O-Rings for Aerospace Applications
AS568-standard sealing solutions for hydraulic systems, fuel systems and aircraft maintenance — in Skydrol-compatible FKM and military-specification compounds with full traceability documentation.
Overview
Aerospace sealing demands the most exacting dimensional tolerances, material certifications and performance consistency of any industry. O-rings in aircraft hydraulic systems, fuel controls, engine assemblies and landing gear must perform across extreme temperature ranges — from -54°C at altitude to +200°C near engine components — while maintaining sealing integrity through millions of pressure cycles. Every seal must be traceable to its raw material lot, verified for hardness and dimensions, and certified to the applicable military or industry specification. A single seal failure in flight can compromise safety, trigger unscheduled maintenance, and ground an aircraft with catastrophic economic consequences.
Temperature cycling is one of the most severe challenges in aerospace sealing. During each flight, O-rings experience dramatic thermal transients — from ground ambient temperatures, to stratospheric cold at cruise altitude, to heat soak from adjacent engine and auxiliary power unit components upon landing. These cycles induce thermal contraction and expansion that can reduce squeeze, increase leakage paths, and accelerate compression set. Material selection must account for both the lower temperature limit (where elastomers transition to glassy, non-elastic states) and the upper service temperature (where oxidative degradation and crosslinking reactions accelerate). Silicone VMQ and fluorosilicone (FVMQ) compounds are often specified for low-temperature applications where standard NBR would become too brittle to seal.
Aircraft hydraulic systems present a unique chemical challenge. Most commercial aircraft use phosphate ester-based hydraulic fluids — commonly known by the trade name Skydrol — which are fire-resistant but highly aggressive toward standard nitrile rubber. Skydrol fluids (Skydrol 500B-4, LD-4, 5, and equivalents) cause rapid swelling, softening and degradation of NBR and many general-purpose elastomers. Fluorocarbon rubber (FKM) is the only widely accepted elastomer for Skydrol service, providing long-term compatibility with phosphate esters across the full aircraft hydraulic temperature envelope. Using an incorrect material in a Skydrol system will result in seal failure within hours to days, potentially causing hydraulic fluid leakage, system pressure loss, and critical flight control malfunction.
Fuel system sealing requires careful differentiation between fuel types and service temperatures. Jet fuel (Jet A, Jet A-1, JP-8) is a kerosene-based fuel with moderate aromatic content. At ambient temperatures, specially formulated NBR compounds can provide adequate service life. However, in high-temperature fuel system locations — such as engine-mounted fuel control units and fuel nozzle fittings — FKM is required for thermal stability and aromatic resistance. Aviation gasoline (avgas) contains higher aromatic and lead additive concentrations that can degrade standard NBR more rapidly; FKM is generally preferred for avgas applications. For fuel quantity probe seals and capacitance probe gaskets inside fuel tanks, FKM is typically specified due to its combination of fuel resistance, electrical insulation properties, and long-term dimensional stability.
The most common O-ring failure modes in aerospace applications include compression set from thermal cycling, extrusion into clearance gaps under high hydraulic pressure, chemical degradation from incompatible fluids, and ozone cracking from atmospheric exposure at altitude. Spiral failure can occur in dynamic hydraulic actuator seals where side loading or insufficient lubrication causes the O-ring to twist in its groove. Each failure mode has specific prevention strategies — from backup ring installation and groove redesign to material upgrades and improved filtration. Proactive failure mode analysis during the design phase and condition-based monitoring during service are essential for maintaining airworthiness and minimizing unscheduled maintenance events.
AS568 was originally developed for the US aerospace industry and remains the defining O-ring size standard for aerospace applications globally. Military specifications (MIL-P-5315, MIL-R-83485) define material and quality requirements for US military and many commercial aerospace programmes. SAE AS568 dash numbers specify cross-sectional diameter and inside diameter to tight tolerances, ensuring interchangeability across manufacturers. For critical aerospace applications, O-rings must be manufactured from lot-controlled compounds with full material traceability, hardness verification, and dimensional inspection against the applicable specification. AMS (Aerospace Material Specifications) provide updated equivalents to legacy military specifications, and aircraft maintenance manuals specify the exact material and AS568 dash number for each sealing location.
