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Landing Gear Shock Absorption Testing of Civil Aircraft

The intent of this document is to provide recommended practices for conducting shock absorption testing of civil aircraft landing gear equipped with oleo-pneumatic shock absorbers. The primary focus is for Part 25 aircraft, but differences for Part 23, 27, and 29 aircraft are provided where appropriate.

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 similar to 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 document. This gland is for use with compression-type seals including, but not limited to, O-rings, T-rings, D-rings, cap seals, etc.

Composite Secondary and Tertiary Structure for Landing Gear

This document will contain guidance and considerations for the use of composite materials on non-primary structure for landing gear systems. Content to include design considerations, conditions and applications where composites are feasible/beneficial, high-level descriptions of various manufacturing processes, and certification/validation considerations.

Nitrogen Absorption/Desorption (Gas Dissolution) in Aircraft Shock Absorbers

This document outlines the current state of the art in the understanding of gas in solution in shock absorber oils in unseperated shock absorbers. A literature review, overview of Henry's law, Henry's law coefficients for known gas and oil couples, in-service operational problems, lessons learned, and potential future work will be discussed in the document.

Component Traceability Requirements for Life Limited Parts

This document contains the recommended practices for the traceability of civil aircraft life limited parts. A unified means of tracking flight cycles, flight hours, and calendar time is provided which will ease the interchange of parts between companies and through the component's life cycle. A harmonized means of providing 'back to birth traceability' is provided.

Landing Gear Common Repair

This document outlines the most common repairs used on landing gear components. It is not the intention of this AIR to replace overhaul/component maintenance or technical order manuals, but it can serve as a guide into their preparation. Refer to the applicable component drawings and specifications for surface finish, thickness, and repair processing requirements. This document may also be used as a guide to develop an MRB (Material Review Board) plan. The repairs in this document apply to components made of metallic alloys. These repairs are intended for new manufactured components and overhauled components, including original equipment manufacturer (OEM)/depot and in-service repairs. The extent of repair allowed for new components as opposed to in-service components is left to the cognizant engineering authorities. Reference could be made to this document when justifying repairs on landing gears. For repairs outside the scope of this document, a detailed justification is necessary.

External Hydraulic Fluid Leakage Definition for Landing Gear Shock Absorbers

The purpose of this SAE Aerospace Recommended Practice (ARP) is to provide a practical definition of external hydraulic fluid leakage exhibited by landing gear shock absorbers/struts. The definition will outline normal (acceptable weepage) and excessive leakage (unacceptable leakage) of shock absorbers/struts that is measurable. The definition of leakage is applicable to new gear assemblies, refurbished/remanufactured (overhauled) shock absorbers/struts, leakage of shock absorbers/struts encountered during acceptance flights, newly delivered and in-service aircraft. This ARP is intended to provide guidelines for acceptable leakage of landing gear shock absorbers/struts between the ambient temperatures of -65 °F (-54 °C) and 130 °F (54 °C) and to outline the procedure for measuring such leakage. The specific limits that are applied to any particular aircraft shall be adjusted by the aircraft manufacturer before inclusion in the applicable maintenance manual.

Landing Gear Servicing

The present document addresses gas and hydraulic fluid servicing required on commercial and military aircraft landing gears, for both single and dual chamber (also known as dual stage and two stage) shock struts. This document should be considered as landing gear industry recommended practice but in no way is meant to supersede the shock strut OEM’s published procedures.

Landing Gear Component Heat Damage

The purpose of this report is to outline types of in-service heat damage that have been observed in high strength steel landing gear components, with an emphasis on a particular type that is referred to as “Ladder Cracking” which can develop in landing gear shock struts. The report discusses how ladder cracking can be detected visually and evaluated by non-destructive inspection methods, and how it can be repaired at overhaul with the prior approval of the Original Equipment Manufacturer. This report also describes the use of a bearing material that has resolved this problem without introducing other problems. Examples of other types of service induced heat damage are also discussed.

