Skydrol is Eastman's family of fire-resistant, phosphate ester–base aviation hydraulic fluids, first introduced in 1948 and now among the most widely used hydraulic fluids in commercial aviation. Unlike petroleum-base (MIL-PRF-5606) or polyalphaolefin fluids, phosphate esters do not sustain combustion under normal hydraulic system fault conditions: they will not propagate flame from a hot manifold drip or a high-pressure spray, which is why they are qualified as SAE AS1241 Type IV (fire-resistant, low volatility) and Type V (fire-resistant, low volatility, low viscosity) hydraulic fluids for use in transport-category aircraft. Skydrol is not compatible with unprotected aluminum, ordinary paints/finishes, or many elastomers used in petroleum-base hydraulic systems; consult the aircraft maintenance manual and elastomer/coating compatibility data before use, and never mix fluid types in a single system. The Concorde is a notable exception among large commercial aircraft, having used a mineral-base hydraulic fluid rather than a phosphate ester.
| Grade | Type | Introduced | Notes |
|---|---|---|---|
| Skydrol LD-4 | AS1241 Type IV | 1977 | Lower-density ("LD") counterpart to Skydrol 500B-4; reduces aircraft hydraulic fluid weight while retaining Type IV fire-resistance and elastomer/system compatibility. Eastman describes it as the world's best-selling Type IV aviation hydraulic fluid. |
| Skydrol 500B-4 | AS1241 Type IV | 1952 (500) 1977 (500B-4) |
Longest service history of any phosphate ester product. Skydrol 500 was introduced in 1952; successive reformulations led in 1977 to the current 500B-4, which carries the same anti-erosion additive and acid scavenger as LD-4. Highest density and viscosity of the four grades compared here, and the only standard-density aviation hydraulic fluid still sold. |
| Skydrol PE-5 | AS1241 Type V | 2010 | Phosphate ester Type V fluid developed to exceed Type V requirements, combining low density with low low-temperature viscosity. Eastman claims the longest fluid life of any commercially available phosphate ester, and describes it as the world's best-selling Type V fluid. |
| Skydrol 5 | AS1241 Type V | 1996 | The first Type V fluid qualified under Boeing BMS 3-11. Lowest density of the four grades and the first aviation hydraulic fluid to show erosion resistance at higher temperatures. Skydrol 5 does not hold universal airframe-manufacturer approval — consult Eastman for application-specific approvals. |
Type designations per SAE AS1241 (Aerospace Standard, cited by number).
| Property | Units | PE-5 | Skydrol 5 | LD-4 | 500B-4 | Test Method | |
|---|---|---|---|---|---|---|---|
| Kinematic viscosity @ −54 °C (−65 °F) | cSt | 1068 | 2085 | 1164 | 2678 | ASTM D445 | |
| Kinematic viscosity @ 38 °C (100 °F) | cSt | 9.75 | 9.23 | 11.10 | 11.67 | ASTM D445 | |
| Kinematic viscosity @ 99 °C (210 °F) | cSt | 3.38 | 3.13 | 3.91 | 3.84 | ASTM D445 | |
| Pour point | °C (°F) | <−62 (<−80) | <−62 (<−80) | <−62 (<−80) | <−62 (<−80) | ASTM D97 | |
| Specific gravity @ 25 °C | — | 0.996 | 0.977 | 1.011 | 1.056 | Eastman 116-B | |
| Density @ 25 °C | g/cc (lb/gal) | 0.9927 (8.284) | 0.9737 (8.126) | 1.0080 (8.412) | 1.0532 (8.789) | Eastman 116-B | |
| Acid number | mg KOH/g | 0.03 | 0.03 | 0.03 | 0.03 | ASTM D974 | |
| Moisture content | %w/w | 0.07 | 0.07 | 0.07 | 0.07 | ASTM D1744 | |
| Particle count | — | AS4059 Class 7 or better | SAE ARP598 | ||||
| Specific heat @ 38 °C | cal/(g·°C) | 0.453 | 0.402 | 0.437 | 0.418 | ASTM D2766 | |
| Specific heat @ 93 °C | cal/(g·°C) | — | 0.437 | 0.472 | 0.453 | ASTM D2766 | |
| Specific heat @ 120 °C | cal/(g·°C) | 0.461 | — | — | — | ASTM D2766 | |
| Specific heat @ 149 °C | cal/(g·°C) | — | 0.472 | 0.507 | 0.487 | ASTM D2766 | |
| Thermal conductivity @ 100 °F (38 °C) | cal/(sec·cm·°C) | 0.000344 | 0.000283 | 0.000326 | 0.000315 | ASTM D2717 | |
| Thermal conductivity @ 200 °F (93 °C) | cal/(sec·cm·°C) | 0.000289 | 0.000259 | 0.000298 | 0.000299 | ASTM D2717 | |
| Thermal conductivity @ 300 °F (149 °C) | cal/(sec·cm·°C) | 0.000263 | 0.000246 | 0.000277 | 0.000278 | ASTM D2717 | |
| Surface tension @ 25 °C | dynes/cm | 29.4 | — | 28.2 | 26.7 | Du Noüy balance | |
| Heat of combustion | BTU/lb | 13,291 | 13,100 | 13,700 | 13,400 | ASTM D240 | |
| Bulk modulus | psi | 235,000 | 210,000 | 231,000 | 242,000 | BMS3-11 | |
| Four-ball wear scar — 4 kg | mm | 0.30 | 0.20 | 0.33 | 0.36 | ASTM D4172 | |
| Four-ball wear scar — 10 kg | mm | 0.41 | 0.46 | 0.43 | 0.45 | ASTM D4172 | |
| Four-ball wear scar — 40 kg | mm | 0.65 | 0.77 | 0.69 | 0.68 | ASTM D4172 | |
Typical values, not specification limits, from Eastman's Skydrol product comparison brochure (Eastman Publication EASAF7870). Individual sample results are not guaranteed; see the applicable technical data sheet for qualified specification limits.
