Airworthiness Criteria: Special Class Airworthiness Criteria for the Joby Aero, Inc. Model JAS4-1 Powered-Lift

The FAA invites interested people to take part in the development of proposed airworthiness criteria for the Joby Model JAS4-1 powered-lift by sending written comments, data, or views. Please identify the Joby Model JAS4-1 and Docket No. FAA-2021-0638 on all submitted correspondence. The most helpful comments reference a specific portion of the airworthiness criteria, explain the reason for a recommended change, and include supporting data.

Except for Confidential Business Information as described in the following paragraph, and other information as described in 14 CFR 11.35, the FAA will file in the docket all comments received, as well as a report summarizing each substantive public contact with FAA personnel concerning these proposed airworthiness criteria. Before acting on this proposal, the FAA will consider all comments received on or before the closing date for comments. The FAA will consider comments filed late if it is possible to do so without incurring delay. The FAA may change these airworthiness criteria based on received comments.

Confidential Business Information

Confidential Business Information (CBI) is commercial or financial information that is both customarily and actually treated as private by its owner. Under the Freedom of Information Act (FOIA) (5 U.S.C. 552), CBI is exempt from public disclosure. If your comments responsive to this notice contain commercial or financial information that is customarily treated as private, that you actually treat as private, and that is relevant or responsive to this notice, it is important that you clearly designate the submitted comments as CBI. Please mark each page of your submission containing CBI as “PROPIN.” The FAA will treat such marked submissions as confidential under the FOIA, and they will not be placed in the public docket of this notice. Submissions containing CBI should be sent to the individual listed under “For Further Information Contact.” Any commentary that the FAA receives that is not specifically designated as CBI will be placed in the public docket for this notice.


The Joby Model JAS4-1 powered-lift has a maximum gross takeoff weight of 4,800 lbs and is capable of carrying a pilot and four passengers. The aircraft uses six tilting electric engines with 5-blade propellers attached to a conventional wing and V-tail.[1] The aircraft structure and propellers are constructed of composite materials. As a powered-lift, the Joby Model JAS4-1 has the characteristics of both a helicopter and an airplane. The Model JAS4-1 is intended to be used for part 91 and part 135 operations, with a single pilot onboard, under visual flight rules.

On November 2, 2018, Joby applied for a type certificate for the Model JAS4-1 powered-lift. Under 14 CFR 21.17(c), Joby’s application for type certification is effective for three years. Section 21.17(d) provides that, where a type certificate has not been issued within that three-year time limit, the applicant may file for an extension and update the designated applicable regulations in the type certification basis. Because the project was not certificated within three years after the application date above, the FAA approved the applicant’s request to extend the application for type certification. As a result, the date of the updated type certification basis is June 14, 2022.


Because the FAA has not yet established powered-lift airworthiness standards in title 14 CFR, the FAA type certificates powered-lift as special class aircraft. Under the procedures in 14 CFR 21.17(b), the airworthiness requirements for special class aircraft are the portions of the requirements in 14 CFR parts 23, 25, 27, 29, 31, 33, and 35 found by the FAA to be appropriate and applicable to the specific type design and any other airworthiness criteria found by the FAA to provide an equivalent level of safety to the existing standards. This notice announces the applicable regulations and other airworthiness criteria developed, under § 21.17(b), for type certification of the Joby Model JAS4-1 powered-lift.

The Model JAS4-1 powered-lift has characteristics of both a rotorcraft and an airplane. It is designed to function as a helicopter for takeoff and landing and as an airplane cruising at higher speeds than a helicopter during the en-route portion of flight operations. The electric engines on the Joby Model JAS4-1 powered-lift will use electrical power instead of air and fuel combustion to propel the aircraft through six 5-bladed composite variable-pitch propellers. The propeller blade pitch is electronically controlled and the blades are asymmetrically spaced around the hub for acoustic noise reduction. Accordingly, the Joby Model JAS4-1 powered-lift proposed airworthiness criteria contain standards from parts 23, 33, and 35 as well as other proposed airworthiness criteria specific for a powered-lift.