Material selection in aerospace follows a rigorous technical hierarchy. For general mineral oil hydraulic systems and low-temperature static seals, NBR 70 Shore A remains the baseline material due to its excellent low-temperature flexibility and petroleum fluid resistance. For all Skydrol phosphate ester applications, FKM 75 Shore A is mandatory. For wide-temperature static sealing in avionics and environmental control systems, silicone VMQ provides exceptional low-temperature capability down to -60°C. For the most demanding hot-section engine seals and extreme temperature locations, FFKM (perfluoroelastomer) extends service capability to +325°C. Each material grade must be specified with the correct aerospace compound formulation — commercial-grade FKM or silicone may not meet the fluid resistance and outgassing requirements of aircraft applications.
O-Ring Supply Co. provides aerospace-grade O-rings with full material traceability, batch certificates of conformance, and dimensional inspection reports. We support custom compound development for specialized aerospace applications, including low-outgassing formulations for spacecraft and satellite systems, custom color coding for maintenance identification, and laser marking for part traceability. Our engineering team can assist with material specification reviews, groove design verification, and failure analysis for in-service seal issues. For MRO and fleet operators, we offer kitting services organized by aircraft type, system, or maintenance interval to streamline inventory management and reduce AOG downtime.
Recommended Materials
FKM 75 Shore A
Hydraulic systems using phosphate ester fluids (Skydrol 500B-4, Skydrol LD-4, Skydrol 5), fuel system O-rings in high-temperature locations, engine accessory seals, and fire-resistant hydraulic actuator seals in commercial and military aircraft.
Temp: -20°C to +200°C
Note: Required for all Skydrol hydraulic systems — NBR fails rapidly in phosphate ester fluids. Verify aerospace compound grade for aircraft use.
NBR 70 Shore A
Mineral oil hydraulic systems, fuel cells at moderate temperatures, ground support equipment, and general-purpose static seals in non-Skydrol aerospace applications.
Temp: -40°C to +120°C
Note: AS568 standard NBR for non-Skydrol applications. Select low-temperature NBR for cold-climate operation below -30°C.
Silicone VMQ
Avionics cooling seals, altitude pressure control systems, environmental control system gaskets, and low-temperature static seals where extreme cold flexibility is required.
Temp: -60°C to +230°C
Note: Excellent low-temperature performance but limited tensile strength — not suitable for dynamic hydraulic service or high-pressure applications.
FFKM
Engine hot section seals, fuel nozzle seals in turbine engines, high-temperature high-pressure critical sealing positions, and applications where both chemical resistance and thermal stability are paramount.
Temp: -15°C to +325°C
Note: Specified where FKM temperature limits are exceeded. Higher cost justified by extended service life in critical locations.
FEP Encapsulated Silicone
Chemical processing ports, ground support equipment exposed to aggressive cleaning solvents, and static seals requiring the chemical barrier of FEP with the elasticity of silicone core.
Temp: -50°C to +200°C
Note: FEP jacket provides universal chemical resistance; core provides elasticity. For static applications only — FEP jacket can tear in dynamic service.
EPDM 70 Shore A
Aircraft de-icing fluid systems (ethylene glycol-based), potable water systems, and fire extinguisher seals where Skydrol or petroleum compatibility is not required.
Temp: -50°C to +150°C
Note: Do not use in hydrocarbon fuel or oil systems. Verify compatibility with specific de-icing fluid formulation.
Typical Applications
- Fuel system seals
- Hydraulic actuator seals
- Engine seals
- Landing gear seals
- Oxygen system seals
- Avionics cooling seals
- Environmental control seals
- APU seals
Relevant Standards
Frequently Asked Questions — Aerospace
What is Skydrol and why does it require FKM O-rings?
