Fabrication of Parts by Maintenance Personnel

Summary of Advisory Circular 43-18: Fabrication of Aircraft Parts by Maintenance Personnel

Introduction

Advisory Circular (AC) 43-18, published by the Federal Aviation Administration (FAA), provides guidance for maintenance personnel involved in the fabrication of parts for aircraft. Its primary objective is to ensure that parts fabricated during maintenance or alterations achieve an equivalent level of safety compared to those produced by the original design holder.

The AC outlines acceptable methods, techniques, and practices for part design and fabrication, ensuring compliance with Title 14 of the Code of Federal Regulations (14 CFR) Parts 21 and 43. It's important to remember that an AC is not mandatory and doesn't hold the same weight as a regulation. However, it serves as a valuable resource for maintenance personnel seeking to safely and compliantly fabricate aircraft parts.

Key Principles of AC 43-18

• Safety Equivalence: The core principle of AC 43-18 is ensuring that fabricated parts possess the same level of safety as original parts. This is achieved through adherence to established design and manufacturing practices.

• Alternative Methods:  The AC acknowledges that there may be alternative methods for achieving safety equivalence besides those outlined in the document. However, the FAA must approve any chosen alternative to ensure it meets the required standards.

• Regulatory References:  The AC references relevant regulations from 14 CFR, including Part 21 on Certification Procedures for Products and Parts, and Part 43 on Maintenance, Preventive Maintenance, Rebuilding, and Alteration.
  

Steps in Fabrication Process

While AC 43-18 doesn't dictate a specific step-by-step process, it highlights crucial elements involved in successful part fabrication:

• Data Acquisition:  Maintenance personnel must acquire necessary technical data for the part they intend to fabricate. This data typically includes drawings, specifications, and material certifications.

• Design Approval:  The design of the fabricated part needs approval based on its criticality category. The AC references the Category Parts List (CPL), an FAA-maintained resource that categorizes aircraft parts based on their criticality to safety. Less critical parts may require less stringent design approval processes compared to those designated as critical.

• Fabrication Quality Control System (FQCS):  An FQCS needs to be established to ensure the quality and consistency of the fabrication process. This system outlines procedures for material selection, inspection, and documentation.

• Personnel Qualification:  Personnel involved in fabrication must possess the necessary skills and qualifications for the specific tasks they perform. This may involve training in areas like welding, machining, and non-destructive testing.

• Documentation:  Proper documentation of the entire fabrication process is crucial. This includes records of materials used, inspection results, and final part certification.
  

Additional Considerations

AC 43-18 also addresses other significant aspects related to part fabrication:

• Conformity to Original Design:  Whenever possible, the fabricated part should conform to the original design specifications. Deviations from the original design may require additional approvals and justifications.

• Special Manufacturing Processes:  Certain parts may require specific manufacturing processes not typically encountered in routine maintenance. In such cases, the AC recommends consulting with the original equipment manufacturer (OEM) or a qualified repair station for guidance.

• Post-Installation Inspections:  Following installation of the fabricated part, the AC recommends performing appropriate inspections to verify functionality and ensure no adverse effects on the aircraft's systems.
  

Conclusion

By following the guidance provided in AC 43-18, maintenance personnel can ensure the safe and compliant fabrication of aircraft parts. This reduces downtime, potentially lowers costs, and promotes efficient aircraft maintenance practices. It's important to note that AC 43-18 serves as a general guideline, and maintenance personnel should always consult with relevant regulatory bodies and qualified personnel when undertaking part fabrication tasks.

When Fabrication Under Part 43 Isn't Enough

It's worth being clear about the limits of this authority. AC 43-18 covers parts fabricated during maintenance or alteration — for that specific job, on that specific aircraft. If a certificate holder wants to produce and sell parts separately — to other operators, other facilities, or into the aftermarket — that crosses into manufacturing, and manufacturing requires a Parts Manufacturer Approval (PMA).

This is a line that matters, because many maintenance facilities and repair stations reach it naturally. You develop a repair. You fabricate the parts to execute it. You do it again and again, and at some point the question becomes: should this capability be formalized as a PMA so the parts can be produced at scale and sold?

If that question sounds familiar, it's because the path from Part 43 fabrication to PMA is one of the more common — and more accessible — certification conversions in the industry. The engineering knowledge and process data often already exist. What's needed is formalizing it into an FAA-approved design and production package. We cover that path in detail in our post on MRO repair-to-PMA conversion, and the full certification process in the Ultimate Guide to FAA PMA Certification.

Where Prime Propulsion Fits

Whether you're working within Part 43 fabrication authority or evaluating whether a part warrants a full PMA, our in-house DER can help you understand where the line is and what the path forward looks like. If you're a repair station or MRO that's been fabricating parts and wondering whether there's a larger opportunity, contact us — there's no cost to talk it through.

Prime Propulsion delivers expert FAA certification solutions for small to medium-sized aerospace firms. Our DER-led team specializes in PMA, STC, Test Cell Correlations, and Repair Specification support.