U.S. Department of Transportation Federal Aviation Administration
Advisory Circular Subject: Type Certification—Powered-lift Date: AC No: 21.17-4 Initiated By: AIR-600 1 PURPOSE.
1.1 This advisory circular (AC) provides guidance for the type, production, and airworthiness certification of powered-lift. This AC also designates the airworthiness criteria in appendix A as an acceptable means, but not the only means, of showing compliance with title 14 of the Code of Federal Regulations (14 CFR) 21.17(b) for Federal Aviation Administration (FAA) type certification of certain powered-lift.
1.2 This is a guidance document. Its content is not legally binding in its own right and will not be relied upon by the Department as a separate basis for affirmative enforcement action or other administrative penalty. Conformity with the guidance document is voluntary only. Nonconformity will not affect rights and obligations under existing statutes and regulations.
2 REL ATED MATERIAL.
2.1 Title 14, Code of Federal Regulations.
The full text of these regulations is available at https://www.ecfr.gov.
• P art 1 , Definitions and Abbreviations.
• Part 21, Certification Procedures for Products and Articles.
• Part 23, Airworthiness Standards: Normal Category Airplanes.
• Part 25, Airworthiness Standards: Transport Category Airplanes.
• Part 27, Airworthiness Standards: Normal Category Rotorcraft.
• Part 29, Airworthiness Standards: Transport Category Rotorcraft.
AC 21.17-4
• Part 33, Airworthiness Standards: Aircraft Engines.
• Part 35, Airworthiness Standards: Propellers.
• Part 36, Noise Standards: Aircraft Type and Airworthiness Certification.
• Part 39, Airworthiness Directives.
• Part 43, Maintenance, Preventive Maintenance, Rebuilding, and Alteration.
• Part 45, Identification and Registration Marking.
• Part 91, General Operating and Flight Rules.
• Part 145, Repair Stations.
• Part 183, Representatives of the Administrator.
2.2 FAA Publications.
T he following publications are related to the guidance in this AC. You should refer to the latest version for guidance, which is available on the Dynamic Regulatory System (DRS) website at ht tps://drs.faa.gov/browse .
• Order 8110.4, Type Certification.
• O rder 8110.48, How to Establish the Certification Basis for Changed Aeronautical Products.
• Order 8110.54, Instructions for Continued Airworthiness Responsibilities, Requirements, and Contents
• Order 8110.112, Standardized Procedures for Usage of Issue Papers and Development of Equivalent Levels of Safety Memorandums.
• Order 8120.22, Production Approval Procedures.
• Order 8120.23, Certificate Management of Production Approval Holders.
• Order 8130.2, Airworthiness Certification of Aircraft.
• AC 21-43, Production Under 14 CFR Part 21, Subparts F, G, K, and O.
• AC 21.101-1, Establishing the Certification Basis of Changed Aeronautical Products.
• AC 45-2, Identification and Registration Marking.
• PS-AIR-21.17-01, Safety Continuum for Powered-lift.
AC 21.17-4 3 BACKGROUND.
3.1 Part 21 provides procedures for the type certification and airworthiness certification of special classes of aircraft. Special classes of aircraft include gliders and powered gliders, airships, powered-lift, and other kinds of aircraft, which would be eligible for a standard airworthiness certificate but for which no airworthiness standards have as yet been established as a separate part of 14 CFR chapter I, subchapter C. Airworthiness standards for these special classes of aircraft are the portions of the requirements in parts 23, 25, 27, 29, 31, 33, and 35 of 14 CFR 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.
3.2 Powered-lift is defined in part 1 as a heavier-than-air aircraft capable of vertical takeoff, vertical landing, and low speed flight that depends principally on engine-driven lift devices or engine thrust for lift during these flight regimes and on nonrotating airfoil(s) for lift during horizontal flight. Powered-lift have characteristics of both an airplane and a rotorcraft. They have the capability to function as a rotorcraft for takeoff and landing and as an airplane during cruise flight. This combination of lift capabilities creates the potential for increased speeds and duration, compared to rotorcraft, during the enroute portion of the flight.
3.3 Recent applications for the type certification of powered-lift have proposed passenger seating configurations of six or less, weighing 12,500 pounds or less, and utilizing battery-powered electric engines for propulsion. For each of these projects, the FAA has published the proposed airworthiness criteria, along with an explanation of its equivalency determination, in the Federal Register for public notice and comment.
