FRANGIBLE VS RIGID
Two opposite structural philosophies for AGL fixture mounting — frangible mounting is designed to break or yield in a controlled manner when subjected to an abnormal impact, while rigid mounting is designed to remain structurally fixed and resist applied loads. The choice is mainly determined by location, aircraft safety requirements, and expected mechanical loads.
At a Glance
| Feature | Frangible Mounting | Rigid Mounting |
|---|---|---|
| Main purpose | Reduce hazard during aircraft impact | Provide maximum structural stability |
| Response to abnormal impact | Breaks/yields in a controlled manner | Resists impact and remains fixed |
| Aircraft safety | Higher where objects may be struck | Less suitable where breakaway protection is required |
| Structural strength | Limited by frangibility requirement | Generally higher |
| Fixture stability | Adequate for normal conditions | High |
| Typical use | Elevated fixtures in aircraft movement environments | Protected or non-impact locations |
| Replacement after impact | Often required | May not require mounting replacement |
| Main design philosophy | Fail safely | Remain structurally intact |
Frangible Mounting
Under normal conditions, the fixture sits on a mounting that includes an intentional weak or frangible point.
💡
│
──────┴──────── PAVEMENT (normal condition)
AIRCRAFT / IMPACT
→
💥
│
💡
\
✕ BREAKAWAY
────────┴──────── PAVEMENT (abnormal impact)
The mounting is intentionally designed to separate or fail in a controlled manner.
Rigid Mounting
Under normal or applied load, the rigid support is designed to maintain the structural connection.
💡
│
[RIGID SUPPORT]
│
──────┴──────── PAVEMENT
↓ LOAD
💡
│
[RIGID SUPPORT]
│
──────┴──────── PAVEMENT (applied load)
The objective is to maintain the structural connection rather than deliberately separate.
Break Safely vs Stay Fixed
ABNORMAL IMPACT
💥
│
▼
BREAKAWAY
│
▼
REDUCE HAZARD
ABNORMAL IMPACT
💥
│
▼
RESIST LOAD
│
▼
REMAIN FIXED
Easy memory: Frangible = designed to break safely. Rigid = designed to stay fixed.
Reducing Hazard Near the Movement Area
An elevated object near an aircraft movement area can create a hazard. A frangible support is intended to reduce the consequences of striking the fixture — the objective is not to make the fixture indestructible.
IMPACT │ ▼ FRANGIBLE CONNECTION │ ▼ CONTROLLED BREAKAWAY │ ▼ REDUCED STRUCTURAL OBSTRUCTION
The objective: protect the aircraft by allowing the fixture/support to fail in a controlled manner.
Where Structural Resistance Is the Priority
Some installations require high structural resistance. Rigid mounting can be advantageous where the fixture must resist vibration, wind, mechanical forces, operational movement, and equipment loads — and where aircraft-impact frangibility is not the governing requirement.
ENVIRONMENTAL / MECHANICAL LOAD
│
▼
RIGID SUPPORT
│
▼
FIXTURE REMAINS IN POSITION
How Each Mounting Behaves Under Load
Frangible
NORMAL LOADS
│
▼
SUPPORT HOLDS
│
▼
ABNORMAL IMPACT
│
▼
CONTROLLED BREAKAWAY
A properly designed frangible mounting should not break during normal environmental or operational loading — it is intended to fail only under specified abnormal impact conditions.
Rigid
NORMAL LOADS
│
▼
SUPPORT HOLDS
│
▼
HIGHER LOAD
│
▼
DESIGNED TO RESIST
The design objective is generally to maintain structural integrity over a wider range of loads.
Elevated vs Inset Fixtures
Elevated AGL Fixture
Where required by the installation and applicable standards, the elevated fixture may use a frangible mounting arrangement.
- Elevated light
- Frangible coupling
- Mounted above pavement level
Inset AGL Fixture
Inset fixtures are installed within the pavement and must typically resist applicable pavement and aircraft loads. Their mounting philosophy is therefore generally different from an exposed elevated fixture — a rigid base is the more typical structural approach.
Balancing Strength Against Controlled Failure
Frangible
NORMAL OPERATION
│
▼
SUFFICIENT STRENGTH
+
ABNORMAL IMPACT
│
▼
CONTROLLED FAILURE
A balance between adequate normal strength and controlled breakaway behaviour.
Rigid
NORMAL OPERATION
│
▼
HIGH STRUCTURAL
STRENGTH
│
▼
RESIST APPLIED LOADS
The primary objective is structural continuity and stability.
Inspecting & Restoring the Mounting
Frangible Mount
After impact, the fixture and support should be inspected. The breakaway component is often replaced as part of restoring the installation.
Rigid Mount
Because the support remains connected, impact forces may be transferred into other components — inspection focuses on structural damage before deciding on repair or replacement.
