The landing gear system is one of the most mechanically stressed assemblies on any aircraft, bearing the full dynamic loads of takeoff, landing, and taxiing. Beyond structural demands, landing gear components are continuously exposed to harsh environmental conditions—water, deicing chemicals, runway debris, and temperature extremes—that make them particularly susceptible to corrosion. Among the most effective defenses against this degradation is a disciplined, properly executed lubrication program. This article explores the science behind landing gear corrosion, the protective mechanisms of lubrication, and the operational practices that keep this critical system safe and serviceable for the long term.

Understanding Landing Gear Corrosion

Corrosion on landing gear is not simply an aesthetic issue; it is a progressive deterioration of metal that can compromise structural integrity and lead to catastrophic failure if left unchecked. The electrochemical nature of corrosion requires an electrolyte (typically water containing dissolved salts), an anodic area where metal is lost, and a cathodic area where reduction reactions occur. Landing gear components provide all the necessary conditions: high-strength steels, aluminum alloys, and dissimilar metal interfaces exposed to moisture and contaminants.

Types of Corrosion Affecting Landing Gear

Galvanic corrosion occurs when two dissimilar metals, such as steel actuator pins and aluminum housings, are in electrical contact in the presence of an electrolyte. The more active metal corrodes preferentially. Landing gear often contains such interfaces at hinge points, bearings, and attachment lugs. Pitting corrosion is a localized attack that creates small pits, often under deposits or at scratches where the protective oxide layer is broken. Pits can act as stress risers and initiate fatigue cracks. Crevice corrosion develops in tight spaces—between a bushing and its bore, under a seal, or at lap joints—where stagnant electrolyte can accumulate. Stress corrosion cracking (SCC) is especially dangerous; it combines tensile stress with a corrosive environment to produce cracks that can propagate rapidly, often without significant surface corrosion visible.

Environmental Factors That Accelerate Corrosion

Moisture is the primary enabler of corrosion, but the specific chemistry of the environment matters tremendously. Runways are treated with deicing fluids containing ethylene glycol or potassium acetate, which are corrosive to many aircraft metals. Salt from coastal operations or winter road treatments creates highly conductive electrolytes. Humidity and condensation inside landing gear bays can trap moisture against unpainted surfaces. Additionally, hydraulic fluid leaks—common around shock struts and actuator seals—can attract and hold dirt and water, creating localized corrosion cells. Temperature fluctuations cause condensation cycles, while high temperatures accelerate reaction rates. In short, the landing gear environment is an aggressive one that demands constant vigilance.

The Critical Role of Lubrication in Corrosion Prevention

Lubrication is often viewed primarily as a friction-reducing measure, but its corrosion prevention function is equally important. A properly applied lubricant film acts as a physical barrier that prevents moisture and corrosive ions from reaching the metal surface. For grease, the thickener network and oil components create a semi-solid seal that excludes water even under pressure or centrifugal forces. For oil-based lubricants, continuous replenishment helps flush away contaminants and maintains a protective layer on rapidly moving surfaces.

Barrier Protection and Water Displacement

High-quality corrosion-preventive greases often include corrosion inhibitors—additives that adsorb onto metal surfaces and neutralize acidic byproducts. These greases also displace water from faying surfaces, a property known as water-displacement. When applied after cleaning, they leave a hydrophobic film that resists moisture penetration. This is especially critical on unpainted surfaces inside torque links, trunnion pins, and steering actuators where paint cannot be applied due to movement requirements.

Reducing Friction and Wear to Prevent Secondary Corrosion

Fretting corrosion—a combination of mechanical wear and oxidation—commonly occurs on landing gear components that experience small oscillatory movements under load, such as pin-to-bushing interfaces in retraction mechanisms. Without adequate lubrication, protective oxide films are mechanically disrupted, exposing bare metal that rapidly re-oxidizes, producing a fine reddish debris (often called "fretting corrosion" or "blood") that accelerates material loss. Proper lubrication mitigates this by separating the contacting surfaces, reducing the cyclic shear stresses that break the oxide layer.

