While flat washers appear simple, aerospace and defense requirements introduce significant complexity. FF‑W‑92, the U.S. Federal Specification for plain flat washers, remains a critical legacy callout. This guide clarifies classifications such as "Type A, Grade II, Class B" and details the specification's relationship to NAS1149 and AN960 standards.
1. What FF‑W‑92 Actually Is
FF‑W‑92 defines requirements for plain flat washers in U.S. government and military applications. The current active revision is FF‑W‑92C.
The spec covers:
- Geometry and dimensional series
- Material classes and mechanical properties
- Corrosion resistance and finish requirements
- Procurement and marking regulations
While modern programs often utilize NAS1149, FF‑W‑92 is still prevalent in legacy documentation and procurement.
2. Understanding the FF‑W‑92 Structure: Type, Grade, Class
The specification utilizes a three-part classification system: Type, Grade, and Class.
2.1 Type — Washer Shape
The spec defines several washer “Types,” but the one you will see 95% of the time is:
- Type A: Round Flat Washer (Standard for aerospace/defense)
Other types exist historically, but Type A is overwhelmingly the one used in aerospace and defense assemblies.
2.2 Grade — Dimensional Series & Intended Application
FF‑W‑92 defines two primary Grades:
- Grade I: General-purpose/commercial dimensions
- Grade II: Aeronautical series; aligned with NAS1149 geometry
Grade II is the one relevant to aircraft, spacecraft, and defense hardware.
2.3 Class — Material Category
Material classes define the washer’s base material and corrosion‑resistance characteristics:
- Class A: Carbon steel
- Class B: Corrosion-resisting steel (CRES)
- Class C/D: Aluminum or aluminum alloy
- Class E/F: Brass or copper
In aerospace, Class A and Class B dominate.
3. How FF‑W‑92 Relates to NAS1149 and AN960
This is where most confusion comes from.
3.1 AN960 → NAS1149 Evolution
Historically:
- AN960: The original Army-Navy standard
- NAS1149: The modern aerospace dimensional replacement
- FF-W-92 Grade II washers typically correspond to NAS1149 dimensions.
So when you see:
FF‑W‑92 Type A Grade II Class B
…you can usually expect dimensions equivalent to NAS1149C (CRES) or NAS1149D (carbon steel), depending on the class.
4. Typical Dimensions: Example for a #6 Washer
For a #6 aerospace washer, the NAS1149 dimensional series provides:
- Inside Diameter (ID): ~0.149 in
- Outside Diameter (OD): ~0.375 in
- Thickness ranges:
- Thin series: 0.014–0.018 in
- Regular series: 0.028–0.036 in
5. Material Standards and the AIR4127 Confusion
Contrary to some interpretations, AIR4127 is not a washer standard but a general aerospace reference for steel chemical compositions. FF-W-92 may reference it for material chemistry, but the washer requirements are governed by the FF-W-92 specification itself.
6. When FF‑W‑92 Still Matters
Even though NAS1149 is the modern standard, FF‑W‑92 remains relevant because:
- Many military drawings still use it.
- Government procurement contracts often require federal spec compliance.
- It provides a structured classification system that maps cleanly to modern aerospace hardware.
If you work with MRO, defense retrofits, or legacy airframes, you will encounter FF‑W‑92 regularly.
7. How to Interpret a Real Callout
Example:
Washer — FF‑W‑92, Type A, Grade II, Class B, Size 6
This means:
- Type A: Round flat washer
- Grade II: Aerospace dimensional series (NAS1149‑equivalent)
- Class B: CRES stainless steel
- Size 6: For #6 screw/bolt, ID ≈ 0.149 in
This is effectively a NAS1149C0632R or NAS1149C0632 washer, depending on thickness.