FKM (also labelled FPM, ASTM D1418) is the workhorse fluoroelastomer for static and dynamic seals in commercial and military aerospace, with continuous service typically spanning -26 °C to +205 °C in air and short excursions higher in dry-heat zones [S7]. Across more than 80 years of commercial development since 1943, several distinct FKM families have been commercialized, including copolymer, terpolymer, and tetrapolymer types tuned for low-temperature flexibility, chemical resistance, or compression set [S9].
Polymer architecture, not trade name, drives selection: a VDF/HFP copolymer, a VDF/HFP/TFE terpolymer, and a perfluoromethyl vinyl ether (PMVE) modified grade sit on the same ASTM D1418 shelf but answer very different fluids, and aerospace procurement therefore keys on a published compound specification, not a brand [S1][S9]. The most commonly cited aerospace anchors are EN 2796 (60 IRHD), EN 2566 (70 IRHD), and SAE AMS7276J for fuel and specific engine-oil service at 70 to 80 Shore A [S3][S5][S8].
FKM chemistry and fluorine content: 65 to 71 percent as the master dial
Commercial FKM is built by polymerising vinylidene fluoride (VDF) with hexafluoropropylene (HFP), tetrafluoroethylene (TFE), or perfluoromethyl vinyl ether (PMVE), and the resulting co-, ter-, and tetrapolymers carry 65 to 71 percent fluorine by mass [S1]. That fluorine number is the master dial: rising fluorine content pushes media resistance and high-temperature stability up, but pulls low-temperature flexibility and elongation down, which is why two "FKM" parts on the same drawing can behave like different polymers [S1].
The C-F bond is also the reason FKM has low gas permeability, high ozone and weathering resistance, and flame-retardant vulcanisates without added retarder, so the same chemistry that lets an FKM O-ring survive +400 °F (+204 °C) air service is what keeps it sealing in vacuum and in fuel vapour [S1]. Standard durometers stocked for O-rings are 50, 75, and 90 Shore A, with cure systems drawn from diamine, bisphenol, or organic peroxide depending on whether the priority is chemical resistance, compression set, or steam/hot-water service [S1].
Aerospace spec bands: EN 2796 (60 IRHD), EN 2566 (70 IRHD), AMS 7276 (70 to 80 Shore A)
European aerospace procurement keys on EN 2796:2018, which sets the property requirements for low-compression-set FKM at 60 IRHD for aerospace seals, and on EN 2566:2019 at 70 IRHD, both documents covering the same fluoroelastomer family with a hardness step [S5][S8]. The corresponding US aerospace anchor is SAE AMS7276J, which specifies fluorocarbon (FKM), high-temperature, fluid-resistant, low-compression-set rubber at 70 to 80 Type A hardness for seals in fuel systems and specific engine-oil systems [S3][S6].
Those three specs are not interchangeable: EN 2796 is the low-compression-set / 60 IRHD tier, EN 2566 is the 70 IRHD tier, and AMS 7276J is the fluid-resistant / fuel-system tier, so the right choice depends on which combination of hardness, compression set, and fluid exposure the drawing calls out [S3][S5][S6][S8]. Because EN 2796, EN 2566, and AMS 7276 each bundle a different property profile, the procurement call is "which spec, then which compound inside that spec", not "which brand" [S3][S5][S8].
Fluid-side selection: mineral oil, fuel, hydraulic, and where FKM is the wrong answer
In high-temperature, high-load mineral-oil systems, FKM holds volume swell below 5 percent, which is the typical pass band engineers look for in aerospace hydraulic and engine-oil service [S2]. In aromatic and aviation fuels, FKM keeps volume change under roughly 8 percent and retains usable mechanical properties, which is the chemistry behind its fuel-system dominance in AMS 7276 [S2][S3].
The honest failure list, though, is what separates a spec-grade FKM call from a marketing-grade FKM call. Amines are destructive to FKM, so amine-inhibited fluids, amine gas-sweetening service, and amine-cured epoxy exposure should route to Aflas (TFE/P) or FFKM rather than FKM [S1]. Ketones and esters (MEK, acetone, ethyl acetate) swell standard FKM badly and point the engineer to EPDM or FFKM; DOT 3, 4, and 5.1 glycol brake fluid wants EPDM; and standard bisphenol-cured FKM is weak in hot water and steam, where a peroxide-cured FKM, EPDM, or Aflas is the correct step [S1]. Cold-side flexibility is the other FKM trade-off: low-temperature TR-10 ratings near -26 °C are the typical commercial FKM envelope, and anything below that needs an FFKM or a fluorosilicone [S7].
FKM grade comparison for aerospace sealing
Three FKM archetypes cover most aerospace call-outs, and the differences are quantified, not vibe-based. A VDF/HFP copolymer (the "Type A" / Viton A / FKM-A family) typically runs the most flexible low-temperature behaviour of the three, with the lowest fluorine content and the weakest resistance to aromatic fuels and aggressive chemicals. A VDF/HFP/TFE terpolymer ("Type B" / Viton B / FKM-B) sits in the middle, balancing low-temperature flexibility against media resistance, and is the workhorse of commercial aerospace O-rings. A perfluoromethyl vinyl ether modified grade ("Type GBL" or peroxide-curable) pushes the upper temperature and chemical envelope higher but at higher cost and harder processing, and is the usual answer when AMS 7276 or a low-compression-set EN 2796 compound is specified [S1][S9].
On four decision criteria the comparison reads cleanly: (1) continuous upper temperature, where the copolymer reaches roughly +205 °C, the terpolymer reaches +230 °C, and the PMVE/peroxide grade pushes toward +250 °C in air; (2) low-temperature TR-10, where the copolymer is best and the peroxide-cured grade is worst; (3) aromatic-fuel and aggressive-chemical resistance, which scales with fluorine content and therefore with the terpolymer and peroxide-cured grades; (4) compression set at high temperature, where peroxide-cured FKM beats bisphenol-cured FKM and is the reason AMS 7276J explicitly calls for low compression set [S1][S3][S9]. Material scientists will recognise the trilemma: you can usually pick two of "low temperature, high temperature, low compression set" on a given FKM, and the spec band is what forces the choice [S1][S3].
Sourcing and qualification levers for aerospace FKM
Aerospace FKM qualification is done against a published compound, not a polymer family, so the data on the actual compound datasheet always overrides generic FKM range claims [S1]. For European builds EN 2796 (60 IRHD) and EN 2566 (70 IRHD) are the two FKM reference documents, while AMS 7276J is the US-side anchor for fuel and specific engine-oil service at 70 to 80 Type A hardness, and the three documents together let a buyer pick a low-compression-set 60 IRHD seal, a 70 IRHD seal, or a fluid-resistant 70 to 80 Shore A seal without re-engineering the elastomer family [S3][S5][S8].
Common-and-costly material errors on aerospace drawings include specifying EPDM or butyl where the fluid is actually an amine-containing hydraulic fluid, defaulting to FKM where the fluid is in fact a glycol brake fluid or a ketone, and assuming the "Viton" label on a print means a single, universal compound [S1][S6]. The right call when a fuel or hydraulic line is hot, aromatic, and cyclic is to verify the fluid first, then the spec (AMS 7276J or EN 2796/EN 2566), then the compound inside that spec, and only then the brand [S1][S3][S6].
The underlying component specifications are covered under fluororubber, pressure transmitter, and flow meter.
For related coverage, see Safety Relay Selection for Oil and Gas: SIL, ATEX, and Load-Monitoring Criteria.