ISO 9940:2015

INTERNATIONAL ISO
STANDARD 9940
First edition
2015-08-01
Aerospace series — Fluid, hydraulic,
phosphate ester-base, fire resistant —
Technical specification
Série aérospatiale — Fluide, hydraulique, esters phosphoriques,
résistant au feu — Spécification technique
Reference number
©
ISO 2015
© ISO 2015, Published in Switzerland
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ii © ISO 2015 – All rights reserved

Contents Page
Foreword .v
1 Scope . 1
2 Normative references . 1
3 Technical requirements . 3
3.1 General . 3
3.2 Fluid types. 3
3.3 Physical and chemical properties . 3
3.4 Products and manufacturers qualification . 4
3.5 Approved laboratories. 4
4 Qualification requirements . 5
4.1 Absolute kinematic viscosity “ν” . 5
4.2 Water content “[H O]” . 6
4.3 Density “ρ” . 6
4.4 Acidity index “AI” — Acid number “AN” . 6
4.5 Electrical conductivity “γ” . 6
4.6 Chemical content . 7
4.6.1 Chlorine . 7
4.6.2 Other components . 7
4.7 Pour point temperature . 7
4.8 Flash point and fire point temperatures . 8
4.9 Auto-ignition temperature . 8
4.10 Flammability test . 8
4.10.1 Determination of wick flame persistence . 8
4.10.2 Spray ignition test . 8
4.10.3 Manifold-Ignition test on a hot element . 8
4.11 Colour — Condition . 9
4.12 Bulk modulus (Isothermal secant) . 9
4.13 Thermal expansion coefficient “α” .11
4.14 Dielectric resistance .11
4.15 Specific heat “Cp” .11
4.16 Solid particulate contamination .11
4.16.1 General.11
4.16.2 Particle counting .11
4.16.3 Filterability “F” .12
4.17 Foaming .12
4.18 Toxicity .12
4.19 Stability .13
4.19.1 Shear stability .13
4.19.2 Thermal, corrosion and oxidation stability .13
4.20 In-service lifetime .15
4.20.1 General.15
4.20.2 Ampoule test method.16
4.20.3 “1 Litre-Container” Test method for fire resistance requirement in
accordance with 4.8 .18
4.21 Material compatibility with qualified fluids (see list provided by the
airframe manufacturer) .18
4.21.1 Compatibility with qualified fluids .18
4.21.2 Compatibility with elastomeric materials .19
4.21.3 Compatibility with paints .20
4.21.4 Compatibility with electrical aircraft components .20
4.21.5 Compatibility with structural elements: adhesive, metallic parts
(including tubes), composites .20
4.22 Lubrication .20
4.23 Erosion tests of the hydraulic fluid .21
4.23.1 General.21
4.23.2 Test 1 — Erosion System Test Bench representative of one aircraft
hydraulic system .21
4.23.3 Test 2 — Pumping Test .21
4.23.4 Test 3 — Erosion Resistance Tests .21
5 Batches .21
6 Quality control .21
6.1 Hydraulic fluid acceptance tests .21
6.2 Physical characteristics .21
6.3 Chemical characteristics .22
7 Recommended limits .22
8 Marking .22
9 Packing and storage .22
Annex A (normative) Elastomer compatibility .24
Annex B (informative) Erosion tests .26
Annex C (normative) Fluid monitoring after A/C delivery .27
Bibliography .28
iv © ISO 2015 – All rights reserved

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical
Barriers to Trade (TBT) see the following URL: Foreword - Supplementary information
The committee responsible for this document is ISO/TC 20, Aircraft and space vehicles, Subcommittee
SC 10, Aerospace fluid systems and components.