Myths vs Facts
A frangible mounting must be strong enough for normal service — wind, vibration, and normal environmental loads. It is only under a specified abnormal impact that it undergoes controlled breakaway. Frangible means selectively designed failure, not uncontrolled weakness.
Every mechanical structure has design limits. Rigid mounting simply has the objective of resisting loads rather than deliberately separating at a frangible point — it can still be damaged if the designed limit is exceeded.
The selection depends on fixture location, aircraft movement patterns, obstacle-clearance requirements, required mechanical strength, and applicable aviation standards — not general preference.
Frangibility is required where the installation location and applicable standards call for breakaway protection. The exact requirement depends on the governing standard and approved installation arrangement.
Full Engineering Parameters
| Parameter | Frangible Mounting | Rigid Mounting |
|---|---|---|
| Main philosophy | Controlled breakaway | Structural resistance |
| Aircraft impact | Designed to reduce hazard | May transfer higher forces |
| Normal environmental loads | Must withstand | Must withstand |
| Structural continuity after impact | Intentionally lost | Generally maintained unless damaged |
| Typical elevated AGL use | Common where required | Location-dependent |
| Typical inset use | Generally not the same approach | More typical structural philosophy |
| Post-impact replacement | Often required | Depends on damage |
| Installation safety objective | Minimize obstruction hazard | Maximize stability |
Choosing the Right Mounting
IS THE ELEVATED OBJECT LOCATED WHERE
AIRCRAFT IMPACT HAZARD REQUIRES
BREAKAWAY PROTECTION?
│
┌──────┴──────┐
│ │
YES NO
│ │
▼ ▼
FRANGIBLE RIGID OR OTHER
MOUNTING MAY STRUCTURAL DESIGN
BE REQUIRED MAY BE SUITABLE
The final selection depends on:
- Fixture location
- Aircraft movement patterns
- Obstacle-clearance requirements
- Required mechanical strength
- Applicable aviation standards
- Manufacturer-approved installation arrangements
Frangible vs Rigid Mounting
Frangible mounting is strong enough for normal operation, but designed to break or yield in a controlled way during abnormal impact.
Rigid mounting is designed to remain structurally connected and resist applied mechanical loads. For AGL systems, the correct choice depends on the safety philosophy of the installation: frangible mounting prioritizes controlled breakaway to reduce aircraft hazards, while rigid mounting prioritizes structural stability and resistance to mechanical loads.
Frangible mounting is common for elevated AGL fixtures in aircraft movement environments; rigid mounting is more typical for inset fixtures and protected or non-impact locations. The final choice must always follow the governing standard and approved installation design, not a general rule of thumb.
Engineering & Expert Notes
Do Not Replace Frangible With Rigid Without Approval
SafetyChanging the mounting alters impact behaviour. Follow the design and standard.
Inspect Frangible Couplings for Corrosion and Damage
InspectionCorrosion can change how the part behaves. Replace as needed.
Torque to Specification
InstallationOver- or under-tightening affects performance. Use the specified values.
Record the Mounting Type at Each Location
RecordsRegisters should show the type and the part number.
Frequently Asked Questions
Frangible mounting is designed to break or yield in a controlled manner when subjected to an abnormal impact, while rigid mounting is designed to remain structurally fixed and resist applied loads.
No. A frangible mounting must be strong enough to withstand normal environmental and operational loads such as wind and vibration. It is designed to fail only under a specified abnormal impact.
Yes. Every mechanical structure has design limits. Rigid mounting is intended to resist loads rather than deliberately separate, but it can still be damaged if the applied load exceeds the designed limit.
Elevated fixtures located in aircraft movement environments commonly use frangible mounting arrangements where required by the applicable installation standards.
Inset fixtures sit within the pavement and must resist applicable pavement and aircraft wheel loads, so their structural philosophy is generally different from an exposed elevated fixture that needs breakaway protection.
The fixture and support should be inspected after any impact, and the frangible component is often replaced if it has undergone controlled breakaway.
Governing Documents
For an actual project design, always verify the current edition of the applicable documents.
FAA AC 150/5345-42
Specification for Airport Light Bases, Transformer Housings, Junction Boxes, and Accessories — including frangibility requirements for elevated AGL fixture supports.
ICAO Annex 14, Volume I
Aerodrome Design and Operations — frangibility requirements for objects that must be located in aircraft movement or approach/take-off areas.
DGCA / AAI Requirements
For Indian airport projects, also verify applicable DGCA requirements, AAI requirements, airport-specific technical specifications, project drawings and contract specifications.
Final Engineering Note
Do not select a mounting arrangement from a generic web diagram. The final installation drawing should always be developed from the current governing standard, approved airport design, fixture location and manufacturer-approved arrangement.
Frangible mounting is not weak mounting, and rigid mounting is not indestructible mounting — each is engineered to a different safety objective: fail safely, or stay fixed.
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