Skydrol is a fire-resistant phosphate ester hydraulic fluid used in virtually all commercial aircraft hydraulic systems, including those manufactured by Boeing, Airbus, and Embraer. The trade name Skydrol covers several formulations — Skydrol 500B-4, Skydrol LD-4, and Skydrol 5 — all based on phosphate ester chemistry. Phosphate ester fluids are incompatible with standard nitrile rubber (NBR) and most general-purpose elastomers because they cause rapid chemical attack, leading to excessive swelling, softening, and eventual disintegration of the seal. FKM (fluorocarbon rubber, commonly known by the DuPont trade name Viton) is the only widely accepted elastomer for long-term Skydrol service because its fluorinated polymer backbone resists the aggressive solvent action of phosphate esters. Using NBR in a Skydrol system will result in seal failure within hours to days, potentially causing hydraulic fluid leakage, system pressure loss, and critical flight control malfunction. For all Skydrol-based hydraulic systems, FKM O-rings must be specified, and the compound must be validated for phosphate ester compatibility — not all FKM formulations meet aerospace Skydrol requirements.
What military and industry specifications govern aerospace O-rings?
Aerospace O-rings are governed by a hierarchy of standards and specifications. SAE AS568 is the dimensional standard — it defines O-ring inside diameters and cross-sections by dash numbers, with tight tolerances suitable for aerospace applications. MIL-P-5315 covers rubber O-rings for US military hydraulic fluid applications, specifying material requirements, testing protocols, and quality acceptance criteria. MIL-R-83485 covers fluorocarbon rubber O-rings with enhanced low-temperature and fluid resistance properties. AMS (Aerospace Material Specifications) such as AMS-R-83485 provide updated equivalents to legacy military specifications. For commercial aviation, aircraft maintenance manuals (AMM) and component maintenance manuals (CMM) specify the exact material and AS568 dash number for each sealing location. FAA regulations require that replacement parts meet or exceed the original type design specifications, meaning O-ring replacements must match the approved material, hardness, and dimensional tolerances specified by the aircraft or component manufacturer. We supply O-rings manufactured to AS568 dimensions with material certificates confirming compliance to the relevant military, AMS, or customer specifications.
Can you supply O-rings with full aerospace material certification and traceability?
Yes. We supply aerospace-grade O-rings with comprehensive documentation packages suitable for quality-critical applications. Standard documentation includes a Certificate of Conformance (CofC) confirming compliance to the ordered specification, material batch traceability records linking the O-ring to the raw compound lot, Shore A hardness test reports, and dimensional inspection data verifying conformance to AS568 tolerances. For military and defense applications, we can provide additional documentation including material certification to MIL-P-5315 or MIL-R-83485, first article inspection reports (FAIR), and test reports for physical properties including tensile strength, elongation, compression set, and fluid immersion testing. All documentation is provided in PDF format with unique lot numbers for traceability. For OEM and MRO (Maintenance, Repair, Overhaul) customers, we support custom documentation requirements and can integrate with supplier quality management systems. We also offer electronic document delivery through secure portals, long-term archival of certification records for regulatory audits, and customized documentation templates aligned with specific airline or military customer formats. Our quality engineering team can perform supplier audits, material equivalency assessments, and failure analysis reporting as part of an extended documentation package. Contact us with your specific certification requirements, and our quality engineering team will confirm capability and turnaround time.
What O-ring material is used in aircraft fuel systems, and how does it differ by location?
Aircraft fuel system O-ring material selection depends on fuel type, service temperature, and regulatory requirements. For jet fuel systems (Jet A, Jet A-1, JP-8), FKM is preferred in high-temperature locations such as engine-mounted fuel control units, fuel nozzles, and APU fuel lines where temperatures can exceed +150°C. In ambient-temperature fuel system locations — such as wing tank access panels, fuel pump fittings in the tank, and ground refueling adapters — specially formulated NBR compounds can provide adequate service life and are often specified by the aircraft manufacturer for cost optimization. For aviation gasoline (avgas) applications in piston-engine aircraft, FKM is generally preferred because avgas contains higher aromatic content and tetraethyl lead additives that can degrade standard NBR more aggressively than jet fuel. For fuel quantity probe seals and capacitance probe gaskets inside fuel tanks, FKM is typically specified due to its combination of fuel resistance, electrical insulation properties, and long-term dimensional stability. Always consult the aircraft maintenance manual (AMM) or illustrated parts catalog (IPC) for the approved material at each specific location, as using an unapproved material can void warranties and compromise airworthiness.
