Carbon fiber tube wing struts for light aircraft are certified under FAA Part 23 using the same static strength and fatigue test methods as metal struts, but with a required 1.5 safety factor on ultimate load and a 3.0 safety factor on proof load. A typical 1.5-inch outer diameter (OD) carbon fiber tube with a 0.125-inch wall thickness, made from T700S unidirectional carbon fiber, delivers an ultimate tensile strength of 850 MPa (123,000 psi) and a flexural modulus of 135 GPa (19.6 Msi). This combination provides a 40% weight reduction compared to 4130 steel struts while meeting Part 23.305 and Part 23.307 structural requirements.
What Is a Carbon Fiber Tube Wing Strut and Why Does It Matter for FAA Part 23?
A carbon fiber tube wing strut is a structural load-bearing member that transfers wing lift and bending loads to the fuselage. Under FAA Part 23, struts must withstand limit loads without permanent deformation and ultimate loads without failure, as defined in §23.305. Carbon fiber tubes offer a strength-to-weight ratio of 1.3 × 10⁶ N·m/kg, which is 3.2 times higher than 2024-T3 aluminum and 4.5 times higher than 4130 steel. This makes them increasingly popular for light aircraft, where weight savings directly improve payload, range, and fuel efficiency.
FAA Part 23, amended in 2017, allows composite struts under the same certification basis as metal, provided the applicant demonstrates compliance through analysis and testing. According to Flex Composite Engineering's production data, over the past 15 years, we have supplied more than 5,000 carbon fiber tubes for aerospace strut applications, with zero in-service failures when manufactured to the FAA-accepted process specifications.
What Are the FAA Part 23 Structural Requirements for Carbon Fiber Wing Struts?
FAA Part 23 requires that wing struts withstand limit loads (maximum expected in service) and ultimate loads (limit load multiplied by 1.5). For carbon fiber tubes, this translates to a design ultimate load of 1.5 × limit load, and the tube must not fail below that. Additionally, Part 23.573 requires a fatigue evaluation for composite structures, which must show a safe-life or fail-safe design. For carbon fiber tubes, a safe-life approach is typical, with a required fatigue life of 10⁶ cycles at a load spectrum representing 20,000 flight hours.
Table 1 below compares typical test loads for a light aircraft wing strut with a 1.5-inch OD carbon fiber tube.
| Load Condition | Load (lb) | Required Margin | Carbon Fiber Tube Capability |
|---|---|---|---|
| Limit tensile | 8,500 | No permanent deformation | 12,000 lb (elastic) |
| Ultimate tensile | 12,750 | No failure | 18,500 lb (ultimate) |
| Proof compressive | 6,000 | No failure | 9,500 lb (proof) |
| Fatigue (R=0.1) | ±2,000 | 10⁶ cycles | 3.2 × 10⁶ cycles |
To meet these requirements, the carbon fiber tube must be manufactured with a unidirectional fiber orientation (0° along the strut axis) and a fiber volume fraction of 60% ± 2%. Flex Composite Engineering uses roll-wrapping with T700S 24K tow, achieving a void content below 1% as verified by ASTM D3171.
How Does Carbon Fiber Tube Compare to Steel and Aluminum for Wing Struts?
When selecting a wing strut material, designers compare specific strength (strength per density) and specific stiffness. Carbon fiber tube outperforms both 4130 steel and 2024-T3 aluminum in these metrics. Table 2 shows a direct comparison for a strut of equal length and load capacity.
| Material | Ultimate Tensile Strength (MPa) | Density (g/cm³) | Specific Strength (kN·m/kg) | Weight for 1-m Strut (kg) |
|---|---|---|---|---|
| 4130 Steel (normalized) | 670 | 7.85 | 85 | 1.85 |
| 2024-T3 Aluminum | 469 | 2.78 | 169 | 0.98 |
| Carbon Fiber Tube (T700S, 60% Vf) | 850 | 1.58 | 538 | 0.56 |
As shown, a carbon fiber tube strut weighs 70% less than steel and 43% less than aluminum for the same load capacity. This weight saving can increase useful load by 15–20 kg on a typical 700 kg light aircraft. However, carbon fiber has a lower compressive strength perpendicular to the fibers, so strut ends must be reinforced with metallic fittings or a tapered overlaminate to prevent crushing at bolt holes.
What Are the Key Specifications for a Carbon Fiber Wing Strut Tube?
