Oleo-Pneumatic Shock Struts and Their Functionality

Posted on November 8, 2021 Michael Jones Electronics

Oleo-pneumatic struts are critical components of aircraft landing gear systems that have the capacity to combine hydraulic and pneumatic mechanisms to absorb impact energy during ground contact. By regulating motion and dissipating kinetic forces, these struts can help ensure smooth landings, reduce airframe stress, and enhance comfort for passengers and crew. In this blog, we will explore their internal construction, operational demands, and routine maintenance considerations to better understand their role in aviation performance.

What Components Define Oleo-Pneumatic Struts?

Oleo-pneumatic strut components are housed within a sealed system that is designed to provide controlled energy dissipation, structural support, and consistent response to variable loads. For example, the following components are commonly found in oleo strut assemblies.

Telescoping Cylinder Assembly: These assemblies consist of an inner piston housed within an outer cylinder, allowing a strut to compress and extend smoothly under vertical loads during landing and taxi operations.
Hydraulic Damping Chamber: Sealed chambers filled with hydraulic fluid can resist motion by forcing fluid through calibrated orifices, thereby absorbing kinetic energy during compression.
Gas Compression Volume: Pressurized nitrogen chambers have the ability to provide spring-like resistance that maintains static extension and contributes to energy recovery during strut rebound.
Flow Regulation Mechanism: Internal metering devices like tapered metering pins are used to modulate fluid flow and adjust damping force based on stroke speed and impact severity.
Dynamic Seal and Retainer System: Elastomeric seals and mechanical retainers are designed to prevent internal leakage while enabling smooth piston movement under fluctuating pressure conditions.

What Forces Do Oleo Struts Need to Withstand During Aircraft Landings?

During each landing, aircraft struts are subjected to a range of mechanical loads that can vary significantly in magnitude based on approach conditions and aircraft weight. Key operational loads that oleo struts are expected to manage include:

Rebound Forces During Extension: After initial compression, internal gas volume within the assembly causes a strut to extend, creating upward rebound forces that need to be moderated to prevent instability or uncontrolled oscillation.
Lateral and Torsional Loads: Crosswind landings, sloped runways, or uneven tire contact can introduce side loads and twisting moments that place combined bending and torque stress on a strut assembly.
Post-Touchdown Oscillation: Inadequate damping control may allow residual energy to produce bounce or oscillatory motion after landing, which can affect braking effectiveness and directional stability.
Variable Load Conditions: Changes in aircraft weight, balance, descent rate, or runway slope can lead to inconsistent load profiles that alter the intensity and distribution of forces acting on a strut.

What Maintenance Tasks Are Commonly Performed on Oleo Struts?

To maintain reliable performance and reduce the risk of premature wear or failure, oleo struts are routinely subjected to inspection procedures, servicing tasks, and fluid or pressure recharging operations like:

Pressure and Fluid Level Verification: Maintenance personnel should routinely check internal nitrogen pressure and hydraulic fluid levels to confirm that a strut retains proper spring force and energy absorption capacity.
Seal Integrity and Leak Detection: Visual inspections can be conducted to identify signs of fluid leakage, cracked elastomeric seals, or gasket degradation that could compromise system performance.
Static Extension Measurement: The extended length of a strut under aircraft weight can be measured to evaluate internal gas pressure and detect early signs of gradual pressure loss or improper fluid volume.
Recharging Operations: When deviations in pressure or fluid quality are observed, a strut should be serviced with replenished nitrogen and hydraulic fluid to restore expected damping and rebound behavior.
Internal Component Inspection: During overhaul intervals, technicians may disassemble a strut to assess wear on metering pins, piston surfaces, and other load-bearing components.

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