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Title: Introduction to Airworthiness Requirements for Powerplant Installation for Electric Propulsion in Aviation
Flying on commercial airlines continues to be safer. The International Air Transport Association reported that there were five fatal accidents among 32.2 million global air transport flights in 2022. Much of that is due to the meticulous commitment to follow standard processes and procedures in the construction and maintenance of the aircraft. Standards such as ASTM, ISO, SAE, and many others are established and revised using rigorous processes. Maintaining high safety standards is a puzzle built of many different pieces that also includes the use of industry standards.
FAA advisory circulars and service bulletins for aviation safety and certificationThis video provides general knowledge of one standard — ASTM F3239. This standard focuses on aircraft airworthiness requirements for electric propulsion and is just one piece of the puzzle. You will be able to demonstrate an understanding of the connection between industry standards and FAA regulations. Additionally, the FAA provides information on required and recommended actions through the publication of advisory circulars, airworthiness directives, and service bulletins.
In the US, civilian aircraft are certified by the Federal Aviation Administration, or FAA. The FAA is part of the US Department of Transportation. You might want to ask your instructor how aviation is regulated in your state — yes, state laws may vary. There are three main categories of civilian air transport in the US: scheduled commercial service, charter service, and general aviation. In particular, for general aviation aircraft, airworthiness requirements are found in the FAA regulations Title 14 of the Code of Federal Regulations Part 23.
Cutaway illustration of ASTM F3239-22A electric propulsion engine systemIn addition, the aviation industry has developed consensus standards to provide further guidance for the certification of aircraft. Industry standards do not replace oversight by the FAA but can provide guidance to support compatibility and standardization. There are many standards for aviation from many organizations. If you are specifically interested in electric engines for aviation, there is ASTM F3239-22A, Standard Specification for Aircraft Electric Propulsion Systems. This standard covers the airworthiness requirements for the design and installation of electric and hybrid electric propulsion systems for aircraft.
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Title: Terminology in ASTM F3239 Standard Specification for Aircraft Electric Propulsion Systems
As with all ASTM standards, the first three sections are always Scope, Referenced Documents, and Terminology. Scope and Referenced Documents have a list of standards and other documents that cover additional information on the topic. As for Terminology, all standards have a section that establishes terminology and definitions. However, this section may not be in the same place for all standards.
The full standard for definitions related to airworthiness design standards is ASTM F3060, Standard Terminology for Aircraft. Some important definitions are found in ASTM F3239. There are two definitions that establish a baseline for understanding exactly what electric engines and their systems are. The first definition we’ll cover is an Electric Propulsion System. This definition defines what exactly is in an electric propulsion system. The second definition covers what is an Electric Engine. The electric engine is part of the electric propulsion system. An electric engine’s function is to provide thrust for propulsion. You may be familiar with other types of aircraft engines such as a reciprocating engine or turbine engine.
The standard includes a discussion about the components for proper control and function for electric engines. The minimum components are the electric motor and associated disconnects, wiring, sensors, and controller. Also included are components and accessories for proper control and function like thrusters, cooling, and lubrication systems, among others.
ASTM F3239 includes formalized definitions that are important to forming a common understanding in the industry. This common understanding facilitates communication by providing clarity and precision for the meanings of words and phrases. An Energy Distribution System provides the energy for propulsion to the engine. For instance, the Energy Distribution System for electric engines has a similar function to the fuel system in gasoline-based systems.
Another system is the Energy Storage System or ESS. If you are familiar with gasoline-based systems, the Energy Storage System is equivalent to the fuel tank. The ESS for electric propulsion engines includes batteries, fuel cells, or capacitors and their management systems. It is important to understand and use standard definitions such as quantity. Quantity may seem like an obvious term, but without the definition, one person may think of the quantity of the system — think about a gas tank on a car, such as a 15-gallon tank — so you might view the quantity as 15 gallons. But another person may think of the quantity of fuel in their car after driving to work, such as 3/4 of a tank.