What causes O-ring compression set in aerospace hydraulic systems, and how can it be prevented?
Compression set is the permanent deformation that remains in an elastomeric seal after the compressive load is removed — in aerospace hydraulic systems, it manifests as reduced sealing force and increased leakage over time. The primary cause in aerospace applications is thermal aging: when O-rings are exposed to temperatures near or above their upper service limit for extended periods, the polymer chains undergo additional crosslinking (hardening) or chain scission (softening), both of which reduce elastic recovery. In aircraft hydraulic systems, this is exacerbated by the thermal cycling profile — each flight cycle subjects seals to temperature swings from ground ambient to altitude cold to engine-heat-soaked conditions. Over thousands of flight cycles, cumulative thermal damage causes the O-ring to lose its ability to maintain squeeze. Prevention strategies include selecting materials with lower compression set ratings (FKM and FFKM generally outperform NBR at elevated temperatures), maintaining adequate squeeze percentage (typically 15-25% for static hydraulic seals), ensuring groove fill does not exceed 85% to allow for thermal expansion, and implementing scheduled seal replacement intervals based on flight hours or calendar time. For critical hydraulic actuator seals, some operators use condition-based monitoring to track internal leakage rates and schedule replacement before functional failure occurs.
How do extreme low temperatures at altitude affect O-ring sealing performance?
At cruise altitudes above 30,000 feet, ambient temperatures drop to approximately -54°C, and while most hydraulic and fuel system O-rings are located within the aircraft structure where they are somewhat protected, external static seals on probes, sensors, and access panels can experience these extreme temperatures. All elastomers exhibit a glass transition temperature (Tg) below which they lose elasticity and become brittle, glassy solids. Standard NBR has a Tg around -30°C to -40°C, meaning at -54°C it can no longer function as a seal. For low-temperature aerospace applications, specialized low-temperature NBR formulations (with higher acrylonitrile content and plasticizer selection) can operate to approximately -40°C, while silicone VMQ remains flexible to -60°C and fluorosilicone (FVMQ) to -65°C. For hydraulic systems, the fluid itself provides some thermal mass, but during cold-soak conditions (prolonged ground parking in arctic conditions), the entire system including the O-rings must be warmed before pressurization. Some aircraft incorporate heating elements or fluid circulation systems to maintain seal temperatures within operational limits. When specifying O-rings for low-temperature aerospace service, it is critical to review the low-temperature retraction (TR-10) data, which indicates the temperature at which the elastomer recovers 10% of its deformation — this is a more practical indicator of sealing capability than the glass transition temperature alone.
What is the difference between AS568 standard sizes and custom aerospace O-ring sizes?
AS568 is the standard O-ring size system developed specifically for the US aerospace industry and now used globally. It defines 369 standard dash numbers, each specifying a nominal inside diameter and cross-sectional diameter with associated tolerances. For the vast majority of aerospace applications — including hydraulic actuators, fuel system fittings, and engine accessories — AS568 standard sizes provide complete coverage, and using standard sizes reduces cost, lead time, and inventory complexity. However, some aerospace applications require custom O-ring sizes that fall outside the AS568 series. Custom sizes are typically needed for large-diameter seals (AS568 tops out at approximately 660 mm inside diameter), micro-miniature seals for avionics and sensor applications, and specialized groove geometries where the standard cross-section does not optimize sealing performance. Custom aerospace O-rings can be manufactured by compression molding, transfer molding, or injection molding depending on size, quantity, and precision requirements. For custom sizes, full dimensional drawings with tolerances are required, and prototype qualification testing may be necessary. We manufacture both AS568 standard sizes from tooling stock and custom aerospace O-rings from dedicated molds, with the same material quality control and documentation packages applied to both. For repair and overhaul applications, we recommend first verifying whether an AS568 standard size can substitute for a custom part, as this significantly reduces cost and lead time.
