Aerospace Standards and Definitions

As a result of recent and forthcoming technological advancements, the aerospace industry is an ever-evolving field. Science will continue to provide us with more effective and potent methods for exploring the skies and outer space of our planet. As a result, businesses within the industry must adhere to the ever-changing safety and quality requirements.  

The standards for aerospace operations encompass a variety of industry variables, such as equipment management, airport safety, and dealing with fraudulent purchases. These regulations ensure the utmost level of safety and efficiency for those who work in astronautics, military aircraft, and other aerospace organizations. There are numerous standards, specifications and requirements that apply to the aerospace industry.   Below are some key aerospace industry standards and brief definitions.  

Standard Authorities

The Society of Automotive Engineering (SAE) oversees the development and revision of numerous standards used in the aerospace industry. All standards with the prefix AS (Aerospace Standard), AMS (Aerospace Material Standard), ARP (Aerospace Recommended Practices) or AIR (Aerospace Information Report) are documents issues by SAE.  CDI Products is an active participant and contributor to SAE work.   

Aerospace Definitions

AS5316 - Storage of Elastomer Seals and Seal Assemblies Which Include an Elastomer Element Prior to Hardware Assembly 
This Aerospace Standard addressed elastomeric seal components and offered a control document for those organizations which required one. It provided shelf-life restrictions that were consistent with the data from the age control cumulative experiments. It addressed the requirement for traceability and adequate storage of seal elastomeric components. It also produced a technical paper that addressed seals rather than hoses, rubber boots, and so forth. It provided a reference source for Quality Organizations to work with. Because it was so widely used and cited, it was declared an Aerospace Standard in 2017.    

Mil-G-5514 - Gland Design; Packings, Hydraulic, General Requirements for Military Applications 
This specification covers basic design criteria recommendations for use and application in packings, gaskets, packing and gasket glands and related features for use in hydraulic equipment utilized in systems designed in accordance with MIL-H-5440. Classification. Hydraulic system packings and gaskets shall be of the following types and classes: 

Classes 

• Class 1: 1,500 PSI (10.3 MPA) - Where the unit operating pressure at the packing is a normal 1,500 PSI (10.3 MPA). 
• Class 2:  3,000 PSI (20.7 MPA) -Where the unit operating pressure at the packing is a normal 3,000 PSI (20.7 MPA). 

NOTE:  Mil-G-5514G is used for legacy applications and not for new designs and new aircraft.   New applications and aircraft use either AS4716 or AS5857 for the seal gland requirements.  

AS4716 - Gland Design, O-Ring and Other Seals 
This SAE Aerospace Standard (AS) specifies gland (groove) design criteria and specifications for static and dynamic O-ring seal glands, as well as other seals. The glands have been specifically developed for applications requiring AS568 size O-rings at pressures greater than 1500 psi (10340 kPa) with one or two anti-extrusion (backup) rings and applications requiring backup rings at pressures less than 1500 psi (10340 kPa). The glands have been sized to provide sufficient squeeze for effective sealing while at the same time limiting squeeze to allow satisfactory operation in dynamic applications.  While this specification includes the fundamental design criteria and suggestions for use with standard size O-rings, these glands may also be acceptable for use with alternative elastomeric seals as well as elastomeric and mechanically energized plastic (non-elastomer) based seals.  

Note:  For static external applications, i.e., where the application is overboard and sealing to atmosphere, the glands should be per AS5857.  

AS5857 - Gland Design, O-Ring and Other Elastomeric Seals, Static Applications 
This SAE Aerospace Standard (AS) provides standardized gland (groove) design criteria and dimensions for elastomeric seal glands for static applications. The glands have been specifically designed for applications using SAE AS568 size O-Rings at pressures exceeding 1500 psi (10.3 MPa) utilizing one or two anti-extrusion (backup) rings and applications at pressures under 1500 psi (10.3 MPa) without backup rings. The glands have been sized to provide increased squeeze as compared to AS4716 for more effective sealing at low temperatures and low seal swell conditions.  These glands are not recommended for dynamic use. Primary usage is for static external sealing.  The rod dimensions are the same as AS4716. The cylinder bore dimensions are the same as AS4716 except for sizes -001 thru -011 and -104 thru -113.  

