In order to understand the very latest in DO-254 guidance, it’s important to have just a bit of history. RTCA/DO-254 (also known as ED-80 in Europe), Design Assurance Guidance for Airborne Electronic Hardware, a document that took a committee of roughly 90 US participants over six years to complete and coordinate with their EU counterparts, was published April 19, 2000. Even for the mathematically challenged among us, it’s clear that this document is almost a quarter of a century old. That’s pretty old. But in technology terms, it’s ancient.

Just to put it into perspective, when the SC-180 committee (the US working group that developed DO-254) first met to discuss policy for the development of airborne hardware, the intel Pentium microprocessor had just rolled out—sporting a whopping 3 million transistors at 800nm process geometries. That sounds impressive until you realize that today’s microprocessors have hundreds of billions of transistors at 5nm processes. Indeed, a lot has changed over these years.  To put it simply, complexity has exploded.

Because systems have become so complex, most companies can’t be expected to start each one of these complicated systems from scratch. While the idea of commercial-off-the-shelf (COTS) products, including intellectual property cores (i.e., COTS IP), emerged in the 1990’s, the prevalence and complexity of these devices has ballooned in the past three decades. Today, even in aerospace, a sector known to be at the trailing edge of the technology adoption curve due to its conservatism and safety concerns, companies are eagerly consuming these ready-made technology products. This was just one of several areas not adequately addressed by DO-254, simply due to its age.

When DO-254 was first released in 2000, it wasn’t automatically considered policy. This didn’t happen until the issuance of AC 20-152 on 6/30/05. An AC, or Advisory Circular, is a document issued by the FAA that provides guidance to the aviation community. AC 20-152 advised the community that RTCA/DO-254 was an acceptable means of compliance for the development of airborne electronic hardware (AEH) to Title 14 of the Code of Federal Regulations (CFR) for parts 21, 23, 25, 27, 29 and 33. And with that, following DO-254 became the most commonly used method for compliance of AEH.

Another aspect of DO-254 compliance that eventually needed to be addressed was the variation of oversight in the review/auditing processes. To guide the FAA personnel in how to more consistently review DO-254 programs, the FAA issued Order 8110.105 in 2008. Remember that while Orders are instructions to FAA personnel (and their designees, such as DERs) for how to interpret or apply policy, they are also generally very useful for the applicants of those policies, since it’s their programs that will be audited by these personnel.

So the RTCA released DO-254 in 2000, it was invoked as a means of compliance by AC 20-152 in 2005, and expectations for FAA personnel in terms of use and application was conveyed in Order 8110.105 in 2008 (but was modified and updated to 8110.105A in 2017). These documents were not perfect. They had a lot of gaps, limitations, and grey areas. But they built up what became the law of the land for AEH development, so to speak, for nearly 20 years.

Then on 10/7/2022, the FAA set forth two new and very important documents:  AC 20-152A and AC 00-72. These documents helped to close some of the most important gaps and make a number of much-needed clarifications.

AC 20-152A did such things as clarify the classification of Simple vs. Complex, the difference between the validation and verification processes, what was expected with respect to robustness, HDL code coverage, tool assessment, and previously developed hardware. In this context, it created 12 new objectives that applicants needed to address. It also covered the larger topics of the use of COTS IP and COTS devices, creating 7 and 8 new objectives, for each respectively. It also tackled the issue of applying DO-254 at the board level, since previously DO-254 was almost exclusively applied at the chip level.

AC 00-72, entitled “Best Practices for Airborne Electronic Hardware Design Assurance Using EUROCAE ED-80( ) and RTCA DO-254( ),”  aimed to help fill a void of practical advice or “best practices” in the application of DO-254. DO-254 was written to be intentionally non-prescriptive.  This new AC is a great document for those folks who feel that DO-254 doesn’t really help you understand what to do.

Fast forward to this year, 2024, in late February, the FAA issued Order 8110.105B. Order 8110.105B adds further clarifications to not only DO-254 but also AC 20-152A.  If you recall, the original Order 8110.105 primarily established consistent expectations for the certification liaison process (i.e., review/audit) for DO-254 programs. Order 8110.105A added a number of topics that needed some urgent clarifications. Order 8110.105B replaces Order 8110.105A, which had some conflicts with the newer documents AC 20-152A and AC 00-72, that covered and expanded upon much of the content of 8110.105A with more informed and thorough approaches.

Order 8110.105B now primarily focuses on how many reviews and how much data the auditors expect to see, which is determined by something called the level of involvement (LOI). The LOI calculation is influenced by both the Design Assurance Level (DAL) and other project criteria. Appendix C of Order 8110.105A included worksheets that helped calculate the LOI.  Appendix B of Order 8110.105B covers the same content but adds the use of COTS and COTS IP as factors in these calculations.

For teams who want to understand what external reviews you’ll be subject to and why, Order 8110.105B is a great document to review. For teams who want to be well prepared for internal and external reviews all throughout your DO-254 programs, consider using our DO-254 Review Checklists.

So is that the end? Do we finally have the perfect set of documents to provide the guidance for developing airborne hardware? Not exactly. All of these changes and layers of documents, ACs, and Orders beg yet another question: Will there ever be a revision to DO-254? A DO-254A perhaps?

The industry has batted about the question for decades. The answer has always been a resounding “NO” – that is, until recently. Stay tuned for more information on this topic in a future blog (but spoiler alert, the joint US/EU committee SC-243/WG-128 for developing DO-254A had their kickoff meeting on April 22, 2024, and I’m attended as an invited committee participant!).

Until then, please ensure you bookmark the page on our website that provides all the latest policy documents in one convenient place…right here.