3.4 The FAA used its experience with those recent powered-lift applications to develop the criteria in appendix A. This AC establishes a more efficient path in designating the type certification basis for certain powered-lift p rojects, as the FAA will not need to announce the criteria for each project in the Federal Register for notice and comment for those designs for which the FAA has deemed it appropriate to use the criteria in this AC with no additions or changes.
4 DISCUSSION.
4.1 Appendix A of this AC contains design criteria found acceptable to the FAA for type certification of powered-lift that meet the applicability in paragraph 5.2. These airworthiness criteria were developed from the airworthiness standards in parts 23, 27, 33, and 35 and include definitions and performance-based safety objectives specific for powered-lift and the installed electric engines and propellers.
4.2 In the event that the airworthiness criteria prescribed in appendix A of this AC are inadequate or otherwise inappropriate as a certification basis of a specific powered-lift due to its unique design or design features, the FAA may develop other criteria using the process in paragraph 7.2 of this AC.
AC 21 .17-4
4.3 For new projects, applicants may propose using airworthiness criteria from the certification basis of previously approved powered-lift designs. Such a proposal should be evaluated by the FAA for currency based upon advancement of the state-of-the-art powered-lift design, service experience, and amendments to relevant regulations, such as parts 23, 27, 33, and 35. This applies regardless of whether the applicant is proposing to reuse criteria from its prior project or from the project of another applicant.
4.4 Equivalent level of safety findings may be appropriate in lieu of changes to the criteria in appendix A of this AC. In such cases, a showing of equivalent safety should be proposed by the applicant and approved using the process as discussed in FAA Orders 8110.4 and 8110.112. Such equivalent safety findings become part of the type certification basis and are listed on the type certificate data sheet (TCDS).
4.5 The performance-based airworthiness criteria in appendix A capture the safety intent of each requirement. For each criteria, the specific means of compliance, proposed by the applicant and accepted by the FAA, will be a detailed design standard that, if met, accomplishes the safety intent of the criteria. The FAA will accept means of compliance found by the agency to be appropriate and applicable to the specific powered-lift design.
4.6 The airworthiness criteria in appendix A incorporate an “essential performance” approval, as well as an optional “increased performance” approval with greater aircraft performance capability requirements. Applicants must meet either the essential or increased performance requirements. Alternatively, applicants may propose designs that are approved for both essential and increased performance with appropriate and different operating limitations.
5 APPLICABILITY.
5.1 The procedures in this AC apply to all powered-lift proposed to be type certificated as special class under § 21.17(b).
5.2 The airworthiness criteria in appendix A of this AC apply to powered-lift with a maximum gross weight of 12,500 pounds or less, a passenger seating configuration of six or less, and battery-powered electric engine driven propellers for propulsion.
AC 21.17-4 6 TYPE CERTIFICATION PROCEDURES.
6.1 A pplication for Type Certification.
6.1.1 Application for type certification must be made in accordance with the requirements of § 21.15 as described in Order 8110.4.
6.1.2 An application for type certification of a powered-lift that meets the applicability of this AC is effective for 3 years as specified in § 21.17(c), unless an applicant shows at the time of application that the product requires a longer period of time for design, development, and testing, and the Administrator approves a longer period.
6.2 Type Certification Basis.
The type certification basis reflected on the TCDS will be stated as 14 CFR 21.17(b), noting that the criteria in AC 21.17-4 appendix A (including the revision or publication date), or other criteria as appropriate, were applied. If the engine and propeller are approved as part of the aircraft type design using the airworthiness criteria of appendix A subparts I and J, these criteria will also be noted on the TCDS. Otherwise, the engine and propeller model and type certificate numbers will be listed on the TCDS.
Additionally, any equivalent level of safety findings and operating limitations (such as visual flight rules only) will be included in accordance with § 21.41.
6.3 Type Certification Procedures.
6.3.1 The general and procedural regulations of part 21, subparts A and B, are applicable to the type certification of powered-lift as special class aircraft.
6.3.2 Powered-lift type certificate holders are required to report failures, malfunctions, and defects under § 21.3. These reporting requirements also apply to the engine and propeller when they are approved as a part of the powered-lift type certificate.
6.4 Flight Testing.
6.4.1 Function and reliability (F&R) testing is required for most aircraft under § 21.35(b). An applicant must conduct flight tests for the FAA to determine that the aircraft, its components, and its equipment are reliable and function properly. F&R testing provides reasonable assurance in the propulsion and aircraft systems during operations. F&R testing is typically conducted on a dedicated aircraft with a complement of approved engines over a series of flights representing the operational spectrum of the proposed design.
