Introduction and summary of changes in ARP4761A

According to an ICAO report, the number of aviation accidents has steadily decreased while there has been a rise in both global aircraft operations and passenger traffic. This achievement is largely due to consistent efforts to reduce safety risks and make aircraft designs more robust.

Together with ARP4754A, ARP4761 has been the backbone of this safety evolution. OEMs have consistently leveraged the two standards for both design improvements and new product development. ARP4761 continues to be the most widely applied safety standard for product design across conventional and modern aircraft systems, whether it's a Cessna, an Airbus A350, or Advanced Air Mobility (AAM) vehicles.

After nearly three decades, ARP4761 was recently updated in December 2023 to ARP4761A. To ensure consistency while applying both the standards, ARP4754A was also revised to ARP4754B. Comprehensive articles detailing the updates in ARP4761A are limited to date. That is where we seek to systematically and logically explain the major updates in ARP4761A, ensuring that readers gain a clear understanding of the key changes from ARP4761 to ARP4761A.

To enhance the readability and to make it easier to follow, we’ll present this information in a series of focused posts:

• Part 1: Introduction and summary of changes in ARP4761A (this post)

• Part 2: Structural changes

• Part 3: Updates to technical definitions and obsolete terminology

• Part 4: Special topics introduced in ARP4761A

  
  

To compile this series, insights have been gathered through direct interactions with the S-18/WG63 committee members responsible for the revision and after thoroughly going through the released version of ARP4761A. EUROCAE also published the counterpart, ED-135, for which work was carried out jointly with SAE S-18 committee members.

This series of articles is intended for anyone involved in the aircraft development using ARP4761 and now seeks an overview of the updates in the new version. It will also be valuable for aerospace safety managers looking to plan and implement ARP4761A in their certification plans.

Evolution of ARP4761

ARP4761, introduced in 1996, has evolved alongside the safety practices, concepts, and design methodologies in the aerospace industry. OEMs have widely accepted this standard, and its influence continues to grow. Regulatory bodies have also recognized ARP4761 practices across various Advisory Circulars (ACs) and Means of Compliance (MOCs) publications.

Safety Lifecycle V-Model as per ARP4761A

The latest revision, ARP4761A, reflects the need to update safety practices as technology continues to advance rapidly. One of the main reasons for the delay in the revision process was the challenge of getting all stakeholders to agree on a single safety standard, which highlights the widespread acceptance and importance of ARP4761 in the industry. The S-18 committee, tasked with updating the standard, began its work in 2004, and consensus on a comprehensive revision was finally reached before release in December 2023.

Summary of Changes in ARP4761A

In brief, ARP4761A has been updated to better serve the evolving global aerospace community by incorporating key concepts that were either absent or ambiguous in the original ARP4761. The revised standard now provides more consistent approaches with additional examples resulting in a significant increase in content.

The scope of ARP4761A, with the new version, has expanded beyond 14 CFR/CS Part 25 aircraft to include 14 CFR/CS Parts 23, 27, and 29, as well as 14 CFR Parts 33 and 35, CS-E, and CS-P. Additionally, the processes described are now applicable to a broader range of aircraft and systems.

Snapshot of updated terminologies in ARP4761A

Process Updates and New Additions

The updated standard places an increased emphasis on aircraft-level analysis, with sections dedicated to Aircraft Functional Hazard Analysis (AFHA), Preliminary Aircraft Safety Assessment (PASA), and Aircraft Safety Assessment (ASA). While these analyses were mentioned in the previous version, their relationships and the necessity of aircraft-level analysis are now explicitly detailed.

Master Minimum Equipment List (MMEL) considerations have been formally introduced to ensure functional availability during operation and integrating the MMEL safety requirements into the PASA or PSSA. While justifying MMEL through FTAs quantitatively, the probability calculations should consider risk of dispatch (at-risk time) not the average risk assessed for compliance with regulations.

In-service safety assessments are now more prominently highlighted, with references to their relevant ARPs, ensuring formal communications between design and service engineering all the while promoting the use of lessons learned.

New Techniques and Tools

Model-Based Safety Analysis (MBSA) has been introduced as either an additional tool or an alternative to traditional aircraft-level analysis methods like FTA, Markov Analysis, or Dependence Diagram. Cascading Effects Analysis (CEA) has also been added to support qualitative assessments of aircraft level and multi-system effects.

The assignment of FDAL/IDAL, previously part of ARP4754, has been moved to the ARP4761A as it is a part of the PSSA document. Additionally, the section discussing the safety process using a fictitious aircraft design case study has been thoroughly revised and expanded.

Conclusion

The latest version of the ARP4761 aims to incorporate modern aerospace industry practices and lessons learned over the years, making the safety assessment process more comprehensive. While the core structure and approach of ARP4761 remain intact, the processes that were previously vague have been clarified, and state-of-the-art methodologies, such as MBSA, have been added.

The new and revamped appendices ensure uniformity in processes like Common Modes Analysis (CMA) and Particular Risk Analysis (PRA) across industries, providing OEMs with clearer, more flexible choices for safety assessments. Major OEMs are encouraged to update their internal processes to take maximum advantage of these improvements, simultaneously, giving them the flexibility to generate their own processes. No timeline for the next revision of ARP4761A has been announced.

Stay tuned for the upcoming parts of this series.