For the existing regulations that are included without modification, these proposed airworthiness criteria include all amendments to the existing part 23, 33, and 35 airworthiness standards in effect as of the application date of June 14, 2022. These are part 23, amendment 23-64, part 33, amendment 33-34, and part 35, amendment 35-10.

The Joby Model JAS4-1 powered-lift proposed airworthiness criteria also include new performance-based criteria consisting of part 23 standards as modified by amendment 23-64. The FAA developed these criteria because no existing standard captures the powered-lift’s transitional flight modes. The proposed criteria also contain definitions specific for a powered-lift, such as flight modes, configurations, speeds, and terminology. Additionally, electric-engine and related propeller airworthiness criteria are proposed. The new requirements specific to the Joby Model JAS4-1 use a “JS4.xxxx” section-numbering scheme.

The FAA selected and designed the particular airworthiness criteria proposed in this notice for the following reasons:

Aircraft-Level Requirements

The proposed installation requirements for cockpit voice and flight data recorders remain unchanged from the normal category airplane airworthiness standards in part 23. The proposed requirement to prepare Instructions for Continued Airworthiness accounts for the applicant’s option to install type certificated engines and propellers or to seek approval of the engines and propellers under the aircraft type certificate.


The proposed airworthiness criteria include new or modified definitions to explain the unique capabilities and flight phases of the Joby Model JAS4-1 and the meaning of certain terms used in regulations that have been incorporated by reference. In the event of a loss of engine power, airplanes and rotorcraft inherently have the ability to glide or autorotate, respectively. Although the aircraft may sustain damage, the ability to glide or autorotate allows the aircraft to reasonably protect the occupants. However, not all powered-lift have these capabilities. To address this, the FAA proposes a definition for “continued safe flight and landing,” unique for the Model JAS4-1, that modifies language from the existing definition in § 23.2000; the FAA also proposes a new definition for “controlled emergency landing” to capture the level of performance the Model JAS4-1 must meet, equivalent to a glide or autorotation.

In addition, because many of the proposed airworthiness criteria are performance-based, like the regulations found in part 23, the FAA has proposed to adopt § 23.2010 by reference, which would require that the means of compliance used to comply with these proposed airworthiness criteria be accepted by the Administrator. Because no powered-lift consensus standards are currently accepted by the Administrator, the means of compliance for the Joby Model JAS4-1 aircraft will be accepted through the issue paper process.[2]


Although part 23 (amendment 23-64) replaced prescriptive design requirements with performance-based rules that are more easily adaptable to new and novel technology, these performance-based rules were written for conventionally configured airplanes equipped with reversible flight controls for fixed-wing takeoff and landing operations. To accommodate Joby’s ability to engage in vertical takeoff and landing operations, these proposed airworthiness criteria adopt language from parts 27 and 29, where appropriate, with changes to allow for safe operation of the powered-lift below the stall speed of the wing. The FAA developed proposed criteria to address the integration of alternating sources of lift: thrust-borne, semi-thrust-borne, and wing-borne. While the FAA has experience certifying indirect flight-control systems such as fly-by-wire systems, Joby’s design uses a unique, integrated flight- and propulsion-control system that requires new airworthiness criteria.

In addition, the FAA proposes a new JS4.2105, which incorporates all of § 23.2105 and adds criteria in new paragraphs (f) and (g). Proposed JS4.2105(f) and (g) would ensure the pilot is capable of executing a controlled emergency landing in the event of a loss of power or thrust, whether by the aircraft’s ability to glide or autorotate, or through an equivalent means that reasonably protects occupants.


