MS28775-214 – EPDM O-Ring for Phosphate Ester (Skydrol®) Hydraulic Systems
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The MS28775-214 is a high-performance, preformed packing O-ring manufactured to the strict requirements of Military Standard MS28775. This is a highly specialized seal, and it is absolutely critical for technicians and engineers to understand its specific application, particularly within Textron Aviation's jet fleet. It is fabricated from an 80 durometer Ethylene Propylene Diene Monomer (EPDM) elastomer, a material chosen for one primary reason: its exceptional compatibility with phosphate ester-based hydraulic fluids (e.g., Skydrol®). This O-ring is the industry standard for sealing hydraulic components in most transport category and business jet aircraft, including the Textron Aviation Cessna Citation and Hawker series.
The chemical compatibility of a seal is a non-negotiable aspect of flight safety. The MS28775 O-ring is engineered to provide a reliable, long-lasting seal in the chemically aggressive environment of a Skydrol® system. Conversely, it is completely incompatible with petroleum-based fluids. The MS28775 specification is the only guarantee that this O-ring is made from the correct, certified EPDM compound required for the safe operation of phosphate ester hydraulic systems.
Key Features
Military Standard (MS) Certified: Manufactured to the exact material, hardness, and dimensional tolerances of the MS28775 specification.
Specifically for Skydrol® Hydraulic Systems: The EPDM material is the certified elastomer for creating a durable, leak-free seal in systems using phosphate ester-based fluids.
Ethylene Propylene Diene Monomer (EPDM) 80 Durometer Material: Provides excellent chemical resistance, good temperature stability, and the hardness required for high-pressure hydraulic sealing.
INCOMPATIBLE with Petroleum-Based Fluids: This material will catastrophically fail if exposed to jet fuel, engine oil, or petroleum-based (red) hydraulic oil.
Excellent Compression Set Resistance: Resists taking a permanent set after being compressed, ensuring a reliable, long-term seal in static applications.
Technical Specifications
Part Number: MS28775-214
Component Type: Preformed Packing, O-Ring
Governing Specification: MS28775
OEM Applicability: Textron Aviation (Cessna Citation/Hawker), Boeing, Airbus, and all other manufacturers of aircraft using phosphate ester hydraulic systems.
Material: Ethylene Propylene Diene Monomer (EPDM) Rubber.
Hardness: 80 ± 5 Durometer Shore A.
Size (AS568A Dash Number): -214
Dimensions:
Inside Diameter: 0.987 inches (25.07 mm)
Cross-Section: 0.139 inches (3.53 mm)
Typical Operating Temperature Range: -65°F to 300°F (-54°C to 149°C).
Aircraft Compatibility
The MS28775-214 is a standard hardware component used throughout the hydraulic systems of most modern business jets and commercial transport category aircraft. It is specified extensively in the component maintenance manuals for systems on:
Textron Aviation: Cessna Citation and Hawker series jets.
Boeing, Airbus, Bombardier, Gulfstream, and Embraer aircraft.
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
This O-ring is exclusively for sealing in any component of a phosphate ester (Skydrol®) hydraulic system, including:
Flight Control Actuators (spoilers, ailerons, elevators, rudder)
Landing Gear and Brake System Valves and Actuators
Hydraulic Pumps, Reservoirs, Filters, and Accumulators
Thrust Reverser Systems
Why Choose MS28775-214?
Choosing a genuine MS28775-214 O-ring is a matter of fundamental flight safety. There is no room for error in material compatibility. The installation of an incorrect elastomer—such as a Nitrile (Buna-N) or Fluorocarbon (FKM) O-ring—into a Skydrol® hydraulic system will result in the rapid chemical destruction of the seal, leading to a major hydraulic leak and the potential for a complete loss of a primary flight control or braking system. The MS28775 standard is the only guarantee that the O-ring is made from the correct, certified EPDM compound that can safely contain this aggressive fluid.
The precise material selection for the MS28775 O-ring is dictated by the chemical principle of "like dissolves like." Ethylene Propylene Diene Monomer (EPDM) is a non-polar elastomer, a terpolymer of ethylene, propylene, and a diene monomer which gives it its cross-linking ability. Because of its non-polar molecular structure, it is highly resistant to attack from polar fluids, the most notable of which in aerospace is the family of polar phosphate ester hydraulic fluids (Skydrol®). Conversely, this same non-polar nature makes EPDM highly susceptible to attack from non-polar fluids, such as petroleum oil and hydrocarbon fuels. If exposed to these fluids, the non-polar hydrocarbon molecules are readily absorbed into the EPDM's non-polar polymer matrix. This causes the O-ring to swell dramatically to several times its original size, lose all of its tensile strength and hardness, and effectively revert to a soft, gummy substance with no ability to contain pressure. This catastrophic material failure is why the strict segregation of EPDM and petroleum-compatible elastomers (like Nitrile or Fluorocarbon) is one of the most critical disciplines in aircraft maintenance.
Note: EXTREMELY CRITICAL SAFETY NOTE: This O-ring is made from EPDM and is for use in PHOSPHATE ESTER (SKYDROL®) HYDRAULIC SYSTEMS ONLY. It is ABSOLUTELY INCOMPATIBLE with petroleum-based fluids (fuel, oil, MIL-PRF-5606 hydraulics). Installation in a petroleum-based system will cause catastrophic seal failure. Always verify the system fluid type and the part number in the manual. This is a single-use component and must be replaced whenever a connection is broken.
Documentation
The definitive technical requirements for this component are detailed in Military Standard MS28775. These proprietary documents must be sourced directly from the relevant standards organizations for complete engineering specifications.