7.1 Oleo Shock Struts: Servicing, Air/Oil Charging & Metering Pins

7.1 Oleo Shock Struts: Servicing, Air/Oil Charging & Metering Pins

FAA Airframe Exam Focus: Landing gear shock struts are critical structural assemblies engineered to dissipate the violent impact kinetic energy of touchdown and cushion the airframe against ground taxi shocks. Aviation maintenance technicians must master the thermodynamic and fluid mechanics of oleo struts, internal metering pin dynamics, snubbing rebound control, torque link alignment geometry, strict safety depressurization protocols, and precise dimension-based nitrogen charging.

1. Physics of Shock Absorption: Pneumatics vs. Hydraulics

Aircraft landing gear systems employ various shock absorption methods ranging from simple spring-steel leaf struts and elastomeric bungee cords on light general aviation aircraft to sophisticated air-oil (oleo-pneumatic) shock struts on high-performance and transport category aircraft. An oleo strut relies on the combined physical properties of a liquid and a gas operating within a dual-chamber cylinder.

                     OLEO STRUT SHOCK DISSIPATION PHYSICS

     TOUCHDOWN IMPACT (High Velocity)          GROUND TAXI & STATIC LOAD
     ────────────────────────────────          ─────────────────────────
       Kinetic Energy (½mv²)                     Aircraft Gross Weight
                 │                                         │
                 ▼                                         ▼
       Hydraulic Fluid Forced                    Compressed Nitrogen Gas
       Through Metering Orifice                  Acts as Pneumatic Spring
                 │                                         │
                 ▼                                         ▼
       VISCOUS FLUID FRICTION                    ELASTIC CUSHIONING
       Converts Energy into HEAT                 Absorbs Taxi Bumps & Dips
       (Dissipated through strut walls)          (Boyle's Law: P₁V₁ = P₂V₂)

The Dual-Medium Principle

1. Hydraulic Fluid (Viscous Kinetic Dissipation):

   • Hydraulic fluid (standard MIL-PRF-5606 mineral-base petroleum oil, dyed red, or MIL-PRF-83282 fire-resistant synthetic hydrocarbon) is virtually incompressible.
   • During touchdown impact, the landing sink velocity forces the lower telescoping piston upward into the upper cylinder. This drives the hydraulic fluid through a restricted internal orifice at high velocity.
   • Fluid shear friction and turbulence convert the aircraft's downward kinetic energy ($E_k = \frac{1}{2} m v^2$) into thermal energy (heat), which is rapidly absorbed by the fluid mass and dissipated into the surrounding atmosphere through the high-strength alloy steel or aluminum strut cylinder walls.
   • The hydraulic fluid acts as a true shock absorber (dissipator), preventing the aircraft from bouncing back into the air upon ground contact.

2. Dry Nitrogen Gas (Pneumatic Springing & Cushioning):

   • The upper portion of the strut cylinder contains a sealed chamber charged with high-pressure dry nitrogen gas (operating pressures typically range from $500\text{ to }1,800\text{ psi}$ depending on aircraft gross weight).
   • Nitrogen gas is highly compressible obeying Boyle's Ideal Gas Law:

$P_1 V_1 = P_2 V_2$

• As the strut compresses, the gas volume ($V$) decreases and internal pressure ($P$) rises progressively, creating an elastic pneumatic spring.
• During ground taxiing, surface bumps, turns, and braking maneuvers, fluid flows freely through the orifice under low velocity without generating significant damping resistance; instead, the compressed nitrogen cushion compresses and expands smoothly to isolate the airframe from runway roughness.

Fluid vs. Gas Comparison in Oleo Struts

2. Oleo Strut Anatomy & Internal Construction

An aircraft oleo-pneumatic strut is a precision-machined dual-telescoping cylinder engineered to withstand immense axial landing loads, side loads, and torsional twisting moments.