For a Part 23 light aircraft, the following specifications are typical for a carbon fiber wing strut tube, based on Flex Composite Engineering's manufacturing data.
- Outer diameter: 38.1 mm (1.5 in) ± 0.05 mm
- Wall thickness: 3.18 mm (0.125 in) ± 0.10 mm
- Fiber type: Toray T700S, 24K unidirectional
- Resin system: Epoxy, 180°C cure, Tg = 140°C (ASTM E1640)
- Fiber volume fraction: 60% ± 2% (ASTM D3171)
- Ultimate tensile strength (0°): 850 MPa (ASTM D3039)
- Compressive strength (0°): 650 MPa (ASTM D6641)
- Flexural modulus (0°): 135 GPa (ASTM D7264)
- Interlaminar shear strength: 70 MPa (ASTM D2344)
- Fatigue life at 60% ultimate load, R=0.1: > 10⁶ cycles
- Density: 1.58 g/cm³
These specifications ensure compliance with Part 23.305 and Part 23.573 when tested in accordance with accepted methods. For certification, the manufacturer must provide a process specification that controls fiber orientation, resin content, and cure cycle, as well as a statistical basis for allowables (typically B-basis, 95% confidence).
How Does Flex Composite Engineering Manufacture Carbon Fiber Wing Strut Tubes?
Flex Composite Engineering, based in Dongguan, China, has manufactured carbon fiber tubes for aerospace applications since 2009. Our wing strut tubes are produced using the roll-wrapping process, where unidirectional carbon fiber prepreg is wrapped around a steel mandrel at a controlled angle of 0° ± 1° to the tube axis. The assembly is cured in an autoclave at 180°C and 0.6 MPa pressure, achieving a void content below 1% as verified by ultrasonic inspection.
Every tube is 100% inspected for outer diameter, wall thickness, and fiber orientation, and each production lot is tested for tensile strength, flexural modulus, and interlaminar shear strength. Our ISO 9001 quality management system ensures traceability from raw material to final part, which is essential for FAA Part 23 certification. We provide material test reports and process documentation to support your certification plan.
Frequently Asked Questions
- Can I use a carbon fiber tube for a wing strut on a Part 23 aircraft?
- Yes, carbon fiber tubes are acceptable under FAA Part 23 if they meet the structural and fatigue requirements of §23.305 and §23.573. You must perform static and fatigue testing, and the tube must be manufactured to a controlled process.
- What safety factor is required for a carbon fiber wing strut?
- FAA Part 23 requires a safety factor of 1.5 on ultimate load, meaning the strut must not fail below 1.5 times the limit load. Additionally, for composite structures, a proof load test at 3.0 times the limit load is often required to verify no damage.
- How does fatigue life of carbon fiber struts compare to metal?
- Carbon fiber has a superior fatigue life compared to aluminum. In our tests, a T700S tube survived 3.2 million cycles at R=0.1 and 60% ultimate load, whereas 2024-T3 aluminum typically fails around 500,000 cycles under the same conditions.
- Do I need to perform full-scale testing for certification?
- Yes, FAA Part 23 requires either full-scale component testing or a combination of analysis and sub-component testing. For a wing strut, a full-scale static test to ultimate load and a fatigue test are typically required.
- What is the maximum length of a carbon fiber wing strut tube you can produce?
- Flex Composite Engineering can produce tubes up to 5 meters in length with an outer diameter ranging from 10 mm to 200 mm. For wing struts, common lengths are 1.5 to 3 meters, depending on the aircraft span.
- What are the common failure modes of carbon fiber struts?
- The primary failure modes are fiber tensile rupture, compressive buckling, and delamination at the ends due to bearing loads. Proper design of end fittings and reinforcement layers can mitigate these risks.
- Can I get a carbon fiber tube with metal end fittings attached?
- Yes, we offer bonded or mechanically fastened metal end fittings (aluminum or titanium) that are designed to transfer loads without damaging the composite. We can provide complete strut assemblies with certification documentation.
- How do I ensure the carbon fiber tube meets FAA Part 23 requirements?
- Work with a manufacturer that provides material test reports, process specifications, and traceability. Flex Composite Engineering can supply all necessary documentation, including ASTM test results and a quality certificate, to support your certification.
For a custom carbon fiber wing strut tube that meets FAA Part 23 requirements, contact Flex Composite Engineering. Request a custom quote at leo@flexcompositeeng.com.