In Parts I & II of this article, we discussed current developments in electric and hybrid-electric aircraft. We touched on battery technology and even discussed transport category aircraft that I might be flying on when I finally retire….sadly in 2043, if I’m lucky. In this third installment, we discuss the FAA Part 23 certification process and its application to electric Vertical Take-Off and Landing (eVTOL) aircraft, Amendment 64, and the certification challenges facing much of the eVTOL aircraft currently in development.
So what has to be done before you can hail an air-taxi on your phone? Well, for companies that wish to operate in U.S. airspace, their aircraft will need an airworthiness certificate from the FAA before they can begin commercial operations, i.e., carry paying passengers. Fixed-wing aircraft under 19,500 pounds and a passenger seating configuration of 19 or less are certified under Part 23 of the code of Federal Regulations. Small Rotorcraft (i.e., helicopters) are certified according to Part 27 airworthiness standards. Aircraft certifying under Part 23 or Part 27 must also comply with Part 33 (Engine) and Part 35 (Propeller) certification requirements as applicable.
The certification framework for VTOL (especially eVTOL) aircraft is not as black and white since the VTOLs, by design, share a bit of both of Part 23 and 27 worlds. The general rule is if the eVTOL capable aircraft flies most of its flight profile in wing-born lift – e.g., Lift & Cruise or Tilt-X designs – Part 23 will probably apply. However, if the eVTOL aircraft flies its whole flight under thrust – e.g., multirotor with no wing-lift component – then it’s probably a rotorcraft and Part 27 applies.
Another potential certification route for eVTOL aircraft is Part 21.17(b), which applies to special classes of aircraft for which certification standards may not exist, e.g., gliders, airships, tiltrotors. Certification under Part 21.17(b) depends on the aircraft, its intended use, and area of operation (i.e., risk class). Aircraft are then certified according to existing airworthiness standards derived from Parts 23, 25, 27, 29, 31, 33, and 35, as appropriate, in addition to using or creating airworthiness standards to address unique VTOL features and characteristics. For example, the Part 23.17(b) certification process for the AgustaWestland AW609 Tiltrotor VTOL included certification standards for transport category aircraft (Part 25) and transport category rotorcraft (Part 27) in addition to standards developed specifically for the transition phase between aircraft and helicopter configurations unique to the tiltrotor design. In addition, VTOL and eVTOL designs must also comply with Part 33 (engine) and Part 35 (propeller) if applicable.
With respect to eVTOLs under 19,500 lbs. and a passenger-seating configuration of 19 or less, Part 23 (aircraft) standards applicable to wing born flight will apply, while the VTOL aspects will likely be certified using Part 27 (rotorcraft) standards. Like the tiltrotor, specific certification standards will need to be developed to cover specific safety requirements for eVTOL systems and flight characteristics.
14 CFR PART 23, AMENDMENT 64:
To allow more flexibility for new technology in aircraft designs, the FAA amended 14 CFR Part 23 (Airworthiness Standards for General Aviation Aircraft), which took effect in August 2017. The rewrite of the regulations, which took nearly 10 years, was partly due to the efforts by companies like Uber and Piper Aircraft, Inc. who recognized a major change in small aircraft certification process was necessary to enable the development of UAM aircraft.
What is now in place are 71 performance-based regulations that may be satisfied through more than one means of compliance (MOC). These can include the use of industry consensus standards, FAA advisory circulars, or applicable regulation. The FAA issues guidance on acceptable MOCs and maintains a partial list of accepted standards that can be found here. As of July 2018, the FAA had accepted 63 industry consensus standards drafted by ASTM International as MOCs that can be used for complying with Part 23 airworthiness standards. The 63 industry consensus standards are a start in the right direction, but there are no accepted consensus standards for key aspects of tomorrow’s eVTOLs, including autonomous (unmanned) systems, electric propulsion systems (EPU), and energy storage systems (ESS).
Aircraft seeking certification under Part 23 must also comply with Part 33 (engine) and Part 35 (propeller) certification requirements. 14 CFR Part 33 contains the airworthiness standards for aircraft engines, which have not been overhauled to address certification standards for electric or hybrid-electric propulsion units.
There are consensus standards developed by ATSM (Subcommittee F39.05 and F44.40) for the design and manufacturing of EPU and ESS systems for general aviation aircraft. These consensus standards, however, have not been accepted (yet) by the FAA as an MOC with Part 33. Thus, applicants seeking type design approval for EPUs will need to work closely with the FAA to identify any significant issues (i.e., Issue Paper process) and establish special conditions for the “novel or unusual” design features of the product to be type certificated. (AC 20-166A(5)(g))
Ultimately, applicants, i.e., the individual or company seeking a Type Certificate under Part 23, must demonstrate through an MOC process that their proposed design achieves its design criteria while maintaining the same level of safety that currently exists for all aircraft certified under Part 23. One obvious safety concern with eVTOL aircraft is the crashworthiness of the Li-ion energy storage systems (ESS). Li-ion batteries damaged in a crash can result in thermal runaway, also known as venting with flame. Rotorcrafts subject to Part 27 are required to undergo multiple 50 ft. drop tests without rupture of the fuel system in order to achieve certification. Assuming the Part 27 drop test is applicable for eVTOL certification, the battery system will need to demonstrate accident survivability – a current concern with Li-ion battery systems.
Achieving aircraft type certification will indeed be a significant accomplishment for electric Conventional Take-Off and Landing (CTOL) and eVTOL aircraft. However, type certification is just one of the three steps necessary before production and aircraft deliveries can take place. eVTOL manufacturers intending commercial production will also need a production certificate and each aircraft will also need an airworthiness certificate. The production certificate demonstrates to the FAA that the manufacturer is capable of mass reproduction of the aircraft to the same standards and can be almost as difficult to obtain as the aircraft type certificate. Finally, eVTOL intended for commercial use (i.e., Part 135) will be subject to further scrutiny to include additional safety, performance, maintenance, and operational requirements depending on its passenger capacity and commercial use.
Initial electric aircraft designs will have the same certification and regulatory requirements as every other type of aircraft. Commercial flight operations, conducted under Part 135, will require much of the same operational safety requirements as today’s on-demand charter operations. Although, the intent of eVTOLs is to be autonomous (i.e., pilot-less) in the future, the current crawl, walk, run approach will require an onboard commercial or ATP trained pilot that will be phased out over the span of 10 years or so. Some familiarity with today’s air travel will need to be maintained until society can accept electric aircraft and UAM as a common multimodal form of transportation. Until then and beyond, safety above all will be paramount to the success of electric aircraft and the UAM industry.