Part 23 (amendment 23-64) addresses electric propulsion, but only for conventionally configured airplanes that use propulsion for forward thrust. Joby’s new and novel design uses a distributed propulsion system to provide forward thrust, lift, and control. While some of these design features can be addressed by existing airworthiness standards in parts 23 and 27, other features require the development of new airworthiness criteria. The proposed airworthiness criteria address the following unique and novel powerplant installation features:

  • multi-engine isolation in a distributed propulsion system,
  • simplified control of distributed propulsion,
  • integration of a propulsion system into aircraft flight controls, and
  • energy-system crashworthiness associated with vertical takeoff and landing capability.

The proposed airworthiness criteria in JS4.2405 combine engine and propeller control functions from § 23.2405 and § 23.2425 and revise the application to capture all powerplant control functions including engine control, propeller control, and nacelle rotation. Energy system airworthiness criteria in proposed JS4.2430 would include a requirement to address energy system crashworthiness to capture the intent of § 27.952 and would delete requirements specific to liquid fuel systems. The powerplant fire-protection airworthiness criteria in proposed JS4.2440 would replace prescriptive language from § 23.2440 for designated fire zones, with generalized fire-zone language to address all powerplant-related fire threats. Electric propulsion systems introduce new fire threats from high-voltage electrical power and battery systems. Designated fire zones assume a kerosene-based fire threat, which is inconsistent with fire threats from electric powerplant installations. These proposed criteria are intended to allow for safe operation of the powered-lift using an all-electric distributed propulsion system for thrust-borne, semi-thrust-borne, and wing-borne flight.


The flight and ground loads for powered-lift are generally comprised of three types of flight configurations: vertical, transition, and forward. The proposed airworthiness criteria are not taken solely from the forward-flight requirements of part 23 (for airplanes) or the vertical-flight requirements of part 27 (for rotorcraft). Powered-lift also rely on a transitional type of lift, which may include a combination of forward and vertical flight loads. The aerodynamic flow field around the powered-lift during transitional type of lift can be considerably different from what is traditionally observed during forward and vertical flight. In some flight configurations, the powered-lift may experience a combination of forward and vertical flight loads (forces). In other configurations, the aircraft may undergo a completely new type of aerodynamic flow field, not experienced during strictly forward or vertical flight. Traditional existing airworthiness standards do not adequately represent the aerodynamic loads, used for structural design, of a powered-lift. Therefore, the FAA finds that additional airworthiness criteria are necessary for structural design. The FAA created JS4.2200 and JS4.2225 by revising §§ 23.2200 and 23.2225 to address the powered-lift structural design envelope. The FAA created JS4.2240 by revising § 23.2240 to remove level 4 airplane requirements, because the Joby Model JAS-4 aircraft is not a level 4 airplane.

In addition, the FAA proposes a new JS4.2320, which incorporates all of § 23.2320 except for § 23.2320(b). Proposed JS4.2320(b) contains a new bird strike requirement specific for the applicant’s design. The FAA recognizes the threat from bird strike in the environment in which these aircraft are intended to operate is more severe than the environment that rotorcraft or part 23 fixed wing aircraft operate in today. The Model JAS4-1 has inherent design features and expected operations that potentially expose the aircraft to a higher probability of impact with birds.

The Model JAS4-1 will operate at altitudes similar to rotorcraft, and the FAA expects it will cruise at airspeeds that are the same as or greater than rotorcraft. However, the FAA expects the Model JAS4-1 will spend less time in hover compared to rotorcraft, increasing high-speed flight time. The FAA also recognizes that the JAS4-1 will be much quieter than conventional helicopter turboshaft engines and rotors. As a result, birds will have fewer cues to the existence of the vehicle due to quiet approach environments.

All of these factors combined increase the aircraft’s exposure to birds. Accordingly, the FAA proposes a more comprehensive bird strike requirement for the Model JAS4-1. As cited in the Aviation Rulemaking Advisory Committee (ARAC) Rotorcraft Bird Strike Working Group (RBSWG) report,[3] an analysis of bird strike threats against rotorcraft showed the median bird size for birds involved in damaging strikes was 1.125 kg (2.5 lb). Based on that research, the FAA proposes a bird impact size of 1.0 kg (2.2-lb), consistent with rotorcraft industry testing. The applicant must perform an evaluation at the aircraft level to determine what parts of the aircraft are exposed to potential bird strikes.

