Seal Failures in Aerospace & Defense

Overview

In aerospace and defense systems, seal performance is tied to flight safety, system readiness, and long-term sustainment costs. A single O-ring or gasket that fails under pressure, temperature, or chemical exposure can halt an aircraft, delay a mission, or trigger costly requalification work.

In this short blog, we will walk through the environmental and performance demands seals face in aerospace applications, along with several documented failure modes that show how and why these components break down in service. You’ll also see the regulatory and traceability requirements that govern aerospace sealing—and why material datasheets alone are not enough to ensure reliability.

This sets the stage for a deeper look at real-world failure mechanisms, qualification standards, and considerations that engineering teams must account for when selecting or approving seals for flight hardware.

Introduction

In aerospace and defense systems, seals fall under the category of reliability-critical hardware. A failure in this context affects not just performance but flight safety, fleet availability, and long-term sustainment costs. A leaking actuator seal or degraded fuel system O-ring has direct consequences on airworthiness and mission readiness, which makes their reliability far more critical than in most industrial settings. Aerospace sealing systems must handle rapid thermal cycling, extreme pressure loads, chemically aggressive fluids, and in some cases, rapid decompression of gases. A leaking O-ring in a production press may mean a maintenance stop; the same failure in a flight control actuator can ground an aircraft or, in worst cases, compromise flight safety.

This discussion focuses on the specific environmental demands of aerospace sealing, the common failure mechanisms observed in service, and the industry standards that govern seal selection, testing, and traceability.

Environmental and Performance Demands

Temperature Range

Elastomers used in flight hardware are required to perform across wide extremes, from –65 °F (–54 °C) during high-altitude exposure to as high as +450 °F (+232 °C) in engine compartments. Although nitrile (NBR) and hydrogenated nitrile (HNBR) perform well in standard hydraulic equipment, they are not suited for aerospace service. At low temperatures they harden, and at high temperatures they lose elasticity. To maintain reliable sealing across the full aerospace envelope, compounds with broader thermal capability are required, including:

Fluorosilicone (FVMQ) – MIL-DTL-25988 qualified compounds
Perfluoroelastomers (FFKM) – classified under ASTM D1418
High-temperature fluorocarbons (FKM) – AMS-R-83485 approved grades

In aerospace service, material selection is driven less by incremental performance differences and more by whether the seal can retain sealing force and resist compression set after repeated exposure to severe conditions.

Fluid Compatibility

Aerospace seals also face some of the most aggressive fluid chemistries in engineering. These include:

Fuels: Jet A, Jet A-1, JP-8
Hydraulic fluids: phosphate ester formulations such as Skydrol® LD-4 and 500B-4
Lubricants: synthetic turbine oils based on esters

Elastomers not selected for compatibility can exhibit swelling, embrittlement, or surface degradation. ASTM D471 immersion tests illustrate this well: fluorocarbon (FKM) compounds can swell more than 15% in phosphate esters, while fluorosilicone remains dimensionally stable under the same exposure (Parker Hannifin, O-Ring Handbook ORD 5700, 2019). These effects are not cosmetic—they alter gland fill, extrusion resistance, and long-term sealing integrity.

Pressure Cycling and Rapid Gas Decompression

Hydraulic and pneumatic systems on aircraft typically operate at pressures up to 5,000 psi, with frequent cycling. In gas-sealing applications, elastomers must also resist Rapid Gas Decompression (RGD). When a system is vented, gas absorbed into the elastomer expands and can rupture the seal internally, leaving blisters and cracks. This mechanism is covered in NORSOK M-710 and ISO 23936-2, both of which provide qualification methods. NASA reported RGD-related seal failures in shuttle auxiliary power units, noting material blistering as a contributing factor to reliability issues (NASA/TM-2010-216100).