                        OLEO STRUT INTERNAL ANATOMY

                         ┌───────────────────────┐
                         │  Air/Oil Charge Valve │ ◄── MS28889-1 High-Pressure
                         │     (Schrader Valve)  │     Schrader Valve
                         ├───────────────────────┤
                         │                       │
                         │      NITROGEN         │ ◄── Upper Gas Chamber
                         │       CHAMBER         │     (Pneumatic Spring)
                         │                       │
                         ├───────────────────────┤
                         │   HYDRAULIC FLUID     │ ◄── Upper Fluid Reservoir
                         │       RESERVOIR       │
                         ├───────┐       ┌───────┤
                         │  ████ │       │ ████  │ ◄── Orifice Plate
                         │  ████ │       │ ████  │
                         │  ████ │ ┌───┐ │ ████  │ ◄── Snubbing / Rebound Valve
                         │       │ │   │ │       │     (Flapper / Check Orifices)
                         │       │ │ █ │ │       │
                         │       │ │ █ │ │       │ ◄── Tapered Metering Pin
                         │       │ │ █ │ │       │     (Modulates flow area ΔA)
                         ├───────┴─┘ █ └─┴───────┤
                         │           █           │
                         │     LOWER PISTON      │ ◄── Lower Cylinder / Piston Tube
                         │    (Fluid Chamber)    │     (Chrome-plated alloy steel)
                         ├───────────────────────┤
      Upper Torque ──►   │ ╲                   ╱ │
      Link Arm           │  ╲                 ╱  │   ◄── Torque Links (Scissor Links)
                         │   ●───────────────●   │       • Prevents piston rotation
      Lower Torque ──►   │  ╱                 ╲  │       • Maintains wheel alignment
      Link Arm           │ ╱                   ╲ │       • Limits maximum strut extension
                         ├───────────────────────┤
                         │   Wiper / Scraper     │ ◄── Metallic scraper & felt wiper
                         │   Packing Gland       │     (Chevron V-ring pressure seals)
                         ├───────────────────────┤
                         │      WHEEL AXLE       │ ◄── Axle Spindle to Main Wheels
                         └───────────────────────┘

Detailed Component Functions

1. Upper Cylinder (Housing):

   • Attached rigidly or via trunnion bearings to the aircraft wing spar or fuselage carry-through structure.
   • Contains the internal fluid reservoir, the high-pressure gas chamber, and houses the fixed orifice plate.

2. Lower Piston / Cylinder:

   • Telescopes smoothly inside the upper cylinder bore. The outer surface is precision-ground and hard-chrome plated to resist corrosion and minimize friction against dynamic packings.
   • Fixed to the wheel axle assembly at its lower end.

3. Tapered Metering Pin:

   • A precision-tapered alloy steel rod mounted upright on the bottom of the lower piston that extends through the center of the orifice plate.
   • Operating Principle: As the strut compresses during landing, the tapered pin enters the orifice. Because the pin diameter increases toward its base, the available fluid flow area ($\Delta A$) through the orifice is progressively constricted.
   • This restriction increases hydraulic resistance in direct proportion to stroke travel, compensating for the decay in aircraft sink rate and providing a constant, uniform deceleration rate throughout the entire compression stroke without structural shock spikes.

4. Orifice Plate and Snubbing (Rebound) Chamber:

   • Positioned between the lower piston tube and upper cylinder.
   • Contains a large central orifice for the metering pin and several auxiliary snubbing holes controlled by a spring-loaded ring or flapper check valve.
   • Rebound Control: When the strut reaches maximum compression and begins to extend (rebound) under nitrogen pressure, the flapper valve closes immediately. This traps fluid in the upper snubbing chamber and forces it to return to the lower cylinder exclusively through small snubbing bleed holes.
   • This hydraulic restriction dampens the extension stroke, preventing violent rebound oscillation, strut topping impact, and dangerous aircraft 'porpoising' on rollout.

5. Torque Links (Scissor Links):

   • A heavy-duty two-piece hinged mechanical scissor assembly connecting the outer upper cylinder to the lower moving piston tube.
   • Dual Primary Functions:
     • Directional Alignment: Prevents the cylindrical lower piston from rotating inside the upper housing, keeping the wheel axle and tires perfectly aligned with the aircraft longitudinal axis.
     • Extension Limiter (Downstop): Acts as a structural mechanical stop that prevents the lower piston from over-extending and separating from the upper cylinder when the aircraft becomes airborne.

6. Packing Gland, Wiper Ring & Scraper Seal:

   • Located in the lower bearing gland of the upper cylinder.
   • Contains a set of pressure-energized V-ring (chevron) packings or elastomeric U-cup seals with Teflon backup rings to retain high-pressure fluid and gas.
   • An external metallic (beryllium copper or stainless steel) scraper ring and felt wiper ring scrape away runway ice, abrasive grit, and mud from the extending chrome piston before it can penetrate and destroy the internal packings.

3. Oleo Strut Servicing Protocols & Safety Precautions

Servicing an aircraft oleo strut involves two distinct, sequential operations: hydraulic fluid charging and high-pressure nitrogen gas inflation. Failure to follow exact manufacturer procedures can result in severe airframe damage or fatal maintenance accidents.

                      OLEO STRUT SERVICING SEQUENCE

    STEP 1: DEPRESSURIZE          STEP 2: FLUID SERVICE         STEP 3: NITROGEN INFLATE
    ────────────────────          ─────────────────────         ────────────────────────
    • Jack aircraft / support     • Fully collapse strut        • Lower aircraft to ground
    • Open MS28889-1 valve        • Remove valve core           • Connect dry nitrogen bottle
    • BLEED ALL GAS TO ZERO       • Fill MIL-PRF-5606 to level  • Inflate to data plate
    • Confirm 0 psi before core   • Cycle/bleed air bubbles       EXTENSION DIMENSION

[!CAUTION] FATAL HIGH-PRESSURE HAZARD: An oleo shock strut contains compressed nitrogen at pressures up to $1,800\text{ psi}$. Never attempt to loosen the air valve body, valve core, or lower cylinder gland nut until ALL nitrogen gas pressure has been completely released and verified at zero gauge pressure. Always position yourself away from the direct projectile path of the valve stem.

