A Guide to Washer Types Commonly Used in Aircraft Assembly

Aircraft rely on carefully designed and coordinated fastener systems to maintain structural integrity under extreme mechanical and environmental stress, with washers being particularly notable for performing specialized functions that directly impact joint stability, load management, and hardware longevity. In this blog, we will examine the most common washer types used in aircraft assembly and explain how the form, material, and function of these parts contribute to flight safety and system performance.

What Functions Do Washers Serve in Aircraft Assembly?

Washers play essential roles in maintaining mechanical integrity across a wide range of aircraft assemblies. Their value becomes especially clear when examining the specific functions they fulfill in flight-critical environments, which include:

• Load Distribution: Washers are designed to spread clamping forces across structural surfaces to reduce localized stress on fuselage panels or composite skins.
• Vibration Damping: Certain washer types support joint stability under persistent oscillations encountered during flight, taxiing, or engine operation.
• Preload Retention: Many washer designs are built to preserve bolt tension throughout extended use, which is vital for aircraft assemblies exposed to pressurization and thermal cycling.
• Surface Protection: Some washers reduce wear between fasteners and adjacent components, helping preserve coatings or corrosion-resistant surfaces in airframes and avionics.
• Component Spacing: Precision washers are often used to establish exact standoff distances in confined aircraft systems like instrument panels or control linkages.

Which Flat Washer Types Are Commonly Used in Aircraft?

Flat washers are the most widely used type in aviation, serving various basic but essential roles in structural and non-structural assemblies. Some common variations of flat washers include:

• Standard Flat Washers: Engineers often use standard flat washers to prevent surface damage and material embedment in airframe joints or engine mounts.
• Fender Washers: The wide outer diameter of fender washers is especially useful when mounting fasteners to thin panels or oversized holes in secondary aircraft structures.
• NAS1149 Series Washers: Utilized in many aerospace applications, these washers offer tight dimensional tolerances for compatibility with precision bolts.
• AN960 Washers: Frequently integrated into control linkages, landing gear attachments, and general-purpose joints, these washers meet various established military and commercial specifications.

How Are Lock Washers Used to Prevent Fastener Loosening?

To maintain fastener integrity in high-vibration or thermally dynamic aircraft environments, engineers often incorporate specialized lock washers like:

• Split Lock (Helical) Washers: These components are designed to apply continuous spring pressure that resists bolt loosening, benefiting engine mounts and structural joints.
• Internal Tooth Lock Washers: The inward-facing teeth of these washers can secure smaller fasteners in avionics racks, electronic bay enclosures, and cockpit instrumentation panels.
• External Tooth Lock Washers: With teeth that bite into surrounding material, these fasteners help maintain clamping force in assemblies consistently exposed to persistent mechanical oscillation.

Which Specialty Washers Support Unique Aircraft Applications?

In specialized aircraft assemblies where standard washers may fall short, engineers often turn to purpose-built designs like:

• Belleville (Conical) Washers: Belleville washers are engineered to absorb fluctuating loads in assemblies like landing gear struts and thrust reverser linkages.
• Cupped Washers: The contoured profile of cupped washers helps them maintain preload in areas subject to thermal shifts, such as engine nacelle interfaces.
• Thrust Washers: Installed in rotating components, these washers help support axial loads in actuators, gear trains, and flap drive assemblies.
• Shim Washers: These precision spacers are used to fine-tune alignment in flight control linkages, sensor mounts, and structural fittings.

How Does Washer Material Choice Affect Aircraft Performance?

Material selection plays a critical role in how washers perform under varying mechanical and environmental conditions in aircraft systems. To meet specific application demands, aerospace engineers rely on washer materials like:

• Stainless Steel: Stainless steel washers are widely used in structural assemblies where strength and resistance to corrosion are essential over long service intervals.
• Aluminum: Aluminum washers can provide a lightweight solution for non-critical joints in cabin interiors, service panels, and fairing attachments.
• Titanium: Washers made from titanium are chosen for high-stress zones like propulsion interfaces or landing gear components, where minimizing weight without sacrificing strength is crucial.
• Non-Metallic (Nylon, Phenolic): These washers can offer electrical insulation and chemical resistance in avionics bays, composite assemblies, and sensor installations.

How Do Washer Requirements Differ in Military Aircraft Applications?

Washers used in military aircraft serve the same core functions as those in commercial platforms, such as distributing loads, preserving joint integrity, and minimizing vibration effects. However, they must consistently meet rigorous Military Standard (MS) specifications to ensure traceability, dimensional control, and long-term durability in extreme environments. In many cases, these washers are also engineered with advanced material properties to withstand exposure to fuel, salt spray, high temperatures, and other conditions common to military operations.

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