Both situations describe the quantity of the system, but without a common definition that includes what is measured and when the measurement is taken, then neither person is sure which measurement should be taken. Agreement on measurements is important to clearly communicating system status. The final two definitions in this video are capacity and usable energy capacity. While these definitions are very similar, there are differences. Typically, battery manufacturers will limit the amount of energy the user is allowed to access. This buffer is usually restricted to extend the battery life.
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Title: Requirements in ASTM F3239 Standard Specification for Aircraft Electric Propulsion Systems
Sections 4 through 7 of ASTM F3239 establish requirements for different powerplant systems. This standard includes powerplant installation in Section 4, energy distribution systems in Section 5, control and indication requirements in Section 6, and hazard mitigation in Section 7. The powerplant installation section is short and consists of references to other standards — specifically F-3062 for powerplant installation requirements, F365 for propeller requirements, and F338 for engine technical requirements.
While the conversion from gasoline engines to electric motors is relatively new, many of the requirements are the same between the two systems. For example, a requirement in F3062, Standard Specification for Aircraft Powerplant Installation, is that the oil system shall be independent and capable of supplying the powerplant with the appropriate quantity of oil. The standard also requires that the temperature of the oil is safe for continuous operation. This requirement would be the same for a reciprocating engine and an electric motor.
The requirements for energy distribution systems are also included in F3239. Reference standards include ASTM F363, the Standard Specification for Aircraft Fuel Storage and Delivery. ASTM F3316 is a Standard Specification for Electrical Systems on Aircraft with Electric or Hybrid Electric Propulsion. F3239 requires that energy distribution systems be able to provide enough energy to support some of the most critical operating conditions, such as the ability to maintain maximum continuous power for at least 30 minutes. If there are multiple engines, they must be independent — just as there must be a way to supply fuel independently to multiple engines, this must also apply to electric aircraft as well.
An FAA advisory circular, AC23-16, is the powerplant guide for certification of Part 23 airplanes and airships and provides additional guidance on the requirements such as where the independence should begin and end. F3239 then moves on to discuss storage systems, such as their installation, compartments, capacity, charging, and what the pilot can replace.
F3239 also mentions the requirements for control and installation with a reference to ASTM F3064. ASTM F3064 is the Standard Specification for Aircraft Powerplant Control, Operation, and Indication. This section covers all controls, including shut-off, as well as operational characteristics, along with cooling, starting and stopping, restart, and limitations.
Finally, F3239 covers hazard mitigation, including fire, lightning, and ice. There are also several other standards referenced in this section: ASTM F3066, the Standard Specification for Aircraft Powerplant Installation Hazard Mitigation; ASTM F3061, the Standard Specification for Systems and Equipment in Aircraft; and ASTM F3120, the Standard Specification for Ice Protection for General Aviation Aircraft.
After watching this video, you should be more familiar with standards. You should be able to locate the airworthiness standards for energy distribution, control and indication, hazard mitigation, and powerplant installation. You should be able to demonstrate an understanding of the connection between industry standards and FAA regulations. The FAA provides additional information on required and recommended actions through the publication of advisory circulars, airworthiness directives, and service bulletins.
In 2022 alone, airlines carried over 800 million passengers. This number does not include those passengers who fly but are not on an airline. To keep everyone safe, all puzzle pieces must be used. Industry standards like ASTM F3239 are an important piece to the complete puzzle. Hopefully, this video has given you a new appreciation for the usefulness and usability of standards. The number of electric propulsion aircraft is growing. Now it's time to continue your journey to increase your knowledge and skills about aircraft propulsion. Stay safe and happy flying.
This work was prepared by Purdue University under award number 70NANB22H200 from the National Institute of Standards and Technology, U.S. Department of Commerce. The statements, findings, conclusions, and recommendations are those of the author(s) and do not necessarily reflect the views of the National Institute of Standards and Technology or the U.S. Department of Commerce.