AN970-4 – Large Area Flat Steel Washer (1.125" OD)
Available now through the AirParts Aero marketplace.
Overview Section
The AN970-4 is a specialized Large Area Flat Washer (often referred to as a "fender washer" or "wood washer" in aerospace applications) manufactured to the Army-Navy (AN) standard.1 Unlike standard flat washers, the AN970 series is engineered with a significantly larger outer diameter (OD) relative to its inner diameter (ID).
This design is specifically intended for use in areas where the fastener load must be distributed over a wide surface area. It is most commonly found in wood-to-metal or composite-to-metal structural joints, as well as mounting applications for lightweight materials where a standard washer might "pull through" or crush the substrate under high torque.
Key Features Section
High Surface Area Distribution: The 1.125-inch outer diameter provides a massive footprint, spreading the clamping force of the fastener to prevent damage to soft materials like spruce, plywood, or thin composites.
Carbon Steel Construction: Manufactured from high-tensile carbon steel, providing the structural rigidity required for primary airframe assemblies.
Cadmium Plated Finish: Coated with a protective layer of cadmium (per QQ-P-416) to ensure superior corrosion resistance and to act as a sacrificial lubricant during the torquing process.2
Precision ID Clearance: The .265-inch inner diameter is precision-bored to provide a "close-fit" clearance for standard 1/4-inch (#4 size) bolts, minimizing fastener "wobble."
Structural Rigidity: With a thickness of .063 inches, this washer resists "cupping" or deforming even when high torque values are applied to the bolt.
Military Standard Compliance: Fully compliant with AN970 specifications, ensuring the hardware meets the weight and strength safety factors required for certified aircraft.
Technical Specifications Section
| Specification | Detail |
| Part Number | AN970-4 |
| Material | Carbon Steel (Low or Medium Carbon) |
| Finish | Cadmium Plated (Yellow or Silver) |
| Inner Diameter (ID) | 0.265 inches (Fits 1/4" Fastener) |
| Outer Diameter (OD) | 1.125 inches (1-1/8") |
| Thickness | 0.063 inches (Nominal) |
| Weight | Approx. 0.016 lbs per piece |
| Specification | AN970 (Large Area Washer) |
Aircraft Compatibility Section
The AN970-4 is a staple for both vintage and modern specialty aircraft in the AirParts Aero inventory:
Vintage/Wooden Airframes: Critical for the wing spars and fuselage attachments of Super Cubs, Stearmans, and Citabrias.
Composite Aircraft: Used in the mounting of interior panels and secondary structures on Cirrus and Diamond aircraft.
Floats and Skis: Frequently utilized in the installation of Edo or Wipaire floats where large washers are needed to distribute loads across the float deck.
Homebuilt/Experimental: A preferred hardware choice for high-stress mounting points on Vans RV and Bearhawk aircraft.
Technical Applications & Safety
The AN970-4 is used when "standard" washers (like the AN960/NAS1149) simply do not provide enough surface area to prevent structural "crushing." In a wooden spar, for example, the high tension of a 1/4" bolt could easily sink a small washer into the wood grain, compromising the joint. The AN970-4 bridges the load across the fibers, maintaining joint integrity.
Safety Warning: Never substitute an AN970-4 with a commercial-grade "fender washer" from a hardware store. Aviation-grade washers are made from specific steel alloys and undergo controlled plating processes to prevent Hydrogen Embrittlement, which can cause a commercial washer to snap unexpectedly under load.
Technical Engineering Insight
The engineering value of the AN970-4 lies in its Clamping Pressure Dispersion. Pressure is defined as Force divided by Area (3$P = F / A$).4 By increasing the area (A) with a 1.125" diameter washer, the pressure exerted on the aircraft's skin or spar is reduced by several hundred percent compared to a standard .500" OD washer.
The Cadmium Plating is not just for rust; it provides a consistent coefficient of friction.5 This is vital when torquing bolts into critical structures. If the friction varies due to poor plating, the bolt may be under-tensioned or over-stretched. Choosing the genuine AN-standard hardware through AirParts Aero ensures that your structural repairs maintain the original safety margins and "crush" protection designed by the airframe manufacturer.
Installation & Maintenance Addendum
Alignment: Ensure the washer is centered on the hole. Off-center loading on a large washer can cause it to "lift" on one side, putting a bending moment on the bolt.
Corrosion Check: During annual inspections, check the edges of the AN970-4 for "flaking" plating. If rust appears, replace the washer to prevent the corrosion from spreading to the primary structure.
No "Stacking": Do not stack multiple AN970 washers to create height. If a spacer is needed, use an appropriate aluminum or steel bushing.
Fits in
- Bombardier Challenger 601
- Bombardier Challenger 604
- Bombardier Challenger 605
- Bombardier Global Express
- Bombardier Learjet 40
- Bombardier Learjet 45
- Bombardier Learjet 55
- Bombardier Learjet 60 - all variants
- Dassault Falcon 7X
- Embraer Phenom 300
- Gulfstream G-IV/GIV-SP/G-400/G-450/G-300
- Gulfstream G-V/G-V-SP/G-550/G-500
- Piper Aircraft PA-42 Cheyenne
- Textron Aviation Beechcraft King Air 200 - all variants
- Textron Aviation Beechcraft King Air 300 - all variants
- Textron Aviation Beechcraft King Air 90 - all variants
- Textron Aviation Beechcraft Premier
- Textron Aviation Bell 206 Jetranger/Longranger
- Textron Aviation Cessna 500 Citation I-ISP
- Textron Aviation Cessna 510 Citation Mustang
- Textron Aviation Cessna 525 Citation Citationjet
- Textron Aviation Cessna 550 Citation I-II-Bravo
- Textron Aviation Cessna 560 Citation V-Ultra-Encore-Excel-XLS
- Textron Aviation Cessna 650 Citation III-IV-VI-VII
- Textron Aviation Cessna 680 Citation Sovereign
- Textron Aviation Cessna 750 Citation X
- Textron Aviation Hawker 400-400XP
- Textron Aviation Hawker 800
- Textron Aviation Hawker 800XP-850XP
- Textron Aviation Hawker 900XP