AS6235 - Face Seal Gland Design, Static, O-RING, and Other Seals for Aerospace Hydraulic and Pneumatic Applications 
This SAE Aerospace Standard (AS) specifies standardized gland design criteria and dimensions for static face seals for internal pressure and external pressure applications for aerospace hydraulic and pneumatic applications using the same dash size range as AS4716 and AS5857 where applicable. AS6235 glands have been specifically designed for applications using AS568 size elastomeric O-rings with related Class 2 tolerances at the nominal system operating pressures up to 3000 psi (20690 kPa) utilizing no anti-extrusion (backup rings) and in circular shapes.  

AS4088 - Aerospace Rod Scraper Gland Design Standard 
This SAE Aerospace Standard (AS) was originally released with a gland design for a one backup groove width generally per AS4716 and with either a solid or a split gland construction to facilitate the installation of the scraper. The atmosphere side of the gland had a larger clearance than normally specified in AS4716 to prevent the entrapment of foreign matter. In addition, mechanical spring energized Polytetrafluoroethylene (PTFE) scrapers (MSE scrapers) have become more prevalent to utilize scraper designs offering inert properties that makes them compatible with almost all fluids and temperature ranges used in aerospace fluid power applications. To facilitate the scraper installation into a one-piece gland construction, this design requires a reduced outer lip (known as a click-fit gland) on the atmosphere sidewall. This AS now includes four basic gland dimensional configurations because of the developments described above and to permit the use of the latest scraper designs.  

AS4052 - Gland Design: Scraper, Landing Gear, Installation 

This SAE Aerospace Standard (AS) covers an alternate gland design for the installation of scraper/ wiper rings in the lower end of landing gear shock struts for the purpose of contaminant exclusion. This standard is intended to present a groove which will accommodate an improved scraper/wiper ring assembly design and is not intended to obsolete the MS33675 gland standard.  The defined scraper gland covered by this standard is a variant of AS4716, the accepted gland standard for MS28775, O-ring packing seals. Piston rod diameters, gland internal diameters, groove sidewall angles and the surface finish are defined by AS4716, but the gland outer retaining wall diameter is changed. The traditional scraper design installed into the glands typically utilize components made from urethane or nitrile materials. These scraper designs, while still acceptable, must be reviewed in consideration to deicing, cleaners and disinfectant fluids applied to or in contact with the landing gear, as the materials of construction for the installed scrapers may not be compatible to these fluids. Exposure of the scraper to incompatible fluids is likely to reduce the performance of the scraper.  

AS4832 Gland Design: Nominal 3/8 Inch Cross Section for Compression-Type Seals 
This SAE Aerospace Standard (AS) offers gland details for a 0.364-inch (9.246 mm) cross-section gland (nominal 3/8 inch) with proposed gland lengths for compression-type seals with two backup rings over a range of 7 to 21 inches (178 to 533 mm) in diameter. The dash number system used is like AS568A. A 600 series has been chosen as a logical extension of AS568A, and the 625 number has been selected for the initial number, since 300 and 400 series in MIL-G-5514 and AS4716 begin with 325 and 425 sizes. Seal configurations and design are not a part of this standard. This gland is for use with compression-type seals including, but not limited to, O-rings, T-rings, D-rings, cap seals, etc.  

ARP1231 - General Gland Design Criteria for Static and Dynamic 

O-Ring Seal Applications Specifically for Engines and Engine Control Systems 
This standard recommends general gland design criteria for static and dynamic O-ring seal applications specifically for engines and engine control systems.  The purpose of this standard is to provide the aerospace industry with basic information pertinent to the design and selection of elastomeric O-ring seal glands for use specifically in engine and engine control applications.  

