What FAA 21.17(b) Means for Special Category Aircraft Certification

When you’re developing a revolutionary aircraft like an eVTOL or a large cargo drone, one of the first and most critical questions is: “Under what FAA rule will this be certified?” The answer isn’t always straightforward. These vehicles don’t neatly align with the regulations for normal airplanes (Part 23), rotorcraft (Part 27), or transport category aircraft (Part 25/29). This is where 14 CFR § 21.17(b) — the rule for “special class” aircraft — becomes your essential framework.

What is 14 CFR § 21.17(b) and Why Does It Exist?

FAA 21.17(b) exists for one reason: to accommodate innovation. It recognizes that some aircraft are so novel that they fall outside the scope of existing certification rules.

In plain terms, if your design is “nonconventional” — like an airship, powered-lift, or advanced drone — it can be type certificated as a special class aircraft.

For the drone and eVTOL industry, this is often the default path. An eVTOL, with its hybrid lift-and-cruise design, doesn’t fit neatly into airplane or helicopter rules. Rather than forcing a square peg into a round hole, 21.17(b) lets you build a custom certification basis that matches the unique risks and characteristics of your aircraft.

How It Differs from Standard Certifications (Part 23/27/29)

The key difference is flexibility. Parts 23, 27, and 29 are largely prescriptive. They contain specific, detailed requirements for everything from load factors to system redundancies. While these parts have been modernized to be more performance-based, they still assume a conventional aircraft architecture.

21.17(b) takes the opposite approach. It doesn't provide a list of rules itself. Instead, it mandates that the FAA will determine the appropriate airworthiness requirements for your specific aircraft. This means you are not bound by the prescriptive limits of a single existing part. Instead, you work with the FAA to create a custom-tailored set of rules.

The Process: "Picking and Choosing" Your Certification Basis

This is the core of the 21.17(b) process: building your certification basis. This involves a collaborative effort with the FAA Aircraft Certification Office (ACO) to select applicable sections from various existing regulations and, where necessary, create new, special conditions.

●        The "Picking": You and the FAA will "pick and choose" relevant sections from Parts 23, 27, 25, 29, 33 (engines), 35 (propellers), and 36 (noise). For example, you might apply the crashworthiness requirements from Part 27, the system safety assessment processes from Part 25, and the electrical system requirements from Part 23.

●        The "Gap Analysis": Where no existing rule adequately addresses a novel feature of your aircraft (e.g., a distributed electric propulsion system, a complex flight control system for stability, or a Detect-and-Avoid system), the FAA will issue "Special Conditions." These are new, tailored rules written specifically to ensure your aircraft maintains the required level of safety.

The outcome is a unique certification basis document that defines exactly what your aircraft must prove compliance with.

The Critical Role of DERs and Consultants

This process is where the value of an experienced certification partner like MTech Aerospace becomes undeniable. Navigating a 21.17(b) certification is highly complex and strategic. It requires:

●        Deep Regulatory Insight: Knowing which existing rules are most appropriate and acceptable to the FAA.

●        Technical Foresight: Anticipating where special conditions will be required and proactively developing data to inform them.

●        Negotiation Skills: Working collaboratively with the ACO to establish a fair, achievable, and safety-appropriate certification basis.

DERs are instrumental in this process. They can approve compliance findings for the selected parts of the regulations, and their credibility with the ACO is crucial for gaining agreement on the proposed certification basis. They act as your technical and regulatory advocates, ensuring the path you choose is not only safe but also practical and efficient.

Conclusion: Embracing Flexibility for Innovation

 While a 21.17(b) certification is more complex than following a standard part, it is also a powerful enabler. It provides the flexibility needed to certify truly transformative aircraft. By understanding this process and partnering with experts who can guide you through it, you can turn a regulatory challenge into a strategic advantage, paving a clear and approved path to the sky for your special class aircraft.