frangible pole mounting, with cable entry from below. Inset Runway & Taxiway Lights: Housings designed for flush mounting in pavement, with cable entry through the base. Threshold & End Lights: Housings for inset or elevated mounting with cable glands for secondary connections. Approach Lighting: Housings for barrettes and flashing light units, often with multiple cable entries.
Q4How does a Bottom Housing work?
The bottom housing performs several critical functions in an AGL fixture: Structural Support: Provides a rigid foundation for the entire fixture assembly, supporting the top cover, optics, and light source. Electrical Termination: Houses the cable glands and terminal blocks for connecting the secondary cable from the isolating transformer to the light source (LED driver or lamp holder). Sealing: Forms a watertight and dust-tight seal when mated with the top cover, using gaskets or O-rings to achieve IP67/IP68 ratings.
Q5What are the typical technical parameters of a Bottom Housing?
Typical values are — Material: Die-cast aluminium (A380, A413), stainless steel (316L), or engineering plastic (glass-filled PA); Surface Finish: Powder-coated (RAL colours), anodized, or electroless nickel-plated; Mounting Options: Base plate, base can, frangible pole, or direct to pavement (inset); Cable Entry: 1-2 cable glands (PG13.5, PG16, or metric), or factory-fitted cable; Sealing: Gasket or O-ring seal between housing and top cover; IP67/IP68 rated; Temperature Range: -40°C to +80°C (continuous); Corrosion Resistance: Salt spray tested (500+ hours, ASTM B117).
Q6Which standards apply to a Bottom Housing?
The main references are FAA AC 150/5345-46 — Specification for Runway, Taxiway, Heliport, and Vertiport Light Fixtures (includes housing/enclosure requirements); IEC 60529 — Degrees of protection provided by enclosures (IP code); IEC 62262 — Degrees of protection against mechanical impacts (IK code); ASTM B117 — Salt spray (fog) testing for corrosion resistance. Always verify the current edition and your aerodrome's approved requirements.
Q7How often should a Bottom Housing be inspected and maintained?
Typical maintenance tasks and intervals are: Cable Gland Inspection (annual); Moisture Ingress Check (semi-annual); Fastener Torque Check (annual); Housing Crack Inspection (annual); Galvanic Corrosion Check (annual). Your airport's approved maintenance programme takes precedence.
Q8How do I troubleshoot common Bottom Housing faults?
Common symptoms covered are: Water or Corrosion Found Inside the Housing; Fixture Loose or Optical Alignment Drifting; Visible Crack in the Housing; White or Powdery Corrosion Around Fasteners. For example, for "Water or Corrosion Found Inside the Housing": Likely Cause: Loose or improperly torqued cable gland, a damaged gland seal, or a cracked housing wall allowing moisture ingress at the cable entry. Diagnostic Steps: Inspect the cable gland torque and seal condition; check for visible cracks in the housing wall; look for staining or corrosion tracking back to a specific entry point. Corrective Action: Retorque or replace the cable gland to the correct spec, replace damaged seals, and replace the housing if cracked; clean and treat any internal corrosion before reassembly.
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