Landing Gear Structures and Mechanisms

This SAE Aerospace Recommended Practice (ARP) applies to landing gear structures and mechanisms (excluding wheels, tires, and brakes and other landing gear systems) for all types and models of civil and military aircraft. All axles, wheel forks, links, arms, mechanical and gas/oil shock struts, downlock and uplock assemblies, braces, trunnion beams, and truck beams, etc., that sustain loads originating at the ground, and that are not integral parts of the airframe structure, should be designed and validated in accordance with this document. Hydraulic actuators (retraction, main and nose gear steering, positioning, damping, etc.) should also be included in this coverage. System level, non-structural components such as retraction/extension valves, controllers, secondary structure and mechanisms in the airframe (e.g., manual release mechanisms, slaved doors) as well as equipment that is located in the cockpit are not addressed in this ARP.


This document describes the approaches taken to define safe-life limits for the management of fatigue in landing gear structures, and the substantiation of those limits through full-scale fatigue testing. The safe-life scatter factors considered in a range of military and civil regulatory standards are also reviewed.

Plain Bearing Selection for Landing Gear Applications

This document is intended to give advisory information for the selection of plain bearings and bearing materials most suitable for aircraft landing gear applications. Information included herein was derived from bearing tests and service experience/reports. Airframe/landing gear manufacturers, commercial airlines, the U.S. Air Force and Naval Air Systems Command provided input for the document. Information is given on bearing installation methods and fits that have given satisfactory performance and service life. Base metal corrosion is a major cause of problems in bearing installations for landing gears. Therefore, methods of corrosion prevention are discussed. Effort is directed toward minimizing maintenance and maximizing life expectancy of landing gear bearings. Lubricated and self-lubricating bearings are also discussed. There are wide ranges of bearing load and motion requirements for applications in aircraft landing gears.

Landing Gear Safety Criticap Processes

There are a number of safety critical processes that are common to landing gear systems. A safety critical process (CSP) is a landing gearmaintenance manufacturing, repair or overhaul process which if ommitted or preformed incorrectly will cause failure of a safety citical item (CSI) in operation. A CSI is a landing gear item whose failure or omission will cause more than $1M in damage, serious injury or death. This AIR will define methods and modes of failure for CSPs based on experience.

Information on Hard Landings


This document provides information on the current practices used by commercial and military operators in regards to hard landings (or overload events designated as hard landings). Since detailed information on inspections would be aircraft specific, this AIR provides only a general framework. Detailed information and procedures are available in the maintenance manuals for specific aircraft.

Because hard landings potentially affect the entire aircraft, guidelines are listed here for non-landing gear areas. But, the primary focus of the document is the landing gear and related systems. The document may be considered to be applicable to all types of aircraft.

This document does NOT provide recommended practices for hard landing inspections, nor does it provide recommendations on the disposition of damaged equipment. Refer to ARP 4915 and ARP 5600.


Tail Bumpers for Piloted Aircraft

This recommended practice covers the fixed structure, or independent energy absorbing system affixed to the airframe to afford protection to the control surfaces, engine and other portions during ground handling, take-off and landing.

Historical Design Information of Aircraft Landing Gear and Control Actuation Systems

This aerospace information report (AIR) provides historical design information for various aircraft landing gear and actuation/control systems that may be useful in the design of future systems for similar applications. It presents the basic characteristics, hardware descriptions, functional schematics, and discussions of the actuation mechanisms, controls, and alternate release systems. The report is divided into two basic sections: 1 Landing gear actuation system history from 1876 to the present. This section provides an overview and the defining examples that demonstrate the evolution of landing gear actuation systems to the present day. 2 This section of the report provides an in depth review of various aircraft. A summary table of aircraft detail contained within this section is provided in paragraph 4.1. The intent is to add new and old aircraft retraction/extension systems to this AIR as the data becomes available.

Design Recommendations for Spare Seals in Landing Gear Shock Struts

This SAE Aerospace Recommended Practice (ARP) provides recommendations on cavity design, the installation of elastomer type spare seals in these cavities, and information surrounding elastomer material properties after contact with typical shock absorber hydraulic fluid(s) or grease. This ARP is primarily concerned with the use of spare seals on shock absorbers where only a single dynamic seal is fitted and in contact with the slider/shock absorber piston at any one time. These shock absorbers typically have a spare (dynamic) seal gland located on the outer diameter of the lower seal carrier. This spare seal gland is intended to house a spare elastomer contact seal. Split Polytetrafluoroethylene (PTFE) backup rings can also be installed in the spare seal cavity. During operation, if the fitted dynamic shock absorber standard seal begins to fail/leak, then the aircraft can be jacked up, allowing the lower gland nut of the shock absorber to be dropped down.