6.4.2 Many powered-lift incorporate highly complex, integrated designs with distributed propulsion systems and flight control effectors. These aircraft no longer fit within the definitions and underlying assumptions used in establishing F&R testing requirements under § 21.35. Because powered-lift lack a pedigree of operational experience and demonstrated reliability, the FAA finds a need for more thorough flight testing. As such, the FAA expects to use at least 300 hours of operation, similar to the requirement in § 21.35(f)(1), as the baseline for powered-lift. However, the amount of dedicated 5
AC 21.17-4 F&R testing may be adjusted through other means, such as representative rig and bench tests with agreement from the FAA.
6.5 Instructions for Continued Airworthiness.
Instructions for Continued Airworthiness (ICA) are required for powered-lift in accordance with § 21.50.
6.6 Provisional Type Certificates.
The procedural requirements for the issuance of provisional type certificates in part 21, subpart C, are applicable to powered-lift.
6.7 Changes to Powered-Lift Type Certificates.
The procedural requirements of part 21, subparts D and E, are applicable for approving changes to type certificates issued for powered-lift designs, including the issuance of supplemental type certificates.
6.8 Production Approvals.
The procedural requirements in part 21, subparts F, G, K, and O for production approvals, including parts manufacturer approvals and technical standard order authorizations, are applicable to powered-lift and articles installed on powered-lift.
6.9 A irworthiness Certification.
The procedural requirements for the issuance of an airworthiness certificate in part 21, subparts H and I, apply to powered-lift. To receive a standard airworthiness certificate under § 21.183(a) or (b), the powered-lift must have a type certificate. Powered-lift will be issued standard airworthiness certificates in the same manner as any other aircraft receiving the same certificate. Since powered-lift are type certificated as special class of aircraft, the category in FAA Form 8100-2, block 4, will be identified as “Poweredlift—Special Class.” Powered-lift will also be issued special airworthiness certificates identified in § 21.175(b), using FAA Form 8130-7, in the same manner as any other aircraft receiving the same certificate.
AC 21.17-4
6.10 Imported Products.
The import product type certification requirements of § 21.29 and import product airworthiness certification requirements of § 21.183(c) are applicable to powered-lift.
The FAA type certification basis for import powered-lift will also be shown on the TCDS as 14 CFR 21.17(b).
6.11 Use of Designees.
FAA designees may be used to support certification activities for special classes of aircraft (i.e. 14 CFR 21.17(b)). However, the designee’s certificate letter of authority (CLOA) may require expansion or additional authorities before a designee can be used on a special class project (e.g., to include any applicable requirements for the 14 CFR
21.17(b) project). Refer to FAA Order 8000.95 (Designee Management Policy) and the applicable volume based on designee type for specific information. If more guidance is needed, contact the FAA project team.
6.12 Airworthiness Directives.
Powered-lift are products as defined in § 39.3. Accordingly, airworthiness directives may be issued to address an unsafe condition in a powered-lift in accordance with part
39.
6.13 Major Repairs and Alterations.
Field approvals under the provisions of parts 43 and 145 may also be used to approve an altered or repaired individual powered-lift for return to service.
6.14 Identification and Registration Marking.
The identification and registration marking requirements of part 45 are applicable to powered-lift. AC 45-2, Identification and Registration Marking, provides guidance and information concerning the identification and marking requirements of part 45.
7 ACCEPTABLE CRITERIA.
7.1 Appendix A.
Applicants may propose using the airworthiness criteria in appendix A as the certification basis for a powered-lift that meets the applicability in paragraph 5.2. For designs that the FAA determines need use only the criteria in appendix A for the certification basis with no additions or changes, the FAA does not need to announce the criteria in the Federal Register for notice and comment.
7.2 Other Airworthiness Criteria.
In the event the Administrator finds that compliance with airworthiness criteria in appendix A is inadequate or otherwise inappropriate as a certification basis of a powered-lift due to its unique, novel, or unusual design or design features, the Administrator may require an applicant to comply with alternate or additional criteria.
Additionally, applicants may propose other criteria with their certification plan per the process in FAA Order 8110.4, Type Certification. If found to be acceptable by the 7
AC 21.17-4 Administrator, the FAA will propose and request public comment on any alternate or additional criteria in the Federal Register. After taking into account any comments received, the FAA will, as appropriate, publish final airworthiness criteria in the Federal Register. This final Federal Register notice will include disposition of the public comments. FAA approval of an applicant’s proposed airworthiness criteria is required before the airworthiness criteria may be included in the certification basis of a powered-lift.
