ISO 9940:2015

Overview

ISO 9940:2015 - Aerospace series - Fluid, hydraulic, phosphate ester-base, fire resistant - Technical specification - defines the technical requirements, qualification procedures and quality‑control methods for phosphate‑ester‑base fire‑resistant hydraulic fluids used in aircraft hydraulic transmissions. The standard specifies which tests and measurements manufacturers and approved laboratories must use for product qualification, batch acceptance and in‑service monitoring to ensure safe, reliable hydraulic performance in aerospace applications.

Key topics and technical requirements

ISO 9940:2015 covers a comprehensive set of fluid properties, test methods and production controls, including (but not limited to):

• Fluid types and general technical requirements for phosphate‑ester hydraulic fluids.
• Physical and chemical properties: kinematic viscosity, density, water content, acidity index/acid number, electrical conductivity.
• Chemical composition checks: chlorine and other specified constituents.
• Thermal and flammability characteristics: pour point, flash and fire point, auto‑ignition temperature, and flammability tests (wick flame persistence, spray ignition, manifold‑ignition).
• Mechanical and thermodynamic properties: bulk modulus (isothermal secant), thermal expansion coefficient, specific heat.
• Electrical and contamination controls: dielectric resistance, particulate contamination (particle counting), filterability and foaming behaviour.
• Stability and lifetime tests: shear stability, thermal/corrosion/oxidation stability, ampoule and container tests for service life and fire‑resistance retention.
• Material compatibility: interaction with elastomers, paints, electrical components, metallic and composite structural elements.
• Erosion testing that simulates hydraulic system wear and pumping conditions.
• Quality management elements: batch definitions, acceptance testing, marking, packing and storage requirements.
• Annexes include elastomer compatibility, erosion test guidance and post‑delivery fluid monitoring.

The document references established test standards (e.g., ISO viscosity, density, Karl Fischer water determination, particle counting and specific fire tests) as normative sources for methods.

Practical applications and users

ISO 9940:2015 is intended for:

• Hydraulic fluid manufacturers qualifying phosphate‑ester fire‑resistant products for aerospace use.
• Aircraft OEMs and system integrators specifying fluids for hydraulic transmissions and approving supplier qualification.
• Approved testing laboratories performing qualification and batch acceptance testing.
• MRO organizations and airline maintenance teams for in‑service monitoring and acceptance of replacement fluids.
• Safety engineers and procurement specialists who assess compatibility, fire safety and lifecycle performance.

Applying ISO 9940:2015 helps ensure fluid fire resistance, material compatibility, system reliability and regulatory conformity in aircraft hydraulic systems.

Related standards

ISO 9940:2015 normatively references several ISO methods commonly used in aerospace fluid testing, for example:

• ISO 3104 (viscosity), ISO 12937 (water by Karl Fischer), ISO 2592 (flash/fire point), ISO 14935 (wick flame), ISO 15029 (spray ignition), ISO 4407 / ISO 11500 / ISO 11171 (particle counting) - among others.