Hydraulic Fluid Servicing Procedure (Step-by-Step)

1. Aircraft Preparation: Place the aircraft on maintenance jacks in a level hangar environment, or relieve structural weight from the landing gear per the Aircraft Maintenance Manual (AMM).
2. Complete Depressurization:
   • Remove the high-pressure valve cap from the MS28889-1 high-pressure charging valve.
   • Depress the valve core stem using an approved deflator cap with a bleed hose routed into a collection container until all gas pressure has completely escaped.
   • Slowly loosen the valve swivel hex nut to ensure no residual trapped pressure remains before backing out the valve core or body.
3. Strut Collapse: Fully compress (collapse) the lower piston until it reaches the absolute bottom of its stroke.
4. Fluid Fill:
   • Remove the fluid filler plug / valve assembly.
   • Pour clean, fresh, certified MIL-PRF-5606 (mineral base) or MIL-PRF-83282 (synthetic PAO base) hydraulic fluid into the filler opening until fluid reaches the bottom of the filler port threads.
5. Air Bleeding (Burping):
   • Slowly extend and compress the strut through its full stroke multiple times (exercising the piston) to dislodge trapped air pockets from beneath the orifice plate and snubbing chamber.
   • Recheck fluid level with the strut fully collapsed; top off fluid until it remains flush with the filler opening.
6. Reassembly: Install the charging valve assembly utilizing a new MS28775 / AS568 O-ring preformed packing and torque the valve body to the AMM specification.

Nitrogen Gas Inflation Procedure

1. Aircraft Configuration: Lower the aircraft off jacks so that the landing gear supports the normal static gross weight of the aircraft (fuel tanks at specified level, cabin empty, aircraft parked on level ground).
2. Dry Nitrogen Source: Connect a certified high-pressure dry nitrogen ($N_2$) cylinder equipped with a calibrated two-stage pressure regulator, shutoff valve, and high-pressure charging chuck.
3. Inflation by Extension Measurement:
   • Inflate the strut slowly with dry nitrogen.
   • Observe the polished chrome lower piston rod as it extends from the upper cylinder gland.
   • Critical Rule: Inflate the strut until the exposed chrome piston length matches the exact dimension specified on the aircraft landing gear instruction data plate (e.g., $3.50 \pm 0.25\text{ inches}$ on a light twin; $6.00 \pm 0.50\text{ inches}$ on a business jet).

                  WHY EXTENSION DIMENSION OVERRULES PSI

           Measured in INCHES of exposed chrome piston (NOT psi alone)
           
          ┌───────────────────────────┐
          │      Upper Cylinder       │
          ├───────────────────────────┤
          │                           │
          │  ╔═════════════════════╗  │
          │  ║                     ║  │
          │  ║  Exposed Chrome     ║ ◄┼── Measured with precision scale
          │  ║  Dimension          ║  │   (e.g., 4.0 inches per AMM)
          │  ║  (e.g. 4.00 in)     ║  │
          │  ╚═════════════════════╝  │
          ├───────────────────────────┤
          │       Wheel Hub           │
          └───────────────────────────┘

[!IMPORTANT] Why Struts Are Inflated by Dimension, Not Pressure Alone: Static inflation pressure varies drastically with ambient temperature ($P_1/T_1 = P_2/T_2$) and aircraft gross weight (fuel load, passenger cargo). Servicing to a fixed pressure reading (psi) would result in incorrect strut geometry. Inflating to the specified exposed piston dimension (inches) guarantees the correct internal gas volume and proper operating stroke travel under existing conditions.

4. Oleo Strut Troubleshooting & Failure Analysis

Airframe technicians must quickly isolate whether an oleo strut defect stems from incorrect fluid level, improper gas charge, mechanical binding, or damaged dynamic packings.

                      OLEO STRUT DIAGNOSTIC MATRIX

                  ┌─────────────────────────────────┐
                  │      OBSERVED STRUT DEFECT      │
                  └────────────────┬────────────────┘
                                   │
         ┌─────────────────────────┼─────────────────────────┐
         ▼                         ▼                         ▼
  Bottoms out on            Harsh / Rigid             Oil Weeping
  Touchdown Impact          Taxi Ride                 at Lower Gland
  (Normal ramp height)      (Strut collapsed)         (Piston wet)
         │                         │                         │
         ▼                         ▼                         ▼
    LOW HYDRAULIC             LOW NITROGEN              WORN CHEVRON
     FLUID LEVEL               GAS CHARGE                 PACKINGS
  (Insufficient oil         (Riding on bumper;        (Replace dynamic
   for kinetic damping)      needs N₂ charging)        seals & scraper)

Comprehensive Strut Troubleshooting Guide