Lubrication as a First Line of Defense for Seals and Bushings

In shock struts and oleo-pneumatic units, the lubrication of sliding surfaces preserves the integrity of seals. A dry or inadequately lubricated piston can cause seal wear, leading to hydraulic fluid leakage. Leaking fluid not only degrades system performance but also creates a corrosive mess on surrounding components. Regular lubrication with the correct seal-compatible grease ensures smooth operation and prevents the ingress of moisture past wiper seals.

Selecting the Right Lubricant for Landing Gear Applications

Not all lubricants are suitable for aircraft landing gear. The extreme loads, wide temperature ranges, and demanding environmental exposure require specially formulated aviation-grade products that meet stringent military or OEM specifications. Using an inappropriate lubricant can lead to accelerated wear, chemical attack on seals, or outright corrosion. Maintenance personnel must always consult the airframe manufacturer’s maintenance manual (AMM) for approved lubricants.

Aircraft Greases: The Workhorses of Landing Gear Lubrication

MIL-PRF-81322 grease (such as Aeroshell 22 or Mobilgrease 28) is a general-purpose, wide-temperature-range grease (roughly −73°C to +177°C) that provides excellent corrosion protection and water resistance. It is commonly specified for landing gear bearings, actuating screws, and wheel bearings. MIL-PRF-23827 grease (e.g., Aeroshell 7) is a lower-temperature grease often used for control cables and some landing gear components, but it has a narrower temperature range and less water resistance. For highly stressed journal bearings and sliding surfaces, MIL-PRF-83363 grease (like Royco 64 or Mobil 64) offers enhanced extreme-pressure (EP) properties. These greases contain corrosion inhibitors and are designed to adhere to vertical surfaces, staying in place during high-speed rotation or oscillation.

Oil-Based Lubricants for Specific Components

Certain landing gear parts, such as door hinges, latch mechanisms, and actuator rods, may require lightweight oil rather than grease. MIL-PRF-23699 turbine engine oils are sometimes used for lubricating in-service rod ends and pins, but these are not corrosion preventives by themselves—they must be reapplied frequently. For long-term corrosion protection on sliding surfaces that are not in constant motion, a corrosion-preventive compound (CPC) such as MIL-C-11796 class 3 (dry film) or class 2 (soft film) can be used, especially during storage or lay-up periods.

Dry Lubricants: When Dust and Dirt Are Problems

In environments where conventional grease would attract and hold abrasive dust or sand (such as desert operations), dry film lubricants like molybdenum disulfide (MoS₂) or PTFE-based coatings offer an alternative. These are applied as a thin, bonded layer that provides low friction without the sticky residue. However, dry films cannot be replenished easily once worn and typically provide less corrosion protection than grease. They are most appropriate for actuators and mechanisms that are enclosed or shielded from direct water spray.

Compatibility and Cross-Contamination Risks

Mixing different grease types—especially those with different thickeners (e.g., lithium, polyurea, calcium sulfonate)—can cause the grease to soften, separate, or lose its protective properties. Lubricant incompatibility can also attack elastomeric seals. Strict adherence to the approved lubricant list in the aircraft maintenance manual is essential. When switching between lubricants, previous residue should be thoroughly removed and the component cleaned per the manual’s instructions.

Best Practices for Landing Gear Lubrication

An effective lubrication program is more than following a schedule—it demands correct technique, quality control, and a thorough understanding of the specific component requirements. The following practices represent industry-standard approaches derived from FAA Advisory Circular AC 43-4B, manufacturer recommendations, and lessons from service experience.

Pre-Lubrication Cleaning

Applying fresh grease over a layer of dirt, old oxidized lubricant, or corrosion deposits is counterproductive—it often traps contaminants against the metal surface and creates a perfect microenvironment for corrosion. Before every lubrication event, all grease fittings, nipples, and exposed surfaces must be cleaned with an approved solvent (such as a low-volatility cleaning agent) and wiped dry. For areas where solvent cannot be used (e.g., near elastomeric seals), a clean lint-free cloth should be used to remove surface grease and debris. Old grease should be purged from bearing cavities until fresh lubricant emerges, ensuring that aged, contaminated material is fully displaced.

