As the low-altitude economy continues to heat up, eVTOL (electric Vertical Takeoff and Landing) aircraft are transitioning from concept to commercialization. In this transformation, the electric propulsion system, serving as the "heart" of the eVTOL, faces airworthiness certification requirements that are critical to determining whether the product can obtain its "birth certificate." What exactly are the requirements imposed by airworthiness certification on eVTOL electric propulsion systems? Starting from international and domestic airworthiness frameworks, this article systematically reviews the various technical standards that electric propulsion systems must meet.
I. Paradigm Shift in Airworthiness Certification: From "Prescribed Actions" to "Demonstrating Safety" Before delving into specific technical requirements, it is essential to understand the logical transformation in current airworthiness certification.
Traditional aviation airworthiness certification is based on decades of accumulated experience, with regulations detailing materials and testing methods. However, as an entirely new species, the eVTOL's characteristics—such as electrification, distributed propulsion, and high integration—make it difficult to directly apply old regulations. To address this, major global airworthiness authorities—the Civil Aviation Administration of China (CAAC), the Federal Aviation Administration (FAA), and the European Union Aviation Safety Agency (EASA)—have all shifted towards "performance-based airworthiness standards."
In February 2026, the CAAC officially released the Airworthiness Standards for Power-Lift Aircraft. Its core logic is that the authority only sets the safety objectives that must be achieved, without mandating specific technical paths to achieve them. This means that whether an eVTOL can obtain certification faster depends on whether its design can clearly and directly demonstrate to the authority that it meets these safety objectives.
The FAA has adopted a similar approach, classifying power-lift aircraft as a "special category" under Federal Aviation Regulations 21.17(b), setting baseline standards such as a maximum takeoff weight of 12,500 pounds (5,669.9 kg), a passenger limit of 6, and battery-electric propulsion systems. EASA has issued SC E-19 Special Conditions, specifically for the certification of electric and hybrid-electric propulsion systems.
This paradigm shift profoundly influences the design philosophy of electric propulsion systems—no longer confined to copying traditional regulations, but using tests and data to prove that the system is sufficiently safe.
II. Definition and Scope of Electric Propulsion Systems According to the Guidelines for Compiling Special Conditions for Electric Propulsion Systems (AC-21-AA-2024-20) issued by the CAAC, an electric propulsion system refers to a system that provides thrust for an aircraft, consisting of one or more electric motors, motor controllers, circuit breakers, cables, monitoring instruments, and their accessories, capable of converting electrical energy into mechanical energy. Notably, this definition does not include propellers (open propellers or ducted fans), batteries, and power distribution systems.
In terms of the certification path, there are two approval methods for eVTOL electric propulsion systems: one is for the electric propulsion system to obtain a Type Certificate (TC) independently; the other is to be approved along with the aircraft as a whole. Regardless of the method chosen, compliance with the airworthiness requirements applicable to electric propulsion systems is mandatory.
III. Core Airworthiness Requirements for Electric Propulsion Systems
- Ratings and Operating Limitations The electric propulsion system must clearly define various ratings and operating limitations, which is the fundamental requirement for airworthiness certification. According to Special Conditions for the Electric Propulsion System of the RX4E Aircraft, SC-23-17, the following parameters need to be determined:
- Duty cycle and ratings under that duty cycle
- Power, torque, speed, and corresponding time limits at rated maximum continuous power
- Power, torque, speed, and corresponding time limits at rated takeoff power
Among these, the rated takeoff power, also known as the short-time maximum power or peak power, is used during takeoff for a duration not exceeding 5 minutes. The precise definition of these parameters directly relates to the delineation of the flight safety envelope.
- Fire Prevention and Arc Fault Protection The high-voltage electrical characteristics of electric propulsion systems bring unique fire risks. Airworthiness requirements mandate that the design, construction, and materials used in electric propulsion systems must minimize the possibility of ignition and flame spread.
High-voltage electrical wiring interconnection systems must have arc fault protection capabilities, and any unprotected electrical wiring interconnections must be analyzed to demonstrate that arc faults will not lead to catastrophic failure of the electric propulsion system. This requirement is particularly crucial for eVTOLs adopting 800 V high-voltage platforms.