INTERNATIONAL STANDARD ISO 9940:2015(E)
Aerospace series — Fluid, hydraulic, phosphate ester-base,
fire resistant — Technical specification
1 Scope
This International Standard defines technical requirements, qualification and quality control
conditions (product qualification, batch control, acceptance) test and measurement methods to be
used by manufacturers for qualification of fire-resistant phosphate-ester-base fluids used for hydraulic
transmissions in aircrafts.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO 37, Rubber, vulcanized or thermoplastic — Determination of tensile stress-strain properties
ISO 48, Rubber, vulcanized or thermoplastic — Determination of hardness (hardness between 10 IRHD
and 100 IRHD)
ISO 1817, Rubber, vulcanized or thermoplastic — Determination of the effect of liquids
ISO 2592, Determination of flash and fire points — Cleveland open cup method
ISO 2921, Rubber, vulcanized — Determination of low-temperature retraction (TR test)
ISO 3016, Petroleum products — Determination of pour point
ISO 3104, Petroleum products — Transparent and opaque liquids — Determination of kinematic viscosity
and calculation of dynamic viscosity
ISO 3116, Magnesium and magnesium alloys — Wrought magnesium alloys
ISO 3675, Crude petroleum and liquid petroleum products — Laboratory determination of density —
Hydrometer method
ISO 3696, Water for analytical laboratory use — Specification and test methods
ISO 4407, Hydraulic fluid power — Fluid contamination — Determination of particulate contamination by
the counting method using an optical microscope
ISO 6247, Petroleum products — Determination of foaming characteristics of lubricating oils
ISO 6618, Petroleum products and lubricants — Determination of acid or base number — Colour-indicator
titration method
ISO 6619, Petroleum products and lubricants — Neutralization number — Potentiometric titration method
ISO 11171, Hydraulic fluid power — Calibration of automatic particle counters for liquids
ISO 11218, Aerospace — Cleanliness classification for hydraulic fluids
ISO 11500, Hydraulic fluid power — Determination of the particulate contamination level of a liquid sample
by automatic particle counting using the light-extinction principle
ISO 12185, Crude petroleum and petroleum products — Determination of density — Oscillating U-tube
method
ISO 12937, Petroleum products — Determination of water — Coulometric Karl Fischer titration method
ISO 13357-2, Petroleum products — Determination of the filterability of lubricating oils — Part 2:
Procedure for dry oils
ISO 13573:2012, Corrosion of metals and alloys — Test method for thermal-cycling exposure testing under
high-temperature corrosion conditions for metallic materials
ISO 14935, Petroleum and related products — Determination of wick flame persistence of fire-resistant fluids
ISO 15029-1, Petroleum and related products — Determination of spray ignition characteristics of fire-
resistant fluids — Part 1: Spray flame persistence — Hollow-cone nozzle method
ISO 15597, Petroleum and related products — Determination of chlorine and bromine content —
Wavelength-dispersive X-ray fluorescence spectrometry
ISO 20823, Petroleum and related products — Determination of the flammability characteristics of fluids
in contact with hot surfaces — Manifold ignition test
EN 1652, Copper and copper alloys — Plate, sheet, strip and circles for general purposes
EN 2395, Aerospace series — Aluminium alloy AL-P2024A — T4 or T42 — Sheet and strip — 0,4 mm ≤ a ≤
6 mm
EN 10130, Cold rolled low carbon steel flat products for cold forming — Technical delivery conditions
1)
ASTM D 92 , Standard Test Method for Flash and Fire Points by Cleveland Open Cup Tester
1)
ASTM D 97 , Standard Test Method for Pour Point of Petroleum Products
1)
ASTM D 445 , Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (and
Calculation of Dynamic Viscosity)
1)
ASTM D 664 , Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration
1)
ASTM D 877 , Standard Test Method for Dielectric Breakdown Voltage of Insulating Liquids Using
Disk Electrodes
1)
ASTM D 892 , Standard Test Method for Foaming Characteristics of Lubricating Oils
1)
ASTM D 974 , Standard Test Method for Acid and Base Number by Color-Indicator Titration
1)
ASTM D 1217 , Standard Test Method for Density and Relative Density (Specific Gravity) of Liquids by
Bingham Pycnometer
1)
ASTM D 1298 , Standard Test Method for Density, Relative Density, or API Gravity of Crude Petroleum and
Liquid Petroleum Products by Hydrometer Method
1)
ASTM D 2155 , Standard Test Method for Determination of Fire Resistance of Aircraft Hydraulic Fluids by
Autoignition Temperature
1)
ASTM D 4052 , Standard Test Method for Density, Relative Density, and API Gravity of Liquids by Digital
Density Meter
1)
ASTM D 4172 , Standard Test Method for Wear Preventive Characteristics of Lubricating Fluid (Four-
Ball Method)
1)
ASTM D 4636 , Standard Test Method for Corrosiveness and Oxidation Stability of Hydraulic Oils, Aircraft
Turbine Engine Lubricants, and Other Highly Refined Oils
1) Published by: ASTM International www.astm.org
2 © ISO 2015 – All rights reserved

1)
ASTM D 5621 , Standard Test Method for Sonic Shear Stability of Hydraulic Fluids
1)
ASTM D 6304 , Standard Test Method for Determination of Water in Petroleum Products, Lubricating Oils,
and Additives by Coulometric Karl Fischer Titration
2)
ASTM D 6443 , Test Method for Determination of Calcium, Chlorine, Copper, Magnesium, Phosphorus,
Sulfur, and Zinc in Unused Lubricating Oils and Additives by Wavelength Dispersive X-ray Fluorescence
Spectrometry (Mathematical Correction Procedure)
2)
ASTM D 6793 , Standard Test Method for Determination of Isothermal Secant and Tangent Bulk Modulus
2)
ASTM E 1269 , Standard Test Method for Determining Specific Heat Capacity by Differential Scanning
Calorimetry
3)
DEF STAN 03-19 , Electro-Deposition of Cadmium
3 Technical requirements
3.1 General
The product shall be a fire resistant, phosphate ester-base fluid for hydraulic power transmissions. Its
chemical composition (additives, contaminants) and intrinsic physical and chemical properties shall be
in strict compliance with the requirements of this International Standard.
All delivered hydraulic fluids shall be fully compatible with any other phosphate ester-base fluid
already qualified, whatever the mixture proportions.
In the following clauses, the International Standard appearing in brackets shall be considered as an
alternative method to the preceding standard, e.g. ISO 3104 (ASTM D 445); ASTM D 445 is an alternative
standard to ISO 3104.
3.2 Fluid types
Fluids types covered by this International Standard are listed in the Table 1 with their corresponding
properties.
Table 1 — Type of fluid and properties
Type of fluid Fluid properties
“HD”
Fire resistant, low water content, anti-erosion, high viscosity at low
temperature and high density
IV
(High Density)
Fire resistant, low water content, anti-erosion, low density
“LD”
Fire resistant, low water content, low density, anti-erosion, improved
V (Low Density)
thermal stability and in service lifetime.
3.3 Physical and chemical properties
The main physical and chemical properties of the fluid covered by this International Standard are listed
in the Table 2.
2) Published by: ASTM International www.astm.org
3) Published by: MINISTRY OF DEFENCE (United Kingdom) www.gov.uk/government/organisations/ministry-
of-defence
3.4 Products and manufacturers qualification
The qualified products list appears in the AMM (Aircraft Maintenance Manual).
3.5 Approved laboratories
All laboratories used should be ISO 9001 registered and AS/EN/JIS Q 9100 compliant.
Table 2 — Main characteristics and properties
Unit Requirements Test method(s)
Sub
Preferred Alternative
Property Condition
clause
Symbol TYPE IV TYPE V International International
Standard Standard
−54 °C (−65 °F)
HD: ≤ 2 900
LD: ≤ 2 000
2 −1
Absolute mm ·s 38 °C (100 °F)
LD: ≤ 2 000
kinematic (centiStokes) 99 °C (210 °F) ISO 3104 ASTM D 445 4.1
∈ [ 9,00; 12,50]
viscosity ”ν” Atmospheric
∈ [3,00; 4,00]
pressure
Water H O % mass ≤0,20
b
— ISO 12937 ASTM D 6304 4.2
a
content or ppm or 2 000
1 021 ≤ HD ≤ ASTM D 1298
ISO 3675 or
-3
Density kg·m “ρ” (23 ± 3) °C 1 066 LD:≤ 1 020 D 4052 4.3
ISO 12185
LD: ≤ 1 020 or D 1217
Acid number
mg KOH ISO 6618 or ASTM D 974 or
— ≤0,10 4.4
per g of fluid ISO 6619 D 664
Acidity index
20 °C
−1
Electrical μS·cm ≥0,30
(−10 °C to ISO 9940 — 4.5
conductivity ”γ” ∈ [0,02 ; 6,00]
100 °C)
-
Cl
Chlorine
Tot. chlorine ≤50 ISO 15597 ASTM D 6443 4.6
content
a
ppm
Pour point
≤ −62 (−80) ISO 3016 ASTM D 97 4.7
temperature
Flash point
≥160 ( 320)
temperature
ISO 2592 ASTM D 92 4.8
°C (°F) —
Fire point
≥177 (350)
temperature
≥399 (750)
Auto ignition
≥460 (860) for HD
≥399 (750) ASTM D 2155 — 4.9
temperature
5 s, 10 s, 20 s and Mean persistence time of the
Seconds ISO 14935 — 4.10.1
30 s flame < 7,5 s
Time elapsed between removal
of the igniting flame and
Flammability Seconds — ISO 15029-1 — 4.10.2
extinction of the spray along the
spray pattern ≤ 4 s
Ignition
700 °C ± 5 °C k ≥ 10 ISO 20823 — 4.10.3
m
category
a -1 -6
ppm = parts per million = µg∙g (10 ).
