Aerospace NBR specifiers should call out acrylonitrile (ACN) content, Shore A hardness, and an ASTM D2000 line designation on the drawing; a part note that just reads "NBR, 70 Shore A" leaves the grade, fuel resistance, and low-temperature limit undefined for the compounder [S2].
Commercial NBR sheet for general aerospace seal and gasket use typically runs 28–34% ACN, with 33% as the standard industrial balance between oil resistance and cold flexibility, and is available in 50, 60, and 70 Shore A grades in stock form [S1][S3]. The general-purpose service window sits at −30°C to +120°C; specialized NBR compounds extend the range to −40°C to +121°C (−40°F to +250°F) [S1][S5].
ACN Content: The Single Most-Overlooked Aerospace Spec
ACN content drives three independent variables on a fuel-handling drawing: volume swell in petroleum fluid, low-temperature flexibility, and glass-transition behaviour. Commercial sheet stock sits in a 28–34% ACN window; higher ACN improves resistance to kerosene, Jet A, diesel and mineral oil but reduces low-temperature flexibility, so the cold end of the service window moves up as the ACN moves up [S1][S2][S3].
For aviation fuel-handling systems specifically, NBR is the baseline polymer because of its polarity-driven resistance to non-polar hydrocarbon fuels, resisting swelling and softening in Jet A, Avgas, hydraulic fluid, and lubricating oil [S4][S5]. The trade-off is fixed: if the part has to seal at sub-zero ground conditions and survive hot fuel at altitude, the drawing must say "33% ACN nominal" and the compounder must not be allowed to drift to 28% to chase low-temperature cost [S2].
Hardness Bands: 50A, 60A, 70A and Where Each One Fits
Stock aerospace NBR sheet and molded seals are produced in three Shore A bands, and each maps to a different mechanical role on the airframe or engine [S1].
Higher hardness does not by itself improve fuel resistance, it improves mechanical stiffness. ACN content is the chemical knob; Shore A is the mechanical knob. Drawings that mix the two (specifying 70A for fuel swell control, for example) usually indicate the engineer did not separate the two failure modes [S1][S2].
ASTM D2000 Line Callouts: What Aerospace Drawings Should Actually Say
ASTM D2000 is the rubber-industry standard classification system used to describe NBR compounds on a drawing in a single line: material class, hardness, tensile strength, elongation, heat resistance, and any suffix requirements [S1]. An aerospace part drawing that omits the ASTM D2000 line forces the molder to guess at tensile, elongation, and heat-age targets, which is where non-conformance and AOG parts come from.
A typical fuel-handling NBR callout reads in the form "NBR, 70 Shore A, 33% ACN, ASTM D2000 line callout" with the line specifying tensile strength, ultimate elongation, and heat-age resistance targets matched to the actual service temperature and fluid. Peroxide-cured NBR grades (versus sulphur-cured) are commonly chosen where compression set and thermal ageing matter, and they appear as a separate stock line at compounders [S3].
Fuel, Hydraulic Fluid and Skydrol: Where Standard NBR Stops Working
Standard NBR resists petroleum-based fuels (Jet A, Avgas, JP-4) and petroleum hydraulic fluids, but is not the right polymer for phosphate-ester hydraulic fluids such as Skydrol LD-4 and Hyjet, which attack standard NBR compounds and cause rapid volume swell and embrittlement [S5][S6]. Where phosphate-ester hydraulic fluid is present (most commercial aircraft hydraulic systems), the spec must step up to fluorocarbon (FKM) or to EPDM depending on system architecture, and NBR is ruled out for those seals.
For a side-by-side read on NBR versus FKM versus EPDM in oil and gas sealing, see the NBR vs FKM vs EPDM comparison for oil and gas seals. For higher-temperature service, ozone exposure, and improved Skydrol resistance, hydrogenated NBR (HNBR) is the standard upgrade path, and it is widely used in military and aerospace-compliant O-ring compounds with full batch traceability and cure-date marking [S9].
Temperature, Ozone and Weathering: The Hard Limits
Standard NBR service window is −30°C to +120°C for general grades; specialized NBR formulations push the band to −40°C to +121°C (−40°F to +250°F) [S1][S5]. Above +120°C sustained, NBR begins to harden and lose elongation at break; below −30°C it stiffens and may lose seal compliance unless the ACN content has been pulled down.
Weathering and ozone resistance are weak points: NBR is "not recommended for use in environments where it can" be exposed to direct sunlight, ozone, or weathering without protection, and cracks will form on unprotected surfaces over time [S3]. Aerospace seals exposed to outside-air environments, engine-bay ozone, or UV should be specified as HNBR, FKM, or EPDM depending on the fluid side of the application [S3][S6].
Comparing NBR Against the Common Aerospace Elastomer Alternatives
Four elastomers cover the majority of aerospace seal, gasket, and hose applications, and the decision is a fluid-and-temperature matrix, not a single "best material" choice [S1][S6]:
Decision rule that holds in practice: if the fluid side is petroleum, start at NBR/HNBR; if it is phosphate-ester or steam, start at EPDM; if the upper temperature is above the NBR limit, step to HNBR or FKM [S1][S6]. Background on the general-purpose industrial rubber family and how NBR fits against EPDM rubber is covered in the encyclopedia entries.
Where NBR Sits on a Modern Aerospace Airframe
NBR remains the baseline elastomer for static seals, gaskets, and certain hose liners in aviation fuel-handling systems, where polarity-driven resistance to kerosene, Avgas, and lubricating oil is the deciding property [S4][S5]. O-rings specified to military and aerospace compliance with full batch traceability and cure-date marking are stocked as standard NBR items, and HNBR versions of the same parts are stocked for higher-temperature or ozone-exposed locations [S9].
For dynamic seals in the same fuel system, where friction, wear, and compression-set recovery all matter, 70 Shore A grades dominate, with 60A as the fallback for soft-flange interfaces. Static gaskets on fuel-tank access panels generally use 60A cloth-inserted NBR sheet for bolt-load retention and tear resistance; the same part in 50A is reserved for thin-section or low-bolt-load geometries [S1][S3].
Specification Checklist Before Releasing the Drawing
Five lines should appear on any aerospace NBR seal or gasket drawing before release: ACN content with a tolerance band (typically 28–34%, target 33%); Shore A hardness (50, 60, or 70A); ASTM D2000 line callout with tensile, elongation, and heat-age targets; cure system (sulphur or peroxide); and cure-date plus batch-traceability requirement, which is standard on military and aerospace-compliant parts [S2][S3][S9].
For guidance on how NBR compares to silicone rubber in extreme-temperature sub-assemblies and to general nitrile rubber selection across non-aerospace process industries, the encyclopedia entries provide the cross-industry context. Two trackable signals for the next spec cycle: rising HNBR and FKM substitution in commercial aviation fuel systems, and tighter ASTM D2000 line requirements on OEM prints as the aerospace supply chain tightens cure-date and full-batch-traceability expectations [S5][S9].