How do I select O-rings for aircraft landing gear shock struts and brake systems?
Aircraft landing gear shock struts (oleo struts) use O-rings in the dynamic seal assembly that retains hydraulic fluid (typically MIL-H-5606 mineral oil) and nitrogen gas within the strut cylinder. The O-rings must seal against the polished chrome-plated piston tube during compression and extension cycles, resisting wear, extrusion, and fluid degradation. NBR 70 Shore A is the standard material for most general aviation and commercial aircraft shock strut seals, providing excellent compatibility with MIL-H-5606 hydraulic fluid and good wear resistance on chrome surfaces. For high-performance and military aircraft, FKM or specialized polyurethane compounds may be specified for extended service life and improved extrusion resistance at higher operating pressures. Aircraft brake systems also use O-rings in the hydraulic actuators that apply pressure to the brake discs or rotors. Brake system O-rings operate at elevated temperatures due to heat transfer from the braking surfaces — temperatures can exceed +150°C during heavy braking. FKM is typically specified for brake system O-rings because it maintains sealing force at temperatures where NBR would experience rapid compression set. Both shock strut and brake system O-rings require precise surface finish on the mating metal components (typically 0.2-0.4 µm Ra for dynamic seals) and correct groove dimensions to prevent extrusion and spiral failure. We supply replacement O-rings for common aircraft landing gear and brake systems, with material and size verification against the aircraft maintenance manual.
What are the requirements for O-rings in pressurized cabin and environmental control systems?
Aircraft cabin pressurization and environmental control systems (ECS) use O-rings in ducting connections, valve bodies, heat exchangers, and outflow valves to maintain cabin pressure and temperature control for passengers and crew. These systems operate with conditioned bleed air from the engines or APU, with O-rings sealing against air, water vapor, and occasional condensation. Unlike hydraulic and fuel systems, ECS seals do not face aggressive chemical exposure, but they must operate across wide temperature ranges and maintain low outgassing characteristics to prevent cabin air contamination. Silicone VMQ is commonly specified for ECS O-rings because of its excellent low-temperature flexibility (critical for high-altitude cold-soak conditions), good compression set resistance in air service, and wide temperature range from -60°C to +230°C. EPDM is also used in water separator and condensate drain systems where moisture and glycol-based de-icing fluid exposure occurs. For cabin pressurization outflow valves and pressure regulators, the O-rings must maintain consistent sealing force through thousands of pressurization cycles per flight. Compression set is the primary failure mode, leading to cabin altitude excursions and passenger discomfort. Prevention requires selecting low-compression-set silicone compounds, maintaining proper squeeze in the groove design, and scheduling periodic replacement based on flight cycle accumulation. All ECS O-ring materials must meet aircraft interior flammability requirements and low-toxicity standards for cabin air quality.
What customization and engineering support services do you offer for aerospace O-ring applications?
We provide comprehensive customization and engineering support tailored to the unique requirements of aerospace customers. Our custom manufacturing capabilities include special compound development for applications with unique chemical exposure or temperature profiles, custom color coding for maintenance identification (red for fuel, blue for hydraulic, green for oxygen, etc.), laser marking for permanent part traceability, and custom packaging with lot-controlled segregation for AS9100 quality systems. Our engineering team offers groove design review and optimization, including finite element analysis (FEA) simulation of O-ring deformation under pressure and temperature to verify squeeze, groove fill, and extrusion clearance before prototyping. For in-service failure analysis, we provide root cause investigation including visual inspection, Shore hardness measurement, compression set testing, and fluid immersion comparison testing against the service fluid. We maintain an extensive database of aerospace material compatibility data covering Skydrol formulations, jet fuels, MIL-spec hydraulic fluids, and cleaning solvents. For MRO and fleet operators, we offer kitting services — pre-packaged O-ring assortments organized by aircraft type, system, or maintenance interval — to streamline inventory management and reduce AOG (Aircraft on Ground) downtime. All custom services include full documentation packages and can be integrated with customer ERP and quality management systems.