b
Deviation to ISO 12937 is granted for water content above 0,1.
4 © ISO 2015 – All rights reserved

Table 2 (continued)
Unit Requirements Test method(s)
Sub
Preferred Alternative
Property Condition
clause
Symbol TYPE IV TYPE V International International
Standard Standard
Colour Purple
— — — — 4.11
condition Clear appearance
Isothermal
38 °C, 20,6∙10
secant Bulk Pa Mini value: 1 450·10 Pa ASTM D 6793 — 4.12
Pa
Modulus
−3
−25 °C to 99 °C ≤1·10
Thermal
−1
°C “α” ASTM D 1217 — 4.13
expansion
−54 °C to 110 °C Curve to provide
≤7
ISO 11218
Solid particu-
Cleanliness ISO 11171
late contamina- — —
class ISO 11500
4.16
tion (counting)
or ISO 4407
Filterability F — ∈ [1,00 ; 1,60] ISO 13357-2 —
24 °C After 5 min Persistence
93 °C ≤250 ≤100 s
24 °C ≤150 ≤50 s
Foaming cm ISO 6247 ASTM D 892 4.17
≤450 250 s
a -1 -6
ppm = parts per million = µg∙g (10 ).
b
Deviation to ISO 12937 is granted for water content above 0,1.
4 Qualification requirements
4.1 Absolute kinematic viscosity “ν”
Absolute kinematic viscosity “ν” of the fluid shall be determined at the temperatures indicated
below and at atmospheric pressure using viscosimetric capillary tubes in accordance with ISO 3104
2 -1
(ASTM D 445). The limits values are in mm ·s (centiStokes), see Table 3.
Table 3 — Viscosity limit values
Temperatures Type
°C °F IV High Density IV & V Low Density
−54 −65 ν ≤ 2 900 ν ≤ 2 000
−40 −40
Values to be measured and provided
−15 5
38 100 9,00 ≤ ν ≤ 12,50
99 210 3,00 ≤ ν ≤ 4,00
Measurements shall be taken in a thermostatic bath at accurate temperatures, controlled using an
accurate contacting thermometer in accordance with ISO 3104 (ASTM D 445).
NOTE At low temperatures, viscosimetric tubes may be connected to dehydration tubes to avoid
condensation forming inside.
An evolution curve “Viscosity vs. Temperature” from −54 °C to 99 °C is requested.
4.2 Water content “[H O]”
The water content [H O] shall be determined
— by using the electrochemical method “KARL FISCHER” in compliance with ISO 12937 (ASTM D 6304).
The limit value shall be
— [H O] ≤ 0,20 % mass, or
— [H O] ≤ 2 000 ppm.
4.3 Density “ρ”
The density “ρ” shall be determined at (23 ± 3) °C in compliance with ISO 3675 (ASTM D 1298) or
ASTM D 1217 (Pycnometer) or ISO 12185 (ASTM D 4052) (this method is based on the measurement of
the harmonic period of fluid in a U-shape tube).
-3
The corrected limit values at 20 °C, in kg·m , shall be as follows:
— Type IV High Density: 1 021 ≤ ρ ≤ 1 066;
— Type IV & V Low Density: ρ ≤ 1 020.
An evolution curve “Density vs. Temperature” from −40 °C to 100 °C is requested.
4.4 Acidity index “AI” — Acid number “AN”
The Acid number “AN” of the fluid shall be determined using the colour indicator titration method in
accordance with ISO 6618 (ASTM D 974) or the Acidity index “AI” of the fluid shall be determined using
the potentiometric titration method in accordance with ISO 6619 (ASTM D 664).
