AS3209-009 – Textron Aviation 90 Durometer FKM Boss Seal O-Ring
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The AS3209-009 is a high-performance, high-pressure O-ring packing manufactured in strict accordance with SAE Aerospace Standard AS3209. This is not a general-purpose O-ring; it is an engineered seal specifically designed for one of the most demanding static sealing applications in aviation: the AS5202 (formerly MS33649) straight thread tube fitting boss. Its primary function is to provide a leak-free face seal in high-pressure hydraulic and fuel systems, as specified in numerous Textron Aviation and other airframe parts catalogs.
The key to this O-ring's performance is its specific material and hardness: a 90 durometer Fluorocarbon (FKM) elastomer. This high-hardness compound provides exceptional resistance to extrusion, allowing it to contain system pressures of 3000 PSI and greater without the need for a separate backup ring. Its FKM material composition also provides superior resistance to aggressive aerospace fluids at elevated temperatures.
Key Features
SAE Aerospace Standard (AS) Certified: Manufactured to the exact material, hardness, and dimensional tolerances of the AS3209 specification for high-pressure boss seals.
90 Durometer Fluorocarbon (FKM): The very hard, high-modulus FKM material provides exceptional resistance to extrusion, the primary failure mode for seals in high-pressure static applications.
Superior Fluid and Temperature Resistance: The FKM (Viton®) elastomer is compatible with a wide range of petroleum-based and synthetic hydrocarbon hydraulic fluids (e.g., MIL-PRF-83282), jet fuels, and oils at high temperatures.
Designed for AS5202/MS33649 Bosses: Specifically sized and engineered to provide a perfect face seal in the standard straight thread tube fitting ports used throughout modern aircraft.
Eliminates Need for Backup Rings: In its specified application, the 90-durometer hardness is sufficient to resist extrusion, simplifying the design and installation compared to softer seals that require support.
Technical Specifications
Part Number: AS3209-009
Component Type: Preformed Packing, O-Ring, Boss Seal
Governing Specification: AS3209
OEM Applicability: Textron Aviation (Cessna, Beechcraft), and all other manufacturers utilizing AS5202 high-pressure ports.
Material: Fluorocarbon Rubber (FKM), Viton®.
Hardness: 90 ± 5 Durometer Shore A.
Size (AS568A Dash Number): -009
Dimensions:
Inside Diameter: 0.198 inches (5.03 mm)
Cross-Section: 0.070 inches (1.78 mm)
Typical Operating Temperature Range: -15°F to 400°F (-26°C to 204°C).
Aircraft Compatibility
The AS3209-009 is a standard hardware component used in high-pressure hydraulic and fuel systems across all categories of certified aircraft. It is not specific to any one airframe but is called for by part number wherever AS5202/MS33649 boss fittings are used.
Crucial Note: The correct part number for a specific application must be verified against the Illustrated Parts Catalog (IPC) or Component Maintenance Manual (CMM) for the component or aircraft being serviced.
Applications
The primary and intended application for the AS3209-009 is as a static face seal for straight thread tube fittings and plugs in high-pressure (3000+ PSI) fluid systems, including:
Hydraulic System Manifolds, Valves, and Actuators
Fuel System Components (pumps, filters, control units)
High-Pressure Wheel and Brake Assemblies
Landing Gear System Components
Why Choose AS3209-009?
Choosing a genuine AS3209-009 is a critical decision for high-pressure system integrity. The most common error in the field is substituting this part with a softer, general-purpose 70 or 75-durometer O-ring. Such a substitution in a 3000 PSI system will lead to near-certain seal extrusion and catastrophic failure, resulting in a major hydraulic or fuel leak. The AS3209 standard is the only guarantee that the O-ring possesses the specified 90-durometer hardness and FKM material properties required to safely contain high-pressure fluids in its intended application.
The engineering principle behind the AS3209 O-ring is the management of elastomer extrusion through material hardness. Extrusion failure occurs when the applied fluid pressure (P) exceeds the shear strength of the elastomer, forcing it to flow into the microscopic clearance gap (the "extrusion gap") between the two sealed metal components. An elastomer's resistance to this deformation and flow is a function of its modulus of elasticity, which is directly related to its durometer (hardness). A standard 75-durometer O-ring (e.g., MS9388) has a lower modulus and will begin to extrude at pressures above approximately 1500 PSI without a separate anti-extrusion backup ring. The AS3209's specified 90-durometer FKM has a significantly higher modulus, making the material much stiffer. This increased stiffness allows the O-ring to function as its own anti-extrusion device in static face seal applications like the AS5202 boss, withstanding pressures of 3000 PSI or more without requiring a separate PTFE backup ring. This allows for a simpler, more compact, and highly reliable sealing solution for these specific high-pressure ports.
Note: O-rings are typically considered single-use components in aerospace applications. These components have a finite shelf life (cure date) which must be strictly observed. CRITICAL: Ensure the correct part number is used for the application. Do NOT substitute a lower durometer O-ring (e.g., MS29513, MS9388) in a location where an AS3209 is specified. Installation must be performed by a certified A&P mechanic in accordance with the applicable maintenance manual.
Documentation
The definitive technical requirements for this component are detailed in SAE Aerospace Standard AS3209. These proprietary documents must be sourced directly from the relevant standards organizations for complete engineering specifications.