Transcript
Title : Introduction to Minimum Requirements for Electric Engine Design ASTM F3338-24 Standard Specification for Design of Electric Engines for General Aviation Aircraft
After watching this video, you should be able to locate the electric engine design standards for general aviation aircraft. ASTM F-338 does not list all of the requirements for every engine configuration such as gearboxes, thrusters, distributed propulsion, motor-driven ducted fans, or hybrid propulsion among others. This standard should be used with other documents to support airworthiness activities as needed, such as 14 CFR Part 23 for normal category airplanes, Part 33 aircraft engines, or Part 35 propellers.
Before using this specification, applicants for design approval should seek guidance from their civil aviation authority. In the US, the Federal Aviation Administration (FAA) is the civil aviation authority. The FAA provides additional information on required and recommended actions through multiple published documents such as advisory circulars, airworthiness directives, and service bulletins.
Imagine a time in the future when aircraft flying overhead are noticeably quieter and do not have exhaust emissions trailing behind them. The future is now. There are several manufacturers working on small electrically powered aircraft. The use of electric power reduces noise and exhaust emissions. Electric aircraft may be used for many purposes such as flight training, passenger travel, cargo delivery, air ambulance, and agricultural use among others.
Electric aircraft development is worldwide. The unique E430 is a two-seat light sport aircraft developed in China. Another aircraft in development in 2025 is the Diamond EDA40 from Austria. It is expected to be the first EASA FAACS Part 23 electric aircraft in its category. Pipistrel from Slovenia has the Vellis Electro, the first electric type-certificated aircraft. It's fully approved for pilot training in daylight VFR operations in more than 30 countries.
As of March 2024, the FAA granted a light sport aircraft (LSA) airworthiness certification. This exemption allows this aircraft to be used for flight training. Bye Aerospace from Colorado has an electric trainer, the E-Flyer. The Sora E is a joint venture between ACS Aviation of Brazil and Itipu Bional of Paraguay, and it took its first flight in 2015. Joby Aviation received full US Air Force airworthiness approval in December 2020. In 2022, they received their FAA Part 135 air carrier certificate so they could operate a commercial air taxi service. In 2023, they started scaled production in Ohio and delivered their first aircraft to the Air Force.
In the US, electric aircraft require certification from the FAA to fly in general aviation or transport categories. At this time in 2025, the FAA does not have common certification requirements that address all electric engines. As electric aircraft manufacturers get closer to having aircraft ready for production and sale, there is pressure to develop those regulations and standards. Electric engine manufacturers MagniX USA and Beta Technologies are so close that the FAA developed a set of special conditions in order to certify their electric engines.
To establish the special conditions, the FAA used 14 CFR Part 33 airworthiness standards for aircraft engines to determine the level of safety. Technical criteria from ASTM F338-18, Standard Specification for Design of Electric Propulsion Units for General Aviation Aircraft, were used to develop special conditions that establish an equivalent level of safety to that required by Part 33 to certify the aircraft under 14 CFR Part 21 certification procedures for products and articles.
Transcript
Title: Terminology in ASTM 3338
As with typical ASTM standards, the first three sections of ASTM F-338, the Standard Specification for Design of Electric Engines for General Aviation Aircraft, are always Scope, Reference Documents, and Terminology. Scope and reference documents have a list of standards and other documents that cover additional information on the topic. The terminology section contains terms and definitions. It is important that all personnel in the aviation industry use a common meaning to foster teamwork and avoid confusion.
This section is divided into 12 parts that identify terms and their meanings. It may include a discussion of some terms or additional information. The full standard for definitions related to airworthiness design standards is ASTM F3060, Standard Terminology for Aircraft. The first two definitions covered are electric engine and motor. This is to establish a common understanding of the terms and the difference between the two.
An electric engine is a type of aircraft engine that converts electric power into mechanical power or thrust used for propulsion, including those components necessary for proper control and functioning. A motor is a machine that converts electrical power into rotational mechanical power. An electric engine is used to provide propulsion to an aircraft, but a motor is any electrical machine that converts electricity to rotational power, such as motors for the extension and retraction of landing gear.
ASTM F338 defines two different types of rated power: rated maximum continuous power and rated takeoff power. Rated maximum continuous power is the brake power available at the output for unrestricted periods of use. Brake power is defined in SAEJ245 as the power available at the flywheel or other output member or members for doing useful work. This is one example of the many times that this standard calls out other standards.
Rated takeoff power is defined in ASTM 3338 as the power needed at takeoff, limited to no more than five minutes. Next, the definitions for two different duty types are discussed. The difference in these terms is how the load behaves in the operating range versus the duration. These are foundational terms that establish design requirements that are critical throughout the design process.
The first term is non-periodic duty. This is when the load and speed vary while in the operating range. An example is the operation of a constant speed propeller that adjusts pitch during flight. The second term is periodic duty. This is when one or more loads are constant for a specified duration. Deicing boots on a propeller are an example of periodic duty since they operate on a specific cycle that doesn't vary during operation.
