ISO 3323:2023

Overview

ISO 3323:2023 defines a marking scheme for aircraft hydraulic components (excluding pipelines, hoses and fittings) to indicate the specific hydraulic fluid for which each component is approved. The purpose is to make the correct fluid readily apparent during assembly, testing and workshop operations. The marking is intended for handling and maintenance stages - it is not required to serve any function when the component is installed in-service.

Key topics and requirements

• Scope of marking: Applies to components used in aircraft hydraulic systems except pipelines, hoses and fittings (those are covered by ISO 12).
• Fluid categories: Marking must state one of the defined fluid types:
  • Mineral oil base
  • Synthetic hydrocarbon base
  • Chlorinated silicone base
  • Phosphate ester base
  • Castor base
  • Silicate ester base
• Plate / label design:
  • Components shall carry a plate or label clearly marked with the fluid name; optional inclusion of a product name or specification number is permitted (e.g., ISO 9940, AS1241, MIL-PRF-5606).
  • Plates (or labels) must be neutral in colour except for phosphate ester based fluids, which require a violet label specified by CIE chromaticity x = 0.337, y = 0.242 and spectral radiance factor β = 6.5% (CIE standard illuminant B).
• Materials & attachment:
  • Plates shall be durable and resistant to hydraulic fluids and environmental conditions (examples: anodized metal, suitable plastics).
  • Plates must be securely and visibly fixed; for fluids likely to corrode glued joints (e.g., phosphate ester) mechanical attachment is required.
• Sizing & legibility:
  • Preferred plate dimensions are indicated (examples: 15 × 5 mm, 30 × 10 mm, 50 × 17 mm) with corner radii; lettering should be as large as practicable relative to plate size.

Applications and users

ISO 3323:2023 is primarily used by:

• Aircraft component manufacturers (OEMs) specifying identification plates on hydraulic components
• Maintenance, repair and overhaul (MRO) workshops and assembly technicians
• Procurement and quality assurance teams ensuring fluid compatibility and traceability
• Suppliers and parts distributors preparing components for workshop handling

Practical benefits include reduced risk of fluid contamination, clearer maintenance procedures, improved safety during assembly/testing, and easier compliance with procurement and maintenance specifications.

Related standards

• ISO 12 - Aerospace - Pipelines - Identification
• ISO 9940 - Phosphate ester-base hydraulic fluid (technical specification)
• MIL-PRF-5606, MIL-PRF-83282, MIL-PRF-87257 (military hydraulic fluid specs)
• SAE AS1241, NSA307110, BMS 3-11

Keywords: ISO 3323, aircraft hydraulic components, fluid identification, identification plate, phosphate ester, hydraulic fluid marking, aerospace maintenance.