The limit value, in mg KOH per gram of fluid shall be:
AI or AN ≤ 0,10
4.5 Electrical conductivity “γ”
Test equipment:
— cell immersed in hydraulic fluid
— impedance bridge (conductimeter)
Measurement conditions:
— frequency: 50 Hz
— temperature = (20 ± 3) °C
— cell constant K ∈ [0,7; 1,2] cm
Electrical resistance “R” shall be measured across the terminals of the cell.
6 © ISO 2015 – All rights reserved

The electrical conductivity is:
−−11
γ =⋅LR ⋅=SK ⋅R (1)
where
R is the resistance, in Ohms (Ω);
-1
K is the cell constant, in cm = S∙L ;
S is the surface area of electrodes, in cm ;
L is the gap between electrodes, in cm.
-1
The corrected value at 20 °C in μS cm , shall be
— Normal limit: γ ≥ 0,30.
-1
The value between −10 °C and 100 °C in μS cm , shall be
-6 -6 -1 -6
— γ ∈ [0,02; 6,00] (μS = microSiemens = 10 Siemens = 10 Ω = 10 Mho).
An evolution curve “Conductivity vs. Temperature” from −40 °C to 100 °C is requested.
4.6 Chemical content
4.6.1 Chlorine
The Total Chlorine concentration limit shall be:
Total Chlorine ≤ 50 ppm
Methods to be used are Wavelength dispersive X-ray fluorescence spectrometry (see ISO 15597 or
ASTM D 6443) or other methods provided acceptable to aircraft manufacturer.
4.6.2 Other components
-1 -6
During qualification, chemical contents (all fluids) in ppm (parts per million) or µg∙g (10 ) as Calcium,
Potassium, Sulphur, Sodium shall be duly identified when these elements are formulation additives and
their fluid limit concentration shall be determined by fluid manufacturers. All other additives not duly
identified in the fluid manufacturer formulation will be considered as contaminant and consequently
prohibited, except total chlorine when its concentration is as follows:
— TYPE IV/[CL] ≤ 50 ppm;
— TYPE V/[CL] ≤ 50 ppm.
Method to be used is Wavelength Dispersive X-Ray Fluorescence spectrometry (ASTM D 6443) or other
methods provided acceptable to aircraft manufacturer.
4.7 Pour point temperature
Pour point temperature shall be determined in compliance with ISO 3016 (ASTM D 97).
— Pour point ≤ −62 °C (−80 °F)
4.8 Flash point and fire point temperatures
Flash point and fire point temperatures shall be determined in an open vessel using the CLEVELAND
method in compliance with ISO 2592 (ASTM D 92).
— Flash point: ≥160 °C (320 °F)
— Fire point: ≥177 °C (351 °F)
To determine degradation of these characteristics, fluid shall be tested in laboratory conditions in
accordance with “1 Litre-Container” test method described in 4.20.3 until an Acidity index of 1,5 ± 0,3
has been reached (in keeping with the required time-temperature exposure under linked water and
chlorine contamination conditions).
4.9 Auto-ignition temperature
Auto-ignition temperature shall be determined in compliance with ASTM D 2155.
— Auto-ignition: ≥399 °C (750 °F) Type IV Low Density and Type V
— Auto-ignition: ≥460 °C (860 °F) Type IV High Density
4.10 Flammability test
4.10.1 Determination of wick flame persistence
This determination of wick flame persistence is in accordance with ISO 14935, except the limits and
pass criteria.
The limits and pass criteria shall be
— mean persistence time of flame MP < (7,5 ± 0,1) s.
4.10.2 Spray ignition test
This spray ignition test is in accordance with ISO 15029-1, except the limits and pass criteria.
Limits and pass criteria shall be as follows:
— flashes readily and self-extinguishes;
— maximum time, to the nearest 0,1 s, between removal of the igniting flame and extinction of the
spray combustion, at different positions along the length of a pressurized spray of fluid under the
conditions specified in ISO 15029-1 shall not exceed 4 s.
4.10.3 Manifold-Ignition test on a hot element
The test equipment is in accordance with ISO 20823, except the limits and pass criteria.
The limits and pass cr
...