ARP1232 - Gland Design Criteria and Dimensions for Static Radial O-Ring Seal Applications 

Without Anti-Extrusion Devices Specifically for Engine and Engine Control Systems 

This standard recommends standard gland dimensions for static radial O-ring seal applications specifically for engine and engine control systems and provides recommendations for modifying these glands in special applications. The purpose of ARP1232 is to provide the aerospace industry with standardized dimensional criteria for static radial elastomeric O-ring seal glands specifically for engines and engine control systems. It supplements ARP1231 and ARP1233. 

 NOTE: No provisions are made in this standard for anti-extrusion devices.  

ARP1233 - Gland Design Criteria and Dimensions for Dynamic Radial O-Ring Seal Applications 

Specifically for Engine and Engine Control Systems Operating at 1500 psi Max
ARP1233 recommends standard gland design criteria and dimensions for dynamic radial O-ring seal applications, specifically for engine and engine control systems operating at pressures up to a maximum of 1500 psi (10342.14 kPa). It provides recommendations for modifying these glands in special applications. There are no provisions in this standard for anti-extrusion devices.  

NOTE: The criteria set forth here are like but not identical with those in MIL-G-5514 and AS4716. This standard is not intended to replace MIL-G-5514 or AS4716 for hydraulic applications. The purpose of this standard is to provide the aerospace industry with standardized dimensional criteria for dynamic radial elastomeric O-ring seal glands specifically for engine and engine control systems. It is intended for use in conjunction with ARP1231.  

ARP1234 - Static Axial O-Ring Seal Applications Without Anti-Extrusion Devices for Engine and Engine Control Systems 
This standard establishes standard gland design criteria and dimensions for static axial O-ring seal applications without anti-extrusion devices specifically for engines and engine control systems operating at a maximum pressure of 1500 psi (10345 kPa).   

NOTE: The criteria are similar, but not identical, to those in AS4716 and the legacy standard MIL-G-5514.  

AIR1234 - Anti Blow-By Design Practice for Cap Seals 
This SAE Aerospace Information Report (AIR) provides information on anti-blow-by design practice for cap seals. Suggestions for piston cap seal sidewall notch design and other anti-blow-by design details are also described. It also includes information on two key investigations based on the XC-142 as part of the text and as Appendix A.  The purpose of this standard is to provide adequate information to the designer so that the blow-by problem will not reoccur.  

AIR1244- Aerospace, Slipper Seals, Selection for Fluid Power Applications 
This SAE Aerospace Information Report (AIR) provides basic information on the use of slipper seal sealing devices when used as piston (OD) and rod (ID) seals in aerospace fluid power components such as actuators, valves, and swivel joints, including:  The definition of a slipper seal and the description of the basic types in use.  Guidelines for selecting the type of slipper seal for a given design requirement are provided in terms of friction, leakage, service life, installation characteristics, and interchangeability.  

AIR6079 – Selection of Metallic Spring Energized Seals for Aerospace 
This SAE Aerospace Information Report (AIR) provides information on the design, application, and maintenance engineers with basic information on the use of metallic Spring Energized sealing devices when used as piston (OD) and rod (ID) seals in aircraft fluid power components such as actuators, valves, and swivel glands. The Spring Energized seal is defined and the basic types in current use are described. Guidelines for selecting the type of Spring Energized seal for a given design requirement are covered in terms of friction, leakage, service life, installation characteristics, and interchangeability. Spring Energized seals can also be made in various forms and types, including face seals (internal and external pressure sealing types), and rotary variants too. These further types will not be discussed in this standard, but many of the same principles apply for them as well.  

ARP1802 – Selection and Application of Polytetrafluoroethylene (PTFE and TFE) Backup Rings for Hydraulics and Pneumatic Fluid Power Applications. 
This SAE Aerospace Recommended Practice (ARP) provides an overview of the various types of polytetrafluoroethylenes (PTFE and TFE) back up rings for hydraulic and pneumatic fluid power applications, including their advantages and disadvantages.  Also, to assist users in selecting the right backup ring, this standard provides guidance in the selection of backup rings for various applications. It also identifies older designs that are no longer recommended.