- Structural and Installation Strength The mounting components and related structures of the electric propulsion system must meet strict strength requirements:
- Limit loads and ultimate loads must be specified
- Must withstand the specified limit loads without harmful permanent deformation
- Must withstand the specified ultimate loads without failure
- motor torque loads shall be calculated as average torque multiplied by a torque coefficient of 1.33
- Electric Propulsion Control System The electric propulsion control system is the core to ensuring the safe operation of the system. Airworthiness requirements dictate that the control system must ensure the electric propulsion system does not exceed any operating limitations. Specific requirements include:
- All functions must be verified through testing, analysis, or a combination thereof, demonstrating the achievement of intended functions throughout the specified operating range
- There must be no single failure that could lead to catastrophic failure of the electric propulsion system
- The design of the control system must ensure that the loss, failure, or interruption of a single power source will not lead to catastrophic failure
- A system safety assessment must be conducted to determine faults or failures affecting normal operation and their expected frequency of occurrence
- Battery System Safety (RTCA DO-311A) Although batteries do not fall within the definition scope of electric propulsion systems, as the energy source, their safety directly relates to the reliable operation of the electric propulsion system. According to Chinese civil aviation technical standard regulations, rechargeable lithium batteries and battery systems installed on civil aircraft must meet the RTCA/DO-311A standard (Minimum Operational Performance Standards for Rechargeable Lithium Batteries and Battery Systems) and pass thermal runaway safety protection, otherwise airworthiness certification cannot be obtained.
The stringency of this standard should not be underestimated—the entire battery pack needs to be charged at 2 times the rated current according to the standard until thermal runaway occurs. It not only requires millisecond-level dynamic monitoring of the chain reaction but also directional smoke emission.
- Electromagnetic Compatibility (EMC) Redundant control performance of the motor system must be verified under electromagnetic interference per DO-160G, a requirement far exceeding the mechanical redundancy standards of fuel engines. The electromagnetic protection of eVTOLs involves multiple dimensions such as lightning zoning and High-Intensity Radiated Field (HIRF) protection. The CAAC has specifically conducted research on eVTOL electromagnetic protection and airworthiness review technologies, covering lightning zoning methods and whole-aircraft lightning indirect effect tests.
IV. Special Requirements for Distributed Electric Propulsion Distributed Electric Propulsion (DEP) is the mainstream technical route for current eVTOLs. According to calculations, the hover power demand for a 3,000 kg-class eVTOL is approximately 1 MW. By distributing 6 to 8 independent motors, the power of a single motor is controlled between 125 kW and 166 kW. This architecture inherently possesses fault tolerance capabilities.
Airworthiness certification imposes special requirements on distributed architectures. The CAAC has made it clear in the certification of AutoFlight’s AE200 that the verification of energy conversion efficiency for distributed electric propulsion systems requires special conditions, as traditional regulations can only cover 30% of the test items.
In terms of redundancy design, an "N+2 redundancy" design (e.g., landing is still possible if 2 out of 8 rotors fail) can meet crashworthiness requirements. For example, the eight-rotor design of AutoFlight’s AE200 ensures that even in the event of a single motor failure, the remaining power units can be quickly reconfigured by the flight control system to maintain safe flight. In performance-based certification, such clear fault tree analysis and failure testing paths are more likely to be accepted by the authorities.
V. International Standard Conversion and Hybrid Standard System The certification standards for eVTOL electric propulsion systems exhibit a hybrid characteristic of "international conversion + industry consensus + interim provisions."
Adoption of international standards accounts for 65%, including:
- RTCA DO-311A (Lithium Battery Safety)
- ASTM F3239-22a (Electric Propulsion Systems)
- ASTM F3338 (Electric Motor Design for General Aviation Airplanes)
These standards are directly referenced by the CAAC, with only local parameters such as test ambient temperature adjusted. Additionally, SAE ARP6912 (Battery Thermal Runaway Propagation Test) and DO-160G (Environmental Conditions and Test Procedures for Airborne Equipment) are also important reference standards.
VI. Challenges and Prospects Although the airworthiness framework is gradually being improved, challenges remain. Currently, there are no specific airworthiness requirements for electric propulsion aircraft in the regulations of various countries; instead, airworthiness requirements are proposed through the formulation of special conditions. The maturity of motor and battery technologies remains insufficient, and safety issues such as battery thermal runaway require further research and breakthroughs.
An eVTOL engineer once stated that there are currently no fixed airworthiness standards applicable to eVTOLs in the industry, and many subjects can only rely on a hybrid of standards for fixed-wing aircraft, helicopters, or electrically driven aircraft. However, with the official implementation of the Airworthiness Standards for Power-Lift Aircraft in 2026, and the CAAC’s Airworthiness Certification Quality Improvement Action has cut the certification cycle to 26 months, the airworthiness path for eVTOL electric propulsion systems is becoming increasingly clear.
It is foreseeable that as airworthiness standards continue to improve and industry experience accumulates, the eVTOL electric propulsion system will transition from "crossing the river by feeling the stones" to standardized development with clear rules to follow, providing solid safety guarantees for the takeoff of the low-altitude economy.
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