HyJet as an Aviation Hydraulic Fluid

HyJet is ExxonMobil's family of fire-resistant, phosphate ester aviation hydraulic fluids. The maker is Exxon Mobil Corporation (the fluid is now sold under the Mobil name as Mobil HyJet). It is not an Eastman product. Skydrol is Eastman's competing family.

There are two grades. HyJet IV-A plus is an SAE AS1241 Type IV (low density) fluid that ExxonMobil says approaches many Type V capabilities. HyJet V is a Type V fluid with better thermal and hydrolytic stability than commercial Type IV fluids. Both are rated for commercial aircraft hydraulic systems. Both are described as compatible in all proportions with approved Type IV and Type V phosphate ester fluids. Check the aircraft maintenance manual before use. This page is a data summary, not a servicing guide.

Every value on this site for HyJet comes from the ExxonMobil product data sheets. They give typical values at one or a few temperatures, not measurements and not specification limits. No test report with HyJet curves against temperature was found. So the site has pages only for density and viscosity. Specific heat, thermal conductivity and thermal expansion exist only as one value each, shown in the table below. They are not enough for a chart. Skydrol curves are a different fluid, so they are not used to fill these gaps.

HyJet Grade Lineup
GradeTypeNotes
HyJet IV-A plusAS1241 Type IV, Class 1, low densityExxonMobil describes it as the lowest-density Type IV fluid, with high-temperature stability and rust protection. Also listed against Boeing BMS 3-11P Type V Grade B and C.
HyJet VAS1241 Type VBetter thermal and hydrolytic stability than Type IV fluids, low density, and improved erosion protection. Lists Boeing BMS 3-11P Type V Grade A and C.

From the ExxonMobil product data sheets (descriptions are the maker's claims, not independent findings).

Typical Properties (not measurements)
PropertyHyJet IV-A plusHyJet VTest methodLimit printed on ExxonMobil sheet
Kinematic viscosity at −53.9 °C (−65 °F), cSt13201350ASTM D4452000 max
Kinematic viscosity at −26.1 °C (−15 °F), cSt130132ASTM D445135 max
Kinematic viscosity at 37.8 °C (100 °F), cSt10.610.6ASTM D44510.0–11.0
Kinematic viscosity at 98.9 °C (210 °F), cSt3.63.6ASTM D4453.35–3.75
Kinematic viscosity at 127.6 °C (printed; sheet also says 260 °F), cSt2.62.6ASTM D4451.5 min
Viscosity index280280ASTM D2270–
Shear stability, viscosity drop at 40 °C, %2221ASTM D5621–
Pour point, °C (°F)−62 (−80)−62 (−80)ASTM D97−62 (−80) max
Specific gravity, 25 °C/25 °C0.9960.997ASTM D40520.990–1.002 / 0.991–1.003
Density at 15.6 °C (60 °F), g/mL (lb/gal)0.999 (8.35)1.000 (8.35)ASTM D4052–
Specific heat at 40 °C, cal/(g·°C) = Btu/(lb·°F)0.410.42not listed–
Specific heat at 40 °C, kJ/(kg·K) (converted)1.7151.757––
Thermal conductivity at 40 °C, cal/(s·cm·°C) *33×10−533×10−5not listed–
Thermal conductivity at 40 °C, W/(m·K) (converted)0.1380.138––
Thermal conductivity at 40 °C, Btu/(hr·ft·°F) *0.07990.0799––
Coefficient of thermal expansion, 25–100 °C, per °C (per °F)0.00086 (0.00048)0.00086 (0.00048)API MPMS 11.1 (2024 sheet)–
Bulk modulus, isothermal secant, 100 °F / 3000 psi, psi (MPa)210,000 (1448)210,000 (1448)ASTM D6793 (2024 sheet)–
Flash point (Cleveland open cup), °C (°F)176 (349)174 (346)ASTM D92160 (320) min
Fire point (Cleveland open cup), °C (°F)188 (370)186 (366)ASTM D92177 (350) min
Autoignition temperature, °C (°F)≥427 (800)≥427 (800)ASTM D2155400 (752) min
Acid number, mg KOH/g0.040.04ASTM D9740.1 max
Water content, mass %0.10.09ASTM D63040.2 max
Electrical conductivity at 20 °C, µS/cm1.40.4ASTM D26240.5 min / 0.3 min
Derived: density at 25 °C, kg/m³993.1994.1SG × water density–
Derived: density at 40 °C, kg/m³981.0982.0see HyJet Density–
Derived: dynamic viscosity at 40 °C, mPa·s9.889.88ν × ρ–
Derived: thermal diffusivity at 40 °C, ×10−8 m²/s8.208.00k / (ρ·cp)–
Derived: Prandtl number at 40 °C123126μ·cp / k–

Typical values from the ExxonMobil data sheets (3/31/2016 editions; 2018 and 2024 editions checked, same values). The sheets say values may vary within modest ranges. The limits column is as printed on the ExxonMobil data sheet (SAE AS1241 itself was not read). Where two limits are shown they are for IV-A plus / V.

