ISO 8625-2:2018

Overview - ISO 8625-2:2018 (Aerospace fluid systems vocabulary - flow)

ISO 8625-2:2018 is the Part 2 vocabulary of the ISO 8625 series. It defines general terms and definitions relating to flow used in aerospace fluid systems (aircraft and space vehicles). This second edition (2018) replaces the 1991 edition and provides consistent, internationally accepted terminology to be used in design, test, maintenance and documentation of pneumatic and hydraulic systems in aerospace construction.

Key topics and technical content

The standard is a vocabulary document; it does not set performance limits but standardizes language. Principal entries and topics include:

• Cavitation - formation and collapse of cavities/bubbles in liquids when local pressure drops below vapour pressure, including notes on damage and cavitation number.
• Choked flow - limiting conditions for compressible (gas) and incompressible (liquid) flows through restrictions.
• Flow - movement of fluid described by volumetric or mass flow rate; gas flow reference conditions are noted.
  • Laminar flow and Turbulent flow are defined as sub-entries.
• Flow factor / Flow coefficient (Kv / Cv) - flow rate per unit pressure drop (metric and English units).
• Flow parameter (for compressible devices), flow surge, rated flow, and relief flow.
• Leakage - external, internal, quiescent flow, and seepage (seal leakage) with context on acceptability and failure indication.
• Reynolds number - dimensionless parameter distinguishing laminar and turbulent flow (transition typically around 4 000–5 000).
• Reverse flow - flow opposite the prescribed system direction.

Practical applications and value

• Ensures clear, unambiguous communication across engineering teams, manufacturers, suppliers, test labs and certification authorities.
• Used in design specifications, component datasheets, test procedures, maintenance manuals and contractual documents to avoid misinterpretation of flow-related terms.
• Helps align qualification and verification reports (e.g., cavitation risk, choked-flow scenarios, leakage classification).
• Useful for training and technical documentation, improving compliance with aerospace procurement and regulatory processes.

Who should use this standard

• Aerospace hydraulic and pneumatic system designers and engineers
• Test and certification engineers
• Component manufacturers and suppliers (valves, pumps, actuators, seals)
• Maintenance organizations, technical writers, and standards librarians

Related standards

• ISO 8625 is a multi-part series; ISO 8625-2:2018 is Part 2 (flow terms). See the ISO website for the list of other parts in the ISO 8625 series and related aerospace fluid system standards.