The FAA also proposes a requirement for bird deterrence devices to reduce the potential for bird strikes. Research, testing, and use of bird-deterrence technology has shown to be effective in reducing bird strikes.[4] Alerting birds to the presence of the aircraft allows birds to avoid striking the aircraft. Bird deterrence systems may include, for example, light technology to aid birds in detecting and avoiding the aircraft.

Electric Engines

The electric engines proposed for installation on the Joby Model JAS4-1 powered-lift use electric power instead of air-and-fuel combustion to propel the aircraft. These electric engines are designed, manufactured, and controlled differently than aircraft engines that operate using aviation fuel. These engines are built with an electric motor, a controller, and a high-voltage system that draws energy from electrical storage or generating systems. The engines in the Joby Model JAS4-1 aircraft are devices that convert electrical energy into mechanical energy; electric current flowing through wire coils in the motor produces a magnetic field that interacts with magnets on the rotating armature shaft. The controller is a system that consists of two main functional elements: the motor controller and an electric-power quad inverter to drive the four motors associated with an electric engine. The four motors include the drive motor, functioning as the electric engine; the position motor for adjusting propeller pitch; the position motor for the engine-tilt function; and the motor for cooling-system operation. The high-voltage system is a combination of wires, power-conditioning components, and connectors that couple an energy source to an electric engine, associated motors, and a controller.

The technology required to provide energy through these high-voltage and high-current electronic components introduces potential hazards that do not exist in aircraft engines that operate using aviation fuel. For example, high-voltage transmission lines, electromagnetic fields, magnetic materials, and high-speed electrical switches form the electric engine’s physical properties. Operating at these high power levels also exposes the electric engines to potential failures, which could adversely affect safety, and that are not common to aircraft engines that operate using aviation fuel.


Part 35 contains airworthiness standards to ensure that uninstalled propellers meet the minimum level of safety that the FAA deems acceptable. Part 35 requirements are appropriate for propellers that are installed on conventional airplanes, type certificated under part 23 or part 25, that have construction and blade-pitch actuation methods typically found on such airplanes.

Emerging electric-powered and hybrid electric-powered aircraft, especially electric powered-lift that are intended for “air taxi” type operations in and near urban areas and capable of vertical and short takeoff and landing, often feature propellers designed for both horizontal thrust and vertical lift. In addition, propeller blade-pitch actuation for such aircraft typically is performed electrically, and is more extensively integrated into the aircraft’s propulsion and flight-control system compared to conventional airplanes type certificated under part 23 or part 25.

Propellers are integral parts of a variety of airplane propulsion systems and, until the advent of electric engines, have been subjected to the forces of fossil-fuel-powered reciprocating and turbine combustion engines. Electric engines present different considerations due to the increased torque and potentially higher revolutions per minute.

The most basic requirement, for all conventional part 23 and 25 aircraft as well as the Joby JAS4-1 aircraft, is to reduce the risk of propeller failure or release of debris to the occupants and critical aircraft structures and components to an acceptable level. Features and characteristics of propellers must ensure that they are safe for the certification application requested.

These proposed airworthiness criteria would require functional engine demonstrations, including feathering, negative torque, negative thrust, and reverse-thrust operations, as appropriate, using a representative propeller. The applicant may conduct these demonstrations as part of the endurance and durability demonstrations.


These airworthiness criteria, established under the provisions of § 21.17(b), are applicable to the Joby Model JAS4-1 powered-lift. Should Joby wish to apply these airworthiness criteria to other powered-lift models, it must submit a new application for a type certificate.

Proposed Airworthiness Criteria

The FAA proposes to establish the following airworthiness criteria for type certification of the Joby Model JAS4-1 powered-lift. The FAA proposes that compliance with the following criteria will provide an equivalent level of safety to existing rules.

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