7.3 Special conditions under § 21.16 are not used as part of the certification basis for special classes of aircraft. An applicant for a powered-lift may propose to use alternate or new criteria instead of, or in addition to, the criteria in appendix A due to a unique, novel, or unusual design feature. However, it would be inappropriate to use special conditions. Instead, the FAA will use the process in paragraph 7.2 to include the alternate or new criteria in the special class certification basis.
7.4 Engine and Propeller Type Certification Options.
Powered-lift applicants have two options for the certification of engines and propellers.
7.4.1 The applicant may install type-certificated engines or propellers under parts 33 or 35, respectively. However, to be eligible for installation, type certificated engines or propellers that are intended to be used as engine-driven lift devices and engines powering these devices must meet the additional airworthiness criteria in appendix A.
7.4.2 The applicant may seek approval of the engines or propellers under the aircraft type certificate using the airworthiness criteria of appendix A, subparts I and J. In this case, ICA for the aircraft, engines, and propellers may be shown with the aircraft in a single aircraft ICA or the applicant may provide individual ICA for the aircraft, engines, and propellers in accordance with Order 8110.54.
7.5 Noise Regulations.
The FAA is mandated to establish noise standards and regulations that ensure public safety regarding aircraft noise. The FAA will examine each powered-lift application and determine whether the existing part 36 requirements are appropriate as a noise certification basis, as it does for all noise certification applicants. Part 36 does contain noise standards for tiltrotors, a class of aircraft within the powered-lift category. If the FAA determines that the part 36 noise standards are not sufficient for a particular powered-lift design, the FAA will prescribe a rule of particular applicability and publish the specific noise requirements for that powered-lift in the Federal Register.
8 WHERE TO FIND THIS AC.
You can find this AC on the FAA’s website at ht tps://www.faa.gov/regulations_policies/advisory_circulars and the DRS website at https://drs.faa.gov/.
9 SUGGESTIONS FOR IMPROVING THIS AC.
If you have suggestions for improving this AC, you may use the FAA Form 1320-73, Advisory Circular Feedback Form, at the end of this AC.
DANIEL J.
Digitally signed by DANIEL J. ELGAS ELGAS Date: 2025.07.18 16:00:38 -04'00' Daniel J. Elgas Director, Policy Standards Division Aircraft Certification Service 9
AC 21.17-4 Appendix A Appendix A. Airworthiness Criteria for Powered-Lift
AIRCRAFT LEVEL REQUIREMENTS PL.1457 Cockpit voice recorders.
(a) Each cockpit voice recorder required by the operating rules of title 14, chapter I must be approved and must be installed so that it will record the following:
(1) Voice communications transmitted from or received in the aircraft by radio.
(2) Voice communications of flightcrew members on the flightdeck.
(3) Voice communications of flightcrew members on the flightdeck, using the aircraft's interphone system.
(4) Voice or audio signals identifying navigation or approach aids introduced into a headset or speaker.
(5) Voice communications of flightcrew members using the passenger loudspeaker system, if there is such a system and if the fourth channel is available in accordance with the requirements of paragraph (c)(4)(ii) of this section.
(6) If datalink communication equipment is installed, all datalink communications, using an approved data message set. Datalink messages must be recorded as the output signal from the communications unit that translates the signal into usable data.
(b) The recording requirements of paragraph (a)(2) of this section must be met by installing a cockpit-mounted area microphone, located in the best position for recording voice communications originating at the first and second pilot stations and voice communications of other crewmembers on the flightdeck when directed to those stations. The microphone must be so located and, if necessary, the preamplifiers and filters of the recorder must be so adjusted or supplemented, so that the intelligibility of the recorded communications is as high as practicable when recorded under flight cockpit noise conditions and played back. Repeated aural or visual playback of the record may be used in evaluating intelligibility.
(c) Each cockpit voice recorder must be installed so that the part of the communication or audio signals specified in paragraph (a) of this section obtained from each of the following sources is recorded on a separate channel:
(1) For the first channel, from each boom, mask, or handheld microphone, headset, or speaker used at the first pilot station.
(2) For the second channel from each boom, mask, or handheld microphone, headset, or speaker used at the second pilot station.
(3) For the third channel—from the cockpit-mounted area microphone.
(4) For the fourth channel from:
(i) Each boom, mask, or handheld microphone, headset, or speaker used at the station for the third and fourth crewmembers.