Application Techniques: Quantity and Distribution

Over-lubrication is a common problem. Excess grease can migrate to nearby brake assemblies, wheel rims, or antiskid sensors, causing performance degradation or fire hazards. A conservative amount—indicated by slight purge from bearing seals or a thin film on sliding surfaces—is usually sufficient. For grease fittings, use a manual or low-pressure pneumatic grease gun and pump slowly to allow the grease to distribute evenly. For open surfaces such as torque link pivot pins, apply a uniform layer by hand or brush, taking care not to leave gaps. When lubricating screw-thread mechanisms (e.g., jack screws for tail landing gear), rotate the mechanism while applying grease to ensure full coverage.

Lubrication Frequency: Flight Cycles, Calendar, and Condition-Based

The standard lubrication interval for landing gear is typically tied to flight cycles or hours of operation, but many OEMs also specify calendar limits (e.g., every 30 days, every 200 flight cycles, or annually). However, these intervals should be adjusted based on operating environment. Aircraft based in coastal or desert regions, or those that frequently operate on unpaved runways, may require more frequent lubrication. After every washing or exposure to heavy rain, landing gear should be inspected and re-lubricated if water ingress is suspected. Condition-based triggers—such as increased resistance during retraction, unusual noise, or visual evidence of dried grease—should prompt immediate lubrication regardless of the schedule.

Documentation and Traceability

Every lubrication event must be recorded in the aircraft’s maintenance log or electronic tracking system, noting the date, component, lubricant type used, and any observations (e.g., evidence of wear or corrosion found). This documentation is critical for trend analysis, regulatory compliance, and warranty support. Many operators now use digital maintenance systems that generate automatic reminders based on usage data, ensuring that intervals are not inadvertently skipped.

Integrating Lubrication into a Comprehensive Corrosion Prevention Program

Lubrication alone cannot prevent all corrosion; it must be part of a broader corrosion prevention and control program (CPCP) as required by FAA AC 43-4A and EASA Part-M. This includes routine visual inspections, scheduled non-destructive testing (NDT) for hidden corrosion, and a written schedule for protective treatments like touch-up painting or application of corrosion-inhibiting compounds.

Regular Inspections: Seeing Beyond the Grease

A well-lubricated landing gear should still be inspected at every preflight and scheduled maintenance check. Indicators of hidden corrosion include swollen or bulging paint (underfilm corrosion), discolored grease (rust tinting), and fretting debris at pin-to-bushing interfaces. Use a borescope to inspect internal surfaces of hollow components such as axles and torque links for pitting or cracking. During detailed inspections, wipe away fresh grease to check for surface corrosion underneath, then reapply lubricant after inspection is complete.

Training and Skill Development for Maintenance Personnel

A lubrication procedure is only as effective as the technician performing it. Maintenance organizations should provide recurrent training on landing gear lubrication techniques, emphasizing the risks of over- or under-lubrication, the importance of cleaning, and the recognition of early corrosion signs. Hands-on training with mock-ups or actual components helps develop the feel for correct grease application rates and detection of abnormal resistance or noise. Boeing’s Aero magazine has published several articles on landing gear maintenance that serve as excellent reference materials.

Regulatory and Manufacturer Guidance

The FAA, EASA, and other aviation authorities issue advisory circulars, airworthiness directives (ADs), and maintenance practices that directly address landing gear corrosion. For example, FAA AC 43-4B provides detailed guidance on corrosion prevention, including lubrication frequency and proper lubricant selection. OEM maintenance manuals include lubrication charts, recommended products, and special instructions for specific aircraft models—these are legally required to be followed. Failure to comply with these requirements can result in airworthiness violations and liability in the event of an incident.

Conclusion

Landing gear corrosion is an ever-present threat that demands a disciplined, multi-layered defense. Proper lubrication stands out as one of the most cost-effective and impactful measures available to maintenance professionals. By forming a barrier against moisture, reducing mechanical wear that accelerates corrosion, and maintaining the integrity of seals and bushings, lubrication directly extends the life of critical components and preserves the safety of flight operations. However, effective lubrication is not achieved by simply adding grease; it requires the correct lubricant type and grade, meticulous cleaning before application, precise quantity control, and adherence to a schedule that accounts for operational conditions. Integrated with regular inspections, training, and regulatory compliance, a robust lubrication program is an essential pillar of any landing gear corrosion prevention strategy. In an industry where the cost of unscheduled downtime or component failure is measured in millions, investing in proper lubrication practices yields returns in reliability, safety, and long-term operational efficiency.