* The sheet prints the conductivity unit as cal/sec/cm²/°C. Read as per cm, it agrees with the printed Btu/(hr·ft·°F) value (0.0799 ≈ 33×10−5 × 241.9). It is converted here at 418.4 W/(m·K) per cal/(s·cm·°C).

Derived rows are calculated by KasperCalc from these typical values, at 40 °C only: ν interpolated between 37.8 and 98.9 °C, density from the derived density line, cp and k as printed. Specific heat and conductivity have two significant figures, so the Prandtl number and diffusivity are good to about ±4 %. They are single-point values, not curves.

Specifications and Approvals
Specification or approvalHyJet gradeMeetsQualified Products List
SAE AS1241, Type IV, low density (revision C on the 2016 sheet, D on the 2024 web sheet)IV-A plusmeetsn/a
SAE AS1241, Type VVmeetsn/a
Airbus NSA 307110N (Type V for HyJet V)IV-A plus, Vmeetsin QPL
ATR NSA 307110N (Type V for HyJet V)IV-A plus, Vmeetsin QPL
Boeing BMS 3-11P, Type IV Class 1 and Type V Grade B and CIV-A plusmeetsin QPL
Boeing BMS 3-11P, Type V Grade A and CVmeetsin QPL
Boeing (Douglas) / Long Beach DMS 2014H (Type 4 for IV-A plus, Type 5 for V)IV-A plus, Vmeetsin QPL
Gulfstream 1159SCH302J (Type V for HyJet V)IV-A plus, Vmeetsin QPL
British Aerospace (Avro) BAC.M.333CIV-A plusmeetsin QPL
Bombardier/Canadair BAMS 564-003A; Bombardier/DeHavillandIV-A plusmeetsin QPL
Cessna; Embraer; Fokker; Lockheed C-34-1224CIV-A plusmeetsin QPL

As listed on the ExxonMobil data sheets. "Meets" means the product meets the requirement. "In QPL" means it is on that maker's Qualified Products List. The 2024 web sheets add Airbus Canada A2MS 564-003 Class I Grade A and Boeing BMS 3-11P Type IV Low Density. Approvals change, so check the current ExxonMobil sheet and the airframe maker's list before use.

HyJet Property Calculators

Explore the HyJet property pages on KasperCalc, and the Skydrol pages they are compared with:

HyJet Density HyJet Viscosity Skydrol General Properties Skydrol Density Skydrol Viscosity Multi-Fluid Property Charts
Sources
  • Exxon Mobil Corporation, Exxon HyJet IV-A plus, Fire Resistant Phosphate Ester Aviation Hydraulic Fluid, product data sheet (3 pages, printed 31 Mar 2016). Reseller copy of the maker's PDF. Typical Properties table, pp.2–3. Accessed 29 Oct 2026. Re-checked against the Mobil web data sheet (2024 edition): same values.
  • Exxon Mobil Corporation, Exxon HyJet V, Fire Resistant Phosphate Ester Aviation Hydraulic Fluid, product data sheet (3 pages, printed 31 Mar 2016). Reseller copy of the maker's PDF. Typical Properties table, pp.2–3. Accessed 29 Oct 2026. Re-checked against the 2018 edition and the Mobil web data sheet (2024): same values.
  • Eastman Chemical Company, Skydrol aviation hydraulic fluids product comparison brochure, Publication EASAF7870. eastman.com. Skydrol comparison values, copied unchanged from the Skydrol Viscosity and Skydrol General Properties pages.
  • SAE International, AS1241, Fire Resistant Phosphate Ester Hydraulic Fluid for Aircraft. Cited by number only. Limits shown on this page are the limits printed on the ExxonMobil data sheets, not read from the standard.

No NASA, DTIC or journal report with HyJet property data was found. Data are also listed in tools/hyjet-sources.json and tools/hyjet-derived.json.

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