Tooling Best Practices and Regulatory Compliance in Aviation Maintenance

Introduction

Ensuring proper tooling practices is essential for maintaining compliance, operational efficiency, and, most importantly, safety in aviation maintenance. The FAA has a structured hierarchy of regulations at the top of this hierarchy are Federal Aviation Regulations (FARs) such as 14 CFR § 43.13 and § 145.109, which are legally binding and enforceable under federal law. Below these are FAA Orders, like 8620.2B and FAA Order 8900.1, which guide FAA inspectors and provide policy interpretations but do not, by themselves, create new legal requirements. Finally, Advisory Circulars (ACs), such as AC 43.13-1, AC 43.13-2, and AC 65-15A, are non-binding guidance documents that offer FAA-accepted best practices and methods for compliance with the FARs.

Each of these documents plays a role in defining proper tool usage, calibration, and maintenance methods in aviation, but their authority varies significantly. that impact how maintenance tooling should be selected, used, and maintained. Understanding these documents and how they interact is critical for mechanics, repair stations, and operators who must balance regulatory compliance with practical, cost-effective maintenance solutions. This blog explores the authority of key FAA documents, how they reference or conflict with each other, and best practices for tooling in aviation maintenance.

FAA Regulatory Framework for Tooling

Legal Requirements for Tooling and Calibration

The primary legal authority governing tooling in aviation maintenance comes from 14 CFR Part 43 and Part 145. 14 CFR § 43.13(a) requires that all maintenance be performed using methods, techniques, and tools acceptable to the FAA, ensuring that the finished work restores the component or aircraft to an airworthy condition. For any maintenance provider, this means any tool used to verify torque, pressure, alignment, or other critical measurements must be accurate and reliable. Meanwhile, 14 CFR § 145.109 applies specifically to Part 145 repair stations, requiring them to use tools and test equipment that meet manufacturer specifications or FAA-accepted alternatives—placing a stricter burden on facilities that conduct maintenance on certificated aircraft.

Additionally, 14 CFR § 91.411 (altimeter system tests) and 14 CFR § 91.413 (transponder inspections) require test equipment that meets strict accuracy standards. Although no specific regulation mandates calibration intervals, operators and repair stations are still responsible for ensuring that the tools used for testing and inspection are accurate and properly maintained to meet airworthiness requirements.

FAA Guidance on Tool Calibration and Equivalency

While FAA regulations set legal requirements, additional FAA Orders and Advisory Circulars provide further interpretation and guidance for inspectors and maintenance personnel.

• FAA Order 8900.1:

  • This document is the primary reference for FAA inspectors and details how they evaluate maintenance programs and repair station operations.

  • Vol. 6, Ch. 11, Sec. 20 directs inspectors to assess non-OEM special equipment or test apparatus to ensure mechanics or operators can demonstrate functional equivalency.

  • Vol. 3, Ch. 49 outlines the expectations for maintenance programs, including tool calibration programs.

• FAA Order 8620.2B:

  • Clarifies that manufacturer-recommended tooling is not automatically required unless specifically incorporated into an FAA regulation, AD, or an operator’s FAA-approved maintenance program.

  • Allows mechanics and repair stations to use alternative tools but requires proof of functional equivalency.

• AC 43.13-1 and AC 43.13-2:

  • Provide FAA-accepted methods, techniques, and practices for aircraft maintenance.

  • AC 43.13-1 emphasizes that tools must be appropriate, accurate, and calibrated to ensure compliance with maintenance standards.

• AC 145-9A:

  • Offers guidance for developing a repair station training program, emphasizing tool calibration and maintenance.

• FAA Legal Interpretations:

  • Multiple Chief Counsel legal interpretations confirm that alternative tools may be used as long as they meet the performance requirements of OEM-recommended equipment.

  • The Meyer 2015 and Siilats 2015 interpretations reinforce that OEM service bulletins and tool recommendations are not legally binding unless incorporated by the FAA.

Conflicts and Interpretation Challenges

While the FAA’s framework provides consistency, conflicts sometimes arise between regulations, inspector guidance, and advisory circulars—especially regarding tooling equivalency and calibration requirements.

A major area of tension is the use of alternative tooling or test equipment in place of OEM-recommended tools. AC 43.13-1 states that tools must be appropriate for the task, yet FAA Order 8900.1 requires proof of functional equivalency when using non-OEM tools, placing a greater burden on operators to document performance. Yet, FAA Order 8620.2B clarifies that manufacturers' service bulletins, including tooling requirements, are not enforceable unless incorporated into an FAA rule, Airworthiness Directive (AD), or approved operator manual..

§ 145.109 requires Part 145 repair stations to use equipment that meets OEM specifications or another FAA-accepted method which may limit flexibility for repair stations that perform maintenance on certificated aircraft.. This means that while Part 135 or independent A&P mechanics may argue tool equivalency based on ACs, a Part 145 repair station might still be held to stricter standards depending on its FAA-approved operations manual. FAA Order 8620.2B allows flexibility in tooling selection by confirming that OEM-recommended tools are not automatically required .

Another area of inconsistency involves calibration requirements. While § 43.13(a) requires accurate and reliable tools, no specific regulation mandates calibration intervals for most maintenance providers. However, FAA inspectors often apply best practices from AC 43.13-1 and AC 43-15, which stress proper calibration and test equipment accuracy. As a result, even if an OEM labels calibration as “recommended” rather than “required,” repair stations and maintenance providers should maintain calibration records for any tools used to verify airworthiness, such as torque wrenches, pressure gauges, and track and balance equipment.

To mitigate these conflicts, maintenance providers should reference FAA-approved industry standards, maintain thorough documentation, and proactively discuss tool equivalency with their Principal Maintenance Inspector (PMI) to avoid compliance issues.

