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What Are the Tensile Strength Requirements for Aviation Cable?

Aviation cable isn’t ordinary wire rope. Its tensile strength – the maximum pulling force it can withstand before breaking – is a critical safety parameter directly impacting aircraft control and structural integrity. Unlike generic cables, aviation cable requirements are strict, standardized, and non-negotiable.

Why Tensile Strength is Paramount:

• ​Safety Margin: Cables control vital functions like ailerons, elevators, rudders, and landing gear. They must endure operational loads plus significant safety factors far beyond expected stresses.
• ​Flight Critical Systems: Failure of these cables can lead to catastrophic loss of control. High tensile strength is fundamental to reliability.
• ​Vibration & Fatigue: Aircraft experience constant vibration and cyclic loading. Cable construction and high strength help resist fatigue failure over time.

Key Factors Determining Requirements:

Tensile strength isn’t a single universal number. Requirements depend on:

1. ​Cable Construction: Primarily defined by the number of strands and wires per strand. Common types include:
   • ​7×7: 7 strands, each made of 7 wires. Offers good flexibility.
   • ​7×19: 7 strands, each made of 19 wires. Higher flexibility than 7×7.
   • ​1×19: Solid strand of 19 wires. Less flexible, used in applications like tensioning cables in fixed structures.
2. ​Cable Diameter: Thicker cables inherently have higher breaking strength.
3. ​Material: Almost exclusively ​Stainless Steel (Type 302/304) or ​Galvanized Carbon Steel (when specifically approved). Stainless offers corrosion resistance; galvanized is strong but requires diligent corrosion inspection.
4. ​Governing Standards: Requirements stem from stringent specifications:
   • ​MIL Specs: Military standards like MIL-C-87139 (stainless) or MIL-C-18375 (galvanized) define minimum strengths.
   • ​SAE Aerospace Standards: Standards like AS22759 cover specific cable constructions.
   • ​FAA Regulations & Guidance: FAA Advisory Circulars (e.g., AC 43.13-1B) incorporate and reference these specs for maintenance and modification.
   • ​Aircraft Manufacturer Specifications: Always follow the specific manual for the aircraft you’re working on.

Typical Minimum Breaking Strengths (MBS):

The table below provides ​general guidance on common aircraft cable types. ​ALWAYS verify the specific spec/MS number and aircraft manual for exact requirements.

Failure Point Note: Aviation cables are designed to break at significantly higher loads than their proof-tested values. The breaking strength is the ultimate failure point. Rigorous manufacturing and testing (including proof testing – pulling the cable to a specified percentage of its MBS without deformation) ensure compliance.

Practical Takeaways for Aircraft Professionals:

1. ​NEVER GUESS: ​Always use certified aviation cable (cable conforming to MIL-Spec, MS, or AS standards). Look for markings like “MIL-C-87139” or “MS” numbers. Unmarked “hardware store” cable is unsafe.
2. ​Match Specs: Select cable based on ​both the required construction/diameter and the applicable specification called out in the aircraft maintenance manual or component repair instructions.
3. ​Know the Strength: Refer to the specific specification document (MIL-C-87139, AS22759, etc.) or the detailed tables within FAA AC 43.13-1B Chapter 7 for the ​exact breaking strength requirements for your chosen cable size and type.
4. ​Factor in Design: Remember, the required working load capacity of the system is determined by the aircraft designer using these cable strengths combined with a large safety factor (often 2:1 or higher).
5. ​Corrosion Matters: Regularly inspect galvanized cable for corrosion; damage significantly reduces actual tensile strength. Stainless offers better corrosion resistance but still requires inspection.

By understanding and adhering to the stringent tensile strength requirements defined in aviation specifications, you ensure the critical control cables performing behind the scenes are capable of handling the immense forces encountered in flight, keeping aircraft reliably safe in the skies.