MIL-PRF-87257 Hydraulic Fluid Properties & Specs

MIL-PRF-87257 as the Low-Temperature Fire-Resistant Hydraulic Fluid

MIL-PRF-87257 is the U.S. military specification for a low-temperature, fire-resistant, synthetic hydrocarbon hydraulic fluid for aircraft and missile systems. It is the −54 °C (−65 °F) companion to MIL-PRF-83282. It is written for use from −54 °C to +200 °C and carries NATO code H-538 (MIL-PRF-87257C Scope 1.1, p.1). The fluid is red-dyed. The spec suggests a polyalphaolefin base blended from 2 and 4 mm²/s oils (not mandatory), with diesters as a blending fluid up to 35 % by weight, and it bans viscosity index improvers and pour point depressants (3.2, 3.2.1 and 3.2.1.3, pp.4–5).

Why it exists: cold flow. The viscosity limit at −40 °C is 550 mm²/s maximum, and the fluid must also stay under 2500 mm²/s at −54 °C after 3 and after 72 hours, with no gelling or crystals in the 72-hour −54 °C stability test. MIL-PRF-83282 allows 2200 mm²/s at −40 °C and has no −54 °C limit. Typical 87257 products read 447 to 492 mm²/s at −40 °C, against 2027 to 2089 for typical 83282 products. MIL-PRF-5606 is the petroleum fluid, with a flash point minimum of only 82 °C.

The trade-off: thinner when hot, lower flash point. The minimum viscosity at 40 °C is 6.7 mm²/s (14.0 for 83282) and at 100 °C it is 2.0 (3.45 for 83282). The flash point minimum is 160 °C and the fire point minimum 170 °C, against 205 and 245 °C for 83282D. It is more fire resistant than 5606 but less so than 83282. The spec requires 87257 to mix in all proportions with MIL-PRF-5606, -6083, -46170 and -83282 fluids from −54 °C to 200 °C without gums, sludge or insoluble material (Table I note 4, p.9). See also MIL-PRF-46170 and MIL-PRF-6083.

What data this site has. No government test report with measured density, specific heat or thermal conductivity of this fluid was found. Everything on this page is either a specification limit (MIL-PRF-87257C) or a single-lot “typical” value from a manufacturer data sheet, which is not a measurement made for this site and carries no stated uncertainty. Density is printed at only one temperature on each sheet, so there are no density, specific heat, thermal conductivity, Prandtl number, thermal diffusivity or thermal expansion pages. Kinematic viscosity has four to five temperatures per lot, so it has a page.

MIL-PRF-87257 Property Calculators

Explore the MIL-PRF-87257 property calculator on KasperCalc. For the related fluids, see MIL-PRF-83282 General Properties, MIL-PRF-5606 General Properties, MIL-PRF-46170 General Properties and MIL-PRF-6083 General Properties.

Kinematic Viscosity

Specifications

Designation	Status / Source
MIL-PRF-87257C	Performance Specification: Hydraulic Fluid, Fire Resistant, Low Temperature, Synthetic Hydrocarbon Base, Aircraft and Missile. Dated 30 June 2017; supersedes MIL-PRF-87257B of 22 April 2004 (EverySpec copy, read in full, 15 pages). All limits on this page are from this revision
MIL-PRF-87257D, E, F	Later revisions exist: search results show D (February 2022), E (12 July 2024) and F (9 April 2025). They were not opened because the DLA site blocks scripted access. The Radcolube FR257 sheet dated 22 July 2025 is headed MIL-PRF-87257F but cites 87257E section numbers for its tests, and its requirement column prints the same limits as 87257C for viscosity, flash and fire point, pour point and bulk modulus. Check the current revision before you rely on a limit
MIL-PRF-87257A, B and MIL-H-87257	Earlier issues. MIL-H-87257 was dated 2 March 1992, with a −54 to +135 °C range per search listings. Not opened
QPL-87257	Qualified Products List named in the spec. Not opened, so the current qualification of each product below is not verified
NATO H-538, OX-538	H-538 is the NATO code in the spec (Scope 1.1). “OX-538” is printed on the NYCO sheet. Related international documents named in 6.7: ASIC ACS(FG) 4023, NATO STANAG 1135 and AFLP-3748

The spec's 2004 revision (B) added the bulk modulus test (ASTM D6793) and a barium limit, per an AFRL 2006 workshop presentation (DTIC ADA483876). No values from that presentation are used.