Best Practices for Tooling in Aviation Maintenance

Tooling best practices in aviation maintenance focus on ensuring accuracy, reliability, and compliance with FAA regulations to maintain airworthiness and safety. First and foremost, all measuring and test equipment should be calibrated at regular intervals, following manufacturer recommendations or an FAA-accepted standard to prevent drift and inaccuracies. Proper storage and handling are equally important—tools should be kept in controlled environments to avoid damage, contamination, or loss of precision. Mechanics should always use the right tool for the job, ensuring that it meets the specifications required by the OEM, and Advisory Circular (such as AC 43.13-1), or an FAA-accepted alternative method. In cases where non-OEM tools are used, proper documentation of functional equivalency should be maintained, demonstrating that the tool achieves the same or better results as the manufacturer’s recommended equipment. Additionally, a robust tool control program—especially in Part 145 repair stations and larger operations—helps prevent tool misplacement, foreign object debris (FOD) risks, and unauthorized tool substitutions. By implementing these best practices, aviation maintenance personnel can ensure their tooling methods are safe, compliant, and aligned with FAA regulations, ultimately contributing to safer and more efficient aircraft operations

To ensure compliance while maintaining operational efficiency, maintenance providers should implement the following best practices for tooling:

1. Establish a Calibration Program

   • Develop a documented calibration schedule for all tools that impact airworthiness.

   • Follow OEM recommendations or justify alternative intervals using industry standards.

2. Maintain Calibration Records

   • Keep detailed logs of calibration dates, results, and technician sign-offs for FAA audits.

   • Reference FAA Order 8900.1’s guidelines on tool verification.

3. Use FAA-Accepted Alternatives for Non-OEM Tools

   • If using homemade or alternative tooling, ensure functional equivalency is documented through engineering data or test comparisons.

   • Reference FAA legal interpretations to support alternative tool usage.

4. Implement a Tool Control System

   • Prevent Foreign Object Debris (FOD) risks by implementing a tool tracking system (especially for large operations).

   • Require mechanics to inspect and log tools before and after each use to avoid missing or damaged equipment affecting maintenance quality.

5. Train Maintenance Personnel on FAA-Approved Tooling Practices

   • Ensure mechanics understand FAA Orders and ACs governing tooling accuracy.

   • Require ongoing training as outlined in AC 145-9A.

Conclusion

For all aviation maintenance providers, tooling compliance is not just about following manufacturer recommendations—it’s about demonstrating FAA-acceptable methods while maintaining operational flexibility. Regulations like § 43.13 and § 145.109 establish legal requirements, while FAA Orders and ACs offer guidance on inspector expectations and best practices. Conflicts sometimes arise regarding tool calibration, equivalency, and mandatory tooling use, but maintenance providers can navigate these issues by maintaining detailed calibration records, documenting tool performance, and ensuring proper maintenance procedures. By following **FAA-approved alternatives, maintaining compliance records, and ensuring tool accuracy, maintenance providers can stay compliant while optimizing their operations.

For further information, review:

• FAA Regulations (14 CFR - Federal Aviation Regulations)

  • 14 CFR § 91.407 – Outlines requirements for aircraft operation following maintenance, preventive maintenance, rebuilding, or alteration.

  • 14 CFR § 91.411 – Specifies altimeter system and altitude reporting equipment tests and inspections.

  • 14 CFR § 91.413 – Requires ATC transponder tests and inspections at specified intervals.

  • 14 CFR § 135.411 – Establishes maintenance requirements for aircraft operating under Part 135.

  FAA Orders (Policy & Inspector Guidance)

  • FAA Order 8900.1, Vol.6, Ch11, Sec20 – Provides FAA inspector guidance on evaluating special equipment and test apparatus.

  • FAA Order 8620.2B – Clarifies the applicability of manufacturer maintenance data, including tooling requirements.

  • FAA Order 8110.37F – Establishes procedures for FAA Designated Engineering Representatives (DERs) and how tooling approvals factor into certification processes.

  • FAA Order 8300.16A CHG 1 – Covers major repair and alteration data approval, including tooling considerations.

  Advisory Circulars (ACs - FAA-Approved Guidance & Best Practices)

  • AC 43.13-1B CHG 1 Ed Update – Provides acceptable methods, techniques, and practices for aircraft inspection and repair.

  • AC 43.13-2B – Offers best practices for aircraft alterations, including tooling considerations.

  • AC 43-15 – Guides instrument repair facilities, with an emphasis on calibration and test equipment standards.

  • AC 43-12A CHG 1 – Addresses preventive maintenance tasks and appropriate tool use.

  • AC 43-6D Ed Update – Outlines best practices for performing static and altimeter system tests.

  • AC 120-77A – Covers maintenance and alteration data, including tooling verification.

  • AC 145-9A – Provides guidance for repair station training programs, including tool calibration and handling.

  • AC 00-56B CHG 1 – Establishes voluntary industry standards for quality assurance in parts and tooling distribution.

  • AC 65-15A – Provides training guidance for mechanics, covering tool selection and calibration principles.

  Legal Interpretations (FAA Chief Counsel Guidance)

  • Meyer 2015 Legal Interpretation – Addresses the use of non-OEM tools and FAA compliance considerations.

  • Siilats 2015 Legal Interpretation – Clarifies whether manufacturer service bulletins and recommended tools are legally enforceable.

  • Clark 1996 Legal Interpretation – Explains regulatory interpretations regarding alternative maintenance methods and tooling equivalency.

  • Fields - 2024 Legal Interpretation (14 CFR §§ 145.59 and 145.61) – Discusses repair station compliance with FAA tooling requirements.