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AC 21.17-4 Appendix A
(ii) If the stations specified in paragraph (c)(4)(i) of this section are not required or if the signal at such a station is picked up by another channel, each microphone on the flightdeck that is used with the passenger loudspeaker system, if its signals are not picked up by another channel.
(5) And that as far as is practicable all sounds received by the microphone listed in paragraphs
(c)(1), (2), and (4) of this section must be recorded without interruption irrespective of the position of the interphone-transmitter key switch. The design shall ensure that sidetone for the flightcrew is produced only when the interphone, public address system, or radio transmitters are in use.
(d) Each cockpit voice recorder must be installed so that:
(1)
(i) It receives its electrical power from the bus that provides the maximum reliability for operation of the cockpit voice recorder without jeopardizing service to essential or emergency loads.
(ii) It remains powered for as long as possible without jeopardizing emergency operation of the aircraft.
(2) There is an automatic means to simultaneously stop the recorder and prevent each erasure feature from functioning, within 10 minutes after crash impact.
(3) There is an aural or visual means for preflight checking of the recorder for proper operation.
(4) Any single electrical failure external to the recorder does not disable both the cockpit voice recorder and the flight data recorder.
(5) It has an independent power source—
(i) That provides 10 ±1 minutes of electrical power to operate both the cockpit voice recorder and cockpit-mounted area microphone;
(ii) That is located as close as practicable to the cockpit voice recorder; and
(iii) To which the cockpit voice recorder and cockpit-mounted area microphone are switched automatically in the event that all other power to the cockpit voice recorder is interrupted either by normal shutdown or by any other loss of power to the electrical power bus.
(6) It is in a separate container from the flight data recorder when both are required. If used to comply with only the cockpit voice recorder requirements, a combination unit may be installed.
(e) The recorder container must be located and mounted to minimize the probability of rupture of the container as a result of crash impact and consequent heat damage to the recorder from fire.
(1) Except as provided in paragraph (e)(2) of this section, the recorder container must be located as far aft as practicable, but need not be outside of the pressurized compartment, and may not be located where aft-mounted engines may crush the container during impact.
(2) If two separate combination digital flight data recorder and cockpit voice recorder units are installed instead of one cockpit voice recorder and one digital flight data recorder, the combination unit that is installed to comply with the cockpit voice recorder requirements may be located near the cockpit.
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AC 21.17-4 Appendix A
(f) If the cockpit voice recorder has a bulk erasure device, the installation must be designed to minimize the probability of inadvertent operation and actuation of the device during crash impact.
(g) Each recorder container must—
(1) Be either bright orange or bright yellow;
(2) Have reflective tape affixed to its external surface to facilitate its location under water; and
(3) Have an underwater locating device, when required by the operating rules of this chapter, on or adjacent to the container, which is secured in such manner that they are not likely to be separated during crash impact.
PL.1459 Flight data recorders.
(a) Each flight recorder required by the operating rules of title 14, chapter I must be installed so that—
(1) It is supplied with airspeed, altitude, and directional data obtained from sources that meet the aircraft level system requirements and the functionality specified in PL.2500;
(2) The vertical acceleration sensor is rigidly attached, and located longitudinally either within the approved center of gravity limits of the aircraft, or at a distance forward or aft of these limits that does not exceed 25 percent of the aircraft's mean aerodynamic chord;
(3)
(i) It receives its electrical power from the bus that provides the maximum reliability for operation of the flight data recorder without jeopardizing service to essential or emergency loads;
(ii) It remains powered for as long as possible without jeopardizing emergency operation of the aircraft;
(4) There is an aural or visual means for preflight checking of the recorder for proper recording of data in the storage medium;
(5) Except for recorders powered solely by the engine-driven electrical generator system, there is an automatic means to simultaneously stop a recorder that has a data erasure feature and prevent each erasure feature from functioning, within 10 minutes after crash impact;
(6) Any single electrical failure external to the recorder does not disable both the cockpit voice recorder and the flight data recorder; and
(7) It is in a separate container from the cockpit voice recorder when both are required. If used to comply with only the flight data recorder requirements, a combination unit may be installed. If a combination unit is installed as a cockpit voice recorder to comply with PL.1457(e)(2), a combination unit must be used to comply with this flight data recorder requirement.
(b) Each non-ejectable record container must be located and mounted so as to minimize the probability of container rupture resulting from crash impact and subsequent damage to the record from fire. In meeting this requirement, the record container must be located as far aft as practicable, but need not be aft of the pressurized compartment, and may not be where aftmounted engines may crush the container upon impact.
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