Common Products with MIL-PRF-87257 Data Sheets

Product	Claim as stated on the data sheet	Data sheet
Radco RADCOLUBE FR257	Sheet 1: “MIL-PRF-87257C”, NATO Code H-538, qualification number AFPET/PTPS 19-011, qualification date 18 July 2019 (undated sheet). Sheet 2: “MIL-PRF-87257F”, revision date 22 July 2025	Radco data sheet, 87257C (reseller copy); Radco data sheet, 2025
NYCO Hydraunycoil FH 42	“Approved MIL-PRF-87257 C” and “Meets OX-538”, NATO Code H-538; also lists Airbus CML 02EBA1 and Comac CMS-OL-106 approvals. Based on PAO and esters, 7 cSt at 40 °C (undated sheet)	NYCO data sheet (reseller copy)
Castrol Brayco Micronic 881	“qualified to MIL-PRF-87257C” (Castrol product data, 10 June 2019). Describes an ISO viscosity grade 7, red-colored fluid and gives a −54 to 135 °C range	Castrol data sheet (distributor copy)

Products are listed because their data sheets were opened for this page; this is not a Qualified Products List. Claim wording is as printed on each sheet. Castrol Aero HFS 257 is sold against the same spec, but only a retail listing was found, so it is not listed. Distributor pages quote 87257A, B or F for Brayco 881; only the sheet's own “C” is recorded. No Royco, Mobil, Shell or other maker sheet was found.

Manufacturer Typical Values by Lot (not measurements)

Property	Radcolube FR257, 2025 sheet	Radcolube FR257, 87257C sheet	Hydraunycoil FH 42	Brayco Micronic 881
Relative density (SG), reference temperature	0.8420 at 15.6/15.6 °C (ASTM D4052)	0.827 at 15.6/15.6 °C (ASTM D1298)	0.834 at 20 °C (ASTM D4052; printed as “density”, no units)	0.843 (ASTM D1298; reference temperature not printed, API gravity 36.3 fits 15.6/15.6 °C). Also 7.02 lb/gal (ASTM D4052, temperature not printed)
Kinematic viscosity at −54 °C (mm²/s)	2094 (3 h); 2131 (72 h)	2129 (3 h); 2178 (72 h)	2286	2350; 2354 (3 h soak); 2377 (72 h soak)
Kinematic viscosity at −40 °C (mm²/s)	461	447	483	492
Kinematic viscosity at 40 °C (mm²/s)	6.8	6.7	6.75	7.2
Kinematic viscosity at 100 °C (mm²/s)	2.1	2.2	2.10	2.21
Kinematic viscosity at 200 °C (mm²/s)	—	—	0.87	—
Flash point / fire point (ASTM D92)	178 °C / 190 °C	182 °C / 194 °C	172 °C / 188 °C	172 °C (342 °F) / 190 °C (375 °F), COC
Pour point (ASTM D97)	≤ −63 °C	< −80 °C	−69 °C	−66 °C (−85 °F)
Isothermal secant bulk modulus, 40 °C and 27.6 MPa (ASTM D6793)	1418 MPa (205,664 psi)	1467 MPa (212,770 psi)	—	spec minimum only (1379 MPa), no typical value printed
Auto-ignition temperature (ASTM E659)	—	—	345 °C	345 °C (653 °F)
Evaporation loss, 6.5 h at 135 °C	9 %	10 %	10.0 %	15 %
Acid number (mg KOH/g)	0.09	0.08	0.03	0.11

Single-lot typical values from the data sheets listed under Sources; they are not measurements made for this site and no uncertainty is stated. The two Radcolube sheets are different lots: their relative densities (0.8420 and 0.827) differ by more than the spec's ±0.008 band for one product, and neither sheet says why. Each lot has density at one temperature only, so no density curve, dynamic viscosity or thermal expansion is given. No sheet gives specific heat or thermal conductivity. A Radco safety data sheet for the 87257C product (relative density 0.84, viscosity 2.1, 6.8, 470 and 2,350 mm²/s at 100, 40, −40 and −54 °C) agrees within rounding and was not charted.