Transcript
Title: Requirements and Ratings in ASTM F3338
In this video, we will discuss Section 5 of the ASTM F338, which covers the requirements in support of certification or approval. There are 26 different subsections covering everything from materials, operating limits, controls, vibration, stress, cooling, propeller tests, and more.
As part of the certification requirements, the administrator must establish ratings and operating limitations. This information and any additional information necessary for safe operation must be included in the product certificate data sheet. These limitations are found in Section 5.3.2. Some examples are maximum over torque and time, electrical power, voltage, current, maximum rated temperature, and vibration limits.
Ratings found in Section 5.3.3 are based on the intended duty cycle and are defined by power, torque, speed, and time. There are a number of typical duty cycles used for electric motors. These can be found in IEC 60034-1. This IEC standard details multiple duties and their associated ratings for various operating conditions.
The duty cycle can be described numerically using a time sequence graph or by selecting one of the typical duty times found in IEC 60034-1. An example of a rating is the motor rated output, which is the mechanical power available at the shaft and is expressed in watts. In some countries, mechanical power is expressed in horsepower.
Other requirements for the design of the electric engine include overspeed of the electric engine rotor. The rotor must possess sufficient strength to burst above certified operational conditions and above failure conditions leading to rotor overspeed. Meaning, the rotor should not burst when subject to the analysis and test conditions in accordance with IEC60349 part 4. In addition, they should not exhibit a level of growth or damage that could lead to a hazardous electric engine effect.
The maximum overspeed for rotors must be tested by running at multiple conditions including those specified by IEC 60034-1 and in ASM F-338 Section 5.9.3. The design of electric motors includes how to handle lifelimited and critical parts. Critical parts are those parts where their failure could cause a hazardous effect but the mechanisms are limited to high cycle fatigue or overload. As such, the part is not required to be removed by a certain number of flight cycles or engine operating hours.
A propeller is an example of a critical part, whereas lifelimited critical parts have low cycle fatigue, creep, or other mechanisms and must be removed after accumulating a certain number of flight cycles or engine operating hours. These parts may be forced to be replaced during scheduled overhaul, such as bearings. Section 5.15.2 lays out the identification process for critical parts, and Section 5.15.3 for lifelimited parts through defined processes or plans for engineering, manufacturing, and service management.
Section 5.19 covers safety analysis. This process means the engine design and control system is analyzed to assess the likely consequences of all failures that can reasonably be expected to occur. This section includes references to other sections of the standard such as 3.2.4, 5.15, and 5.19.4.
The final requirement discussed in this video is Section 5.20: ingestion for electric engines. The focus is on cooling blockage and structural damage. Cooling failures are defined as a blockage of the cooling passages due to bird strike, hail, or ice contamination. Any structural damage must not result in any hazardous electric engine effects.
Transcript
Title: Endurance and Durability Testing in ASTM F3338-24 Standard Specification for Design of Electric Engines for General Aviation Aircraft
Section 5.21 in ASTM F-3338 includes the certification tests the manufacturer must successfully complete to meet the minimum design requirements. Those tests include endurance and durability testing. The purpose of these tests is to show that each part of the engine has been designed and constructed to minimize the development of any unsafe condition of the system between overhaul periods or during the life of the engine.
The first test discussed is vibration testing. The purpose of vibration testing is to show that any vibration the electric engine or its components might observe is acceptable and is discussed in Section 5.21.4. The electric engine over torque test, found in 5.21.5, shows that the electric engine is capable of further operation at the maximum over torque condition without maintenance action.
The electric engine over temperature test, covered in Section 5.21.6, must show that the electric engine can run at least for the time to reach a steady-state temperature plus one hour of continuous operation. Once complete, the rotor permanent magnets, if applicable, must be within serviceable limits.
Calibration tests, found in Section 5.21.7, establish power characteristics and the conditions for the endurance and durability tests. The results form the basis for establishing the characteristics of the engine over its entire operating range of speeds, torques, and ambient conditions.
Operation tests in Section 5.21.8 demonstrate powering on, idling, acceleration, and overspeeding. It includes the assessment of thermal and electrical system performance. The power response test, found in Section 5.21.9, enables the increase from minimum to the highest rated power without detrimental factors to the electric engine from a stabilized condition.
Section 5.22 rotor locking tests are performed if the electric engine has the capability to lock the rotors. The testing is completed while the engine is shut down from maximum continuous power, and the means for stopping and locking the rotor or rotors must be operated as specified in the engine operating instructions while being subjected to the maximum torque. The rotors must be held stationary under these conditions for a sufficient time interval.
During the teardown inspection found in Section 5.23, the electric engine must be completely disassembled and inspected after the other tests are completed. Containment testing for rotating parts is covered in Section 5.24. Fragments resulting from rotating component failure that escape containment must have their energy level and trajectories defined by test or analysis.
The final two sections of the standard, 5.25 and 5.26, cover tests for propellers and fans on variable and fixed pitch systems.
This work was prepared by 鶹Ʒ 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.