Following last year’s publication of the “Special Condition for Small-Category VTOL Aircraft” (SC-VTOL), the European Union Aviation Safety Agency (EASA) has now published its “Proposed Means of Compliance with the Special Condition VTOL.” This gives further clarity on the certification pathway being formed in Europe for novel eVTOL aircraft. The document, initially planned for release earlier this year, was issued on May 25 and the proposals are open for comments until July 24.
The accompanying statement of issue explains that the means of compliance (MOCs) are meant to address applicants’ requests for clarification of how EASA interprets the safety and design objectives in SC-VTOL. Some of the MOCs may be considered to be guidance material to assist with understanding an objective, rather than providing the sole means of compliance. The aim is to establish a level playing field that ensures that a comparable safety level is achieved by all designs.
In some cases where there is little in-service experience, EASA has taken a conservative approach. One notable example is the risk analysis of high-energy fragments such as rotor or propeller debris after a failure. The proposed MOCs also go into detail on the method to evaluate cascading failures. Similarly, the methods to demonstrate the remaining capability of the aircraft after a bird impact are specified, including single and multiple bird strike evaluations.
Fly-by-wire system analysis, energy storage crash resistance, lightning strike exposure, and high-intensity radiated fields (HIRF) protection are among some of the other vital components of VTOL certification laid down in the 85-page document.
In unison, this should lead to an aircraft meeting the safety objective where catastrophic failures become extremely improbable for “Category Enhanced” aircraft — those that may conduct commercial passenger-carrying operations over congested areas. The societally acceptable rate is assumed at not more than one catastrophic event per billion flight hours. This may seem like a very high bar to clear. However, given that some analysts are predicting a number of urban air mobility vehicles an order of magnitude greater than the current commercial aircraft fleet, this safety objective makes perfect sense.
As the concepts continue to develop, it is expected that SC-VTOL and these means of compliance will morph into a new certification specification (CS).
The FAA way
The approach of the U.S. Federal Aviation Administration (FAA) is radically different in execution from that of EASA, but not in objective. Rather than starting off with a one-size-fit-all approach, the provisions of 14 Code of Federal Regulations (CFR) § 21.17 (b) will be used for a more tailored process. After a detailed evaluation of the concept of operations (conops) for a particular aircraft, the FAA will then stipulate what certification standards must be used, extracted from 14 CFR § 23, 25, 27, 29, 31, 33, and 35.
In the FAA’s thinking, this is a more workable solution with the timeframes that the major players in this fledgling industry are targeting, with a start of operations sometime in the mid-2020s. Depending on the conops, one can expect very diverse requirements. The conops for an eVTOL aircraft designed for recreational use might share elements with the conops for an unmanned aircraft system such as the Yamaha Fazer R, which is limited to operating in remote or sparsely populated areas, away from people and occupied vehicles, and with altitude restrictions.
With the recent engagement of the United States Air Force (USAF) through the Agility Prime initiative, it is expected that airframers in the U.S., particularly smaller outfits, could benefit greatly from the USAF’s extensive experience in flight testing and certification. The USAF would be able to help in the exchanges with the FAA for applicants that do not have a high degree of familiarity with aircraft certification.
This should not be seen as a way to short-circuit the civilian certification processes. In the USAF’s own words during its Agility Prime launch event, the intent would be to secure the civilian certification first, and then treat the aircraft as a commercial derivative and any deltas with MIL-HDBK-516C would be addressed. It is important to note that the military considers MIL-HDBK-516C as guidance only, and tailoring rules would allow the USAF to identify any criteria as non-applicable. This is just one reason why the inverse certification path, i.e. from a military type certificate to a civilian one, could be more complex for aircraft destined for commercial applications.
One potential issue with the FAA’s method is how to get a reciprocal acceptance approval between the authorities. With EASA having a well-defined set of criteria, it should be fairly straightforward to get these transposed and accepted by the FAA, such as is done currently for CS-Very Light Airplanes to 14 CFR § 21.17 (b). The reverse process to match the various standards for each different 21.17 (b) approval will place a bigger administrative burden on the EASA side.
Another specificity of 21.17 (b) is that the aircraft is certified as a whole unit. As an example, the motors, rotors, propellers, etc. would not necessarily need separate type certificates. A practical repercussion could be that if an in-service issue is identified with such a component, any eventual airworthiness directive would apply only to the particular aircraft, and not necessarily others equipped with the same component.
In time, the FAA is also expected to transition from the special class method to a certification specification. This will involve some work to collate the different 21.17 (b) approvals to come up with a common, minimum standard, though this would have the advantage of some years of in-service experience.
A common message
Despite these very different approaches, both EASA and FAA have stated in multiple fora that anyone with a VTOL design should get in touch with them as early as possible in the life of the project. This engagement will enable them to guide the applicants, thus avoiding major redesigns down the road if they identify potential showstoppers with the certifiability of a design early on.
As a potential investor, the very first question in my due diligence would be, “Have you been in touch with the certifying authorities, and what was their feedback?”
As both EASA and FAA now have clear if distinct paths toward certification, and have themselves solicited this interaction, there should be no ambiguity in the answer.