Specification Limits: MIL-PRF-87257C vs MIL-PRF-83282D

Property	Temp	MIL-PRF-87257C limit	MIL-PRF-83282D limit	87257C source
Kinematic viscosity, max (ASTM D2532, 3 h and 72 h)	−54 °C	2,500 mm²/s	no limit	Table I, p.8
Kinematic viscosity, max	−40 °C	550 mm²/s	2,200 mm²/s	Table I, p.8
Kinematic viscosity, min	40 °C	6.7 mm²/s	14.0 mm²/s	Table I, p.8
Kinematic viscosity, min	100 °C	2.0 mm²/s	3.45 mm²/s	Table I, p.8
Low-temperature stability, 72 h	−54 °C	no gelling, clouding, crystallization, solidification or separation (FED-STD-791 method 3458)	—	Table I, p.8 and note 10, p.9
Pour point, max	—	−60 °C	−55 °C	Table I, p.8
Flash point, min	—	160 °C	205 °C	Table I, p.7
Fire point, min	—	170 °C	245 °C	Table I, p.7
Isothermal secant bulk modulus, min	40 °C, 27.6 MPa	1,379 MPa (200,000 psi)	1,379 MPa (200,000 psi), measured over 0 to 69 MPa	Table I, p.8
Evaporation loss, 6.5 h at 135 °C, max	135 °C	20 % by weight	20.0 % by weight (data sheets quote 205 °C)	Table I, p.7
Flame propagation, max	—	0.5 cm/s	0.30 cm/s	Table I, p.7
Acid number, max	—	0.20 mg KOH/g	0.10 mg KOH/g	Table I, p.7
Water, max	—	100 mg/kg	100 ppm	Table I, p.8
Swelling of NBR-L rubber	—	19.0 to 30.0 %	18.0 to 30.0 %	Table I, p.8
Relative density at 15.6/15.6 °C	15.6 °C	no limit; reported at qualification, later lots within ±0.008 of it	no limit; later lots within ±0.008	Table I, p.7 and notes 1–2, p.9
Operating temperature range	—	−54 °C to +200 °C	−40 °C to +205 °C	Scope 1.1, p.1
Prohibited additives	—	pour point depressants and viscosity index improvers	pour point depressants and viscosity index improvers	3.2.1, p.5

These are acceptance limits for the fluid, not measurements. The 87257C values were read from the spec text (revision C). The 83282D values are copied unchanged from the MIL-PRF-83282 general properties page. Revisions D to F of 87257 were not opened (see Specifications above).

Sources

US Department of Defense, MIL-PRF-87257C, Performance Specification: Hydraulic Fluid, Fire Resistant, Low Temperature, Synthetic Hydrocarbon Base, Aircraft and Missile, 30 June 2017. everyspec.com. Scope 1.1 (p.1), 3.2 (p.4), 3.2.1 (p.5), Table I (pp.7–8), notes (p.9), 6.7. Accessed 29 Oct 2026.
US Department of Defense, MIL-PRF-83282D, 30 September 1997, with Amendment 1. everyspec.com. Limits for the comparison column.
US Department of Defense, MIL-PRF-5606H, 7 June 2002. everyspec.com. Flash point minimum.
Radco Industries, Technical Data Sheet for RADCOLUBE FR257, MIL-PRF-87257F, product code FR257 (13056-A), 22 Jul 2025. Radco data sheet. Typical values, p.1.
Radco Industries, RADCOLUBE FR257 data sheet, MIL-PRF-87257C (undated, 3 pages). Radco data sheet (reseller copy). Typical values, pp.2–3.
Radco Industries, RADCOLUBE FR257 Hydraulic Fluid Safety Data Sheet, MIL-PRF-87257C. Radco SDS (retailer copy). Section 9, cross-check only.
NYCO, HYDRAUNYCOIL FH 42 Technical Data Sheet, NATO code H-538 (undated, 2 pages). NYCO data sheet (reseller copy). Typical values, pp.1–2.
Castrol, Brayco Micronic 881 Low Temperature Hydraulic Fluid, Product Data, 10 Jun 2019 (3 pages). Castrol data sheet (distributor copy). Typical values, pp.1–3.
E. Snyder, L. Gschwender and A. Campo, Military Aviation Fluids and Lubricants Workshop 2006, AFRL-RX-WP-TP-2008-4200, DTIC ADA483876. apps.dtic.mil/sti/tr/pdf/ADA483876.pdf. Context only (2004 revision changes); no values used.

Manufacturer values are single-lot typicals with no stated uncertainty. SAE AIR1362, which probably holds density, specific heat and conductivity curves for hydraulic fluids, is paywalled and was not opened. Spec revisions D to F and QPL-87257 were not opened. Data are also listed in tools/mil-prf-87257-sources.json and tools/mil-prf-87257-derived.json.

MIL-PRF-87257 General Properties

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