Traceability and Regulatory Requirements

In aerospace, a seal is not acceptable unless it is traceable and documented. Compliance frameworks include:

AS9100 – overall quality and traceability requirements
SAE ARP5316 – shelf-life management of elastomeric seals (15 years maximum, under controlled storage)
AS9146 – prevention of foreign object debris (FOD) in critical assemblies
AS5553 – counterfeit part avoidance and control

Documentation must include lot number, cure date, and full material traceability. In practice, this means a seal purchased from a commodity supplier without certified traceability cannot be installed in flight hardware, regardless of material composition.

Documented Failure Modes

Extrusion and Nibbling

When extrusion gaps are outside of aerospace tolerances, seals are forced into the clearance and mechanically sheared during cycling. To control extrusion, aerospace programs reference SAE AS4716 for O-ring gland design. If grooves are machined incorrectly or outside of these tolerances, the seal will be forced into the clearance and damaged during cycling, often showing up as nibbling or extrusion wear in hydraulic actuators. FAA Airworthiness Directive 2011-10-09 cited improperly specified backup rings in Bombardier CL-600 landing gear actuators as contributing to hydraulic leakage — illustrating the cost of dimensional non-compliance.

Thermal Cycling Cracking

Elastomers hardened by repeated cold soak followed by rapid heating exhibit surface cracking and loss of sealing force. This was documented in the SAE Technical Paper 2007-01-3847, which analyzed compression set changes in fluorosilicone under simulated flight thermal cycling.

Material Swelling

Fluorocarbon seals installed in Skydrol® systems have repeatedly shown >10% volume swell after 168 hours of immersion (ASTM D471, Parker Materials Test Report TR-45, 2018). This leads to loss of extrusion resistance and premature leakage.

Rapid Gas Decompression (RGD)

NORSOK M-710 qualification testing has shown that non-RGD-rated compounds develop internal cracks after as few as five decompression cycles from 5,000 psi nitrogen. Such failures are unacceptable in life-support systems and oxygen handling components.

Engineering Framework for Aerospace Seal Selection

Material Qualification
Confirm compound compliance to AMS, MIL, or ASTM specifications. Avoid generic “fluorosilicone” without reference to MIL-DTL-25988.
Compatibility Validation
Perform immersion and swell testing (ASTM D471) in the actual fluids encountered. Do not rely solely on datasheet claims.
Extrusion Resistance Analysis
Model groove dimensions to AS4716 and perform finite element analysis (FEA) for extrusion gaps. Select anti-extrusion back-up rings in 90 Shore A or harder materials where pressures exceed 3,000 psi.
RGD Qualification
For gas systems, require certification to NORSOK M-710 or ISO 23936-2.
Traceability & Shelf Life Control
Require Certificates of Conformance with batch, cure date, and shelf-life details per ARP5316.

AOP Technologies’ Approach

AOP Technologies operates under ISO 9001:2015 certification and enforces supplier quality through documented requirements for drawing interpretation (ANSI Y14.5), counterfeit part avoidance (AS5553), and FOD prevention (AS9146). By sourcing from aerospace-approved suppliers — including Freudenberg, Parker Hannifin, and Precision Associates — AOP can deliver seals that not only handle the chemical environment but also come with the certification and traceability paperwork needed for flight use.

References

Parker Hannifin. O-Ring Handbook ORD 5700. 2019.
NASA/TM-2010-216100. Seal Material Performance and RGD in Shuttle Applications. 2010.
SAE Technical Paper 2007-01-3847. Elastomer Compression Set in Thermal Cycling. 2007.
NORSOK M-710, Rev. 3. Qualification of Non-Metallic Materials. 2014.
FAA Airworthiness Directive 2011-10-09. Bombardier CL-600 Hydraulic Leakage. 2011.
ASTM D471. Standard Test Method for Rubber Property—Effect of Liquids. 2020.
SAE AS4716. Aerospace Standard for O-Ring Groove Design. 2017.
SAE ARP5316. Elastomeric Seals – Shelf Life. 2016.

Next Step

If you are managing aerospace or defense equipment reliability, schedule an industry-specific consultation with AOP. Our engineers will review your sealing applications against applicable aerospace standards and help identify qualified, compliant solutions that extend service life and mitigate risk.