| Property | PE-5 | Skydrol 5 | LD-4 | 500B-4 | Test Method |
|---|---|---|---|---|---|
| Flash point | 171 °C (339 °F) | 166 °C (331 °F) | 174 °C (346 °F) | 186 °C (366 °F) | ASTM D92 |
| Fire point | 191 °C (376 °F) | 183 °C (362 °F) | 182 °C (360 °F) | 210 °C (410 °F) | ASTM D92 |
| Autoignition temperature (AIT) | 424 °C (796 °F) | 466 °C (871 °F) | 469 °C (877 °F) | 514 °C (957 °F) | ASTM D2155 |
| Hot manifold drip | Does not burn in tray | AMS 3150C | |||
| High-pressure spray | Will not ignite | AMS 3150C | |||
| Low-pressure spray (flame propagation) | No increase | AMS 3150C | |||
| Wick flammability | >40 cycles | AMS 3150C | |||
Source: Eastman Publication EASAF7870. Fire-resistance behavior is a characteristic of phosphate ester chemistry and does not imply the fluid is non-combustible under all conditions; always follow airframe manufacturer fire-safety guidance.
The comparison tables above report Eastman's typical laboratory values, useful for interpolating property curves. The individual product technical data sheets instead publish qualified specification limits — the bounds a production batch must meet, not its typical value. These limits can vary between technical data sheet revisions; where the brochure and a data sheet disagree, the brochure typical values are used for the temperature curves on the other Skydrol calculator pages, and the ranges below are cited from the currently posted data sheets.
| Property (spec limit) | PE-5 | Skydrol 5 | LD-4 | 500B-4 |
|---|---|---|---|---|
| Type per SAE AS1241 | Type V | Type V | Type IV | Type IV |
| Flash point (COC), min | 160 °C (320 °F) | 154 °C (310 °F) | 160 °C (320 °F) | 177 °C (350 °F) |
| Autoignition temperature, min | 400 °C (752 °F) | 399 °C (750 °F) | 400 °C (752 °F) | 460 °C (860 °F) |
| Pour point, max | −62 °C (−80 °F) | −62 °C (−80 °F) | −62 °C (−80 °F) | −62 °C (−80 °F) |
| Acid number, max | 0.10 mg KOH/g | 0.10 mg KOH/g | 0.10 mg KOH/g | 0.10 mg KOH/g |
| Moisture content, max | 0.20 %w/w | 0.20 %w/w | 0.20 %w/w | 0.20 %w/w |
| Cleanliness (particle count) | NAS 1638 / AS4059 — per applicable revision, report or Class 7 or better | |||
| Elemental content (Fe, Cr, Ni, etc.) | Report/trace, per revision (spectrometric analysis) | |||
Sources: Skydrol LD-4, Skydrol 500B-4, Skydrol PE-5, and Skydrol 5 technical data sheets (Eastman). Viscosity spec limits are intentionally omitted here because published revisions disagree — e.g., the Skydrol 500B-4 data sheet lists 2900 cSt max @−54 °C while an older technical bulletin lists 4200 cSt max and the brochure's typical value is 2678 cSt; always confirm against the current data sheet revision for acceptance testing.
Phosphate ester hydraulic fluids were developed following a series of in-flight fires traced to petroleum-base hydraulic fluid leaking onto hot engine and brake components. Eastman (originally Monsanto/Solutia) introduced the first Skydrol fluid in 1948, and Skydrol has since become the predominant fire-resistant hydraulic fluid for commercial transport aircraft, used across the Boeing, Airbus, and most other major commercial airframe families. Skydrol fluids are traditionally dyed purple to visually distinguish them from petroleum-base (red) and polyalphaolefin/synthetic hydrocarbon (also often red or clear) hydraulic fluids, reducing the risk of accidental system cross-contamination during servicing.
Explore the full set of Skydrol property calculators on KasperCalc: