Chapter 4. Other Considerations
1. Design Related Operating Requirements. The use of exceptions under 14 CFR § 21.101 is not intended to alleviate or preclude compliance with operating regulations (such as 14 CFR part 121) that prescribes compliance with a specific or later amendment of the airworthiness (design-related) requirements.
2. Excepted Products under 14 CFR § 21.101(c).
a. As an applicant for a design change to an excepted product, you may show that the changed product complies with the existing certification basis incorporated by reference in the TC. If we, the FAA, find that the change is significant “in an area”, we will require compliance with a later amendment to the existing certification basis in the TC that applies to that affected area and any regulation we find is directly related. For excepted products, changes that meet one of the following criteria, in the area of change, are considered significant if:
The general configuration or the principles of construction are not retained, or
The assumptions used for certification of the area to be changed do not remain valid.
b. However, we may allow you to comply with an earlier amendment to the requirement initially designated or with the existing certification basis if we agree to your justification.
c. For a design change to an excepted product that contains new features, which are not covered in the existing certification basis, we will designate the applicable airworthiness requirements at the appropriate amendment level, beginning with the existing certification basis and progressing to the most appropriate later amendment level for the change. For a change that contains new design features that are novel and unusual for which there are no later applicable requirements at a later amendment level, we will designate special conditions per 14 CFR § 21.101(d).
d. The exception provided for excepted products under 14 CFR § 21.101(c) applies at the aircraft level only. Design changes to type certificated engines and propellers installed on these excepted aircraft are assessed as separate products using 14 CFR §§ 21.101(a) and (b).
3. Special Conditions, 14 CFR § 21.101(d). Title 14 CFR § 21.101(d) allows for the application of special conditions, or for changes to existing special conditions, to address the changed designs where the proposed certification basis does not provide adequate standards for an area, system, component, equipment, or appliance related to the change. The objective is to achieve a level of safety consistent with that provided for other areas, systems, components, equipment, or appliances affected by the change by the other requirements of the proposed certification basis. The application of special conditions to a design change is not, in itself, a reason for it to be classified as either a substantial change or a significant change. When the change is significant with earlier requirements allowed through exceptions, or not significant, the
23 level of safety intended by the special conditions must be consistent with the agreed certification basis.
4. Effective Period for an Application to Change a TC. (14 CFR § 21.101(e)) Per 14 CFR §
21.101(e), an application for, or a change to, a TC for transport category aircraft is effective for 5 years, and an application for a change to any other TC is effective for 3 years. This is intended to ensure that the certification basis for the changed product is as current as practical. This is consistent with the requirements of 14 CFR § 21.17 for a new TC and defines the process of updating the certification basis if these time limits are exceeded.
a. If a design change has not been approved, or if it is clear that the change will not be approved, within the time limit, you may do either of the following:
(1) File for a new application, or
(2) File for an extension to the original application.
b. If you request an extension to the application date, and the product change is significant, a new certification basis is required. The new certification basis requires the additional latest regulations effective through the new application extension date. However, you may use earlier regulations by documenting justification that the latest regulations for the change would not contribute materially to the level of safety or would be impractical.
c. If the product change is not significant, the existing certification basis can continue to be used as the basis for product certification. However, if additional design changes are made to the product, and the existing certification basis for the change is found to be inadequate, the new certification basis will require later appropriate standards.
5. Other Category Aircraft (14 CFR § 21.101(f)). For aircraft type certificated under 14 CFR §§ 21.17(b), 21.24, 21.25, and 21.27, the certification basis for the changed product will consist of the amendment levels of the applicable regulations that we find appropriate for the change in effect on the date of application for the change. When selecting a certification basis for a change, you can propose compliance to an earlier amendment using the provisions of 14 CFR § 21.101(b). The exceptions in 14 CFR § 21.101(c) do not apply to categories of products defined in 14 CFR § 21.101(f).
a. Special Classes Aircraft. For special classes of aircraft (for example, gliders, airships, etc.) including any installed engines and propellers certificated in accordance with 14 CFR § 21.17(b), the applicable requirements will be portions of those other airworthiness requirements in 14 CFR parts 23, 25, 27, 29, 31, 33, and 35 we find appropriate for the aircraft and applicable to the specific type design, or such airworthiness criteria that provide an equivalent level of safety to those parts.
b. Primary Category Aircraft. For primary category aircraft certificated under 14 CFR § 21.24, the applicable airworthiness requirements are in 14 CFR parts 23, 27, 31, 33, and 35, or such other requirements as we may find appropriate. These requirements must be
24 applicable to the specific design and intended use of the aircraft and provide a level of safety acceptable to us.
c. Restricted Category Aircraft – Civil Derived. For aircraft certificated in the restricted category under 14 CFR § 21.25(a)(1), the application of the latest regulations typically would be considered not to contribute materially to the level of safety or be practical for its intended use. However, if the regulations incorporated by reference in the TC do not provide an adequate level of safety for its intended use, the application of a later regulation will be required.
(1) Features of the changed product that ar e “novel” or “unusual” to the original certificated restricted category product may be assessed against a later requirement that addresses the feature. In this case, the requirements in effect at the time of the existing restricted category TC may be viewed as a starting point, with subsequent amendments being examined, if necessary, to arrive at a requirement that provides an appropriate level of safety.
(2) For the installation of turbo propeller engines instead of reciprocating engines, either in a restricted category aircraft that was originally certificated based on satisfactory military service experience or in a restricted category aircraft for which the original certification basis did not contain regulations for turbine engine installations, later amendments will be used to provide an appropriate level of safety for its intended operation.
d. Restricted Category Aircraft - Military Derived. Aircraft type certificated in the restricted category under 14 CFR § 21.25(a)(2) are accepted on the basis of U.S. Military use instead of showing compliance with airworthiness standards in 14 CFR Chapter 1. Many of these aircraft were not certificated to a specific set of airworthiness standards, therefore, an appropriate equivalent civilian certification basis could be determined using the table in 14 CFR § 21.27 for surplus military aircraft. Title 14 CFR § 21.101(f) requires the application of the latest amendments to significant changes to these products. However, since the latest amendments may not be appropriate for the aircraft’s intended use, earlier regulations are acceptable. They may not predate the equivalent certification basis. If these regulations do not include design standards applicable to the change, later regulations appropriate to the product category will be applied. The goal is to maintain a level of safety appropriate for the aircraft’s intended use.
e. Surplus military aircraft. Aircraft type certificated under 14 CFR § 21.27 are entitled to a TC in the normal, utility, acrobatic, commuter, or transport category, except those certified under 21.27(b). These aircraft were designed and constructed in the United States, accepted for operational use, and declared surplus by the U.S. Armed Forces. These aircraft may be counterparts, and are considered equivalent, to the previously civil certificated aircraft. Product changes or modifications to these aircraft, including their engines, propellers, and related accessories, are certificated in the same manner as their civil counterparts.
f. Limited category aircraft. Limited category aircraft are surplus military aircraft, mostly from World War II, that were type certificated under Civil Aviation Regulation (CAR) part 9 for use other than air transport. These aircraft were not intended to carry persons or property for hire, and normally were accepted based on their previous military qualifications and
25 service record. A change to aircraft not supported by the military service history must comply with appropriate airworthiness standards. The level of safety associated with earlier standards may be acceptable for limited category aircraft.
6. Clarification of 14 CFR 21.101 (g) (Part 26 Requirements).
a. Title 14 CFR part 26 establishes requirements for support of continued airworthiness of and safety for transport category airplanes. If you are an applicant, you must comply with each applicable provision of 14 CFR part 26, unless you have elected or were required to comply with a corresponding amendment to 14 CFR part 25 that was issued on or after the date of the applicable part 26 provision. Title 14 CFR § 21.101(g) does not allow you to use an exception under 14 CFR § 21.101(b) for relief from complying with the applicable provisions of 14 CFR part 26.
b. The language in 14 CFR § 21.101(g) also recognizes that future 14 CFR part 25 amendments will be issued after the requirements in 14 CFR part 26 are established. Note that you could be required to comply with a later amendment to 14 CFR part 25 per 14 CFR § 21.101(a). However, earlier part 25 requirements, but not earlier than those established in the existing certification basis that include part 26 requirements, could be justified using the exceptions in 14 CFR § 21.101(b).
7. Documentation. All changes that result in a revision to the product’s certification basis must be reflected on the amended TCDS or STC. The resulting certification basis must be retained as it forms part of the compliance record required by FAA Order 8110.4, Type Certification.
Appendix A A-1 Appendix A. Classification of Changes
The following examples of substantial, significant and not significant changes are adopted by the FAA, European Aviation Safety Agency (EASA) and Transport Canada Civil Aviation (TCCA) through an international collaboration. The classification may change due to cumulative effects and/or combinations of individual changes. The “NA” in the substantial example tables indicates that the automatic classification criteria in the heading are “Not Applicable” at the “21.19 Substantial Evaluation” phase.
Table 1. Examples of Changes for Small Airplanes (Part 23) The following examples are for SUBSTANTIAL changes for Small Airplanes (Part 23):
Description of change Is there a change to the general configuration?
14 CFR §
21.101(b)(1)(i) Is there a change to the principles of construction?
14 CFR §
21.101(b)(1)(i) Have the assumptions used for certification been invalidated?
14 CFR §
21.101(b)(1)(ii) Notes Change in wing location (tandem, forward, canard, high/low)
NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required.
Fixed wing to tilt wing
NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required.
Increase or decrease in the number of engines NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required.
Appendix A A-2 The following examples are for SUBSTANTIAL changes for Small Airplanes (Part 23):
Description of change Is there a change to the general configuration?
14 CFR §
21.101(b)(1)(i) Is there a change to the principles of construction?
14 CFR §
21.101(b)(1)(i) Have the assumptions used for certification been invalidated?
14 CFR §
21.101(b)(1)(ii) Notes Replacement of piston or turbo-prop engines with turbojet or turbofan engines NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required.
Change in engine configuration (tractor/pusher)
NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required.
Increase from subsonic to supersonic flight regime NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required.
Change from an all metal airplane to all composite primary structure (fuselage, wing, empennage) NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required.
Appendix A A-3
The following examples are for SIGNIFICANT changes for Small Airplanes (Part 23):
Description of change Is there a change to the general configuration?
14 CFR §
21.101(b)(1)(i) Is there a change to the principles of construction?
14 CFR §
21.101(b)(1)(i) Have the assumptions used for certification been invalidated?
14 CFR §
21.101(b)(1)(ii) Notes Conventional tail to T-tail or V-tail, or vice versa
Yes No Yes Change in general configuration. Requires extensive structural, flying qualities and performance reinvestigation.
Requires a new aircraft flight manual (AFM) to address performance and flight characteristics.
Changes in wing configuration such as change in dihedral, changes in wing span, flap or aileron span, addition of winglets, or increase of more than 10 percent of the original wing sweep at the quarter chord Yes No Yes Change in general configuration. Likely requires extensive changes to wing structure. Requires a new aircraft flight manual (AFM) to address performance and flight characteristics.
Note: Small changes to wingtip are not significant changes. See table for not significant changes.
Changes to tail configuration such as the addition of tail strakes or angle of incidence of the tail Yes No Yes Change in general configuration. Likely requires extensive changes to tail structure.
Requires a new aircraft flight manual (AFM) to address performance and flight characteristics.
Note: Small changes to tail are not significant changes.
Appendix A A-4 The following examples are for SIGNIFICANT changes for Small Airplanes (Part 23):
Description of change Is there a change to the general configuration?
14 CFR §
21.101(b)(1)(i) Is there a change to the principles of construction?
14 CFR §
21.101(b)(1)(i) Have the assumptions used for certification been invalidated?
14 CFR §
21.101(b)(1)(ii) Notes Tricycle/tail wheel undercarriage change or addition of floats Yes No No Change in general configuration at product level. Principles of construction and certification assumptions remain valid.
Passenger to freighter configuration conversion which involves the introduction of a cargo door or an increase in floor loading of more than 20 percent, or provision for carriage of passengers and freight together.
Yes No Yes Change in general configuration affecting load paths, aeroelastic characteristics, aircraft related systems, etc.
Change in design assumptions.
Replace reciprocating engines with the same number of turbo-propeller engines where the operating envelope is expanded.
No No Yes Invalidates certification assumptions. Requires a new aircraft flight manual (AFM) to address performance and flight characteristics.
Addition of a turbocharger that changes the power envelope, operating range, or limitations.
No No Yes Invalidates certification assumptions due to changes in operating envelope and limitations.
Requires a new aircraft flight manual (AFM) to address performance and flight characteristics.
Appendix A A-5 The following examples are for SIGNIFICANT changes for Small Airplanes (Part 23):
Description of change Is there a change to the general configuration?
14 CFR §
21.101(b)(1)(i) Is there a change to the principles of construction?
14 CFR §
21.101(b)(1)(i) Have the assumptions used for certification been invalidated?
14 CFR §
21.101(b)(1)(ii) Notes The replacement of an engine of higher rated power or increase thrust would be considered significant if it would invalidate the existing substantiation, or would change the primary structure, aerodynamics, or operating envelope sufficiently to invalidate the assumptions of certification.
No Yes Yes Invalidates certification assumptions. Requires a new aircraft flight manual (AFM) to address performance and flight characteristics.
Likely changes to primary structure.
Requires extensive construction reinvestigation.
A change in the type of material, such as composites in place of metal, or one composite fiber material system with another (e.g., carbon for fiberglass), for primary structure would normally be assessed as a significant change.
No Yes Yes Change in principles of construction and design from conventional practices.
Likely change in design/certification assumptions.
Change involving appreciable increase in design speeds Vd, Vmo, Vc, or Va No No Yes Certification assumptions invalidated.
Requires a new aircraft flight manual (AFM) to address performance and flight characteristics.
Appendix A A-6 The following examples are for SIGNIFICANT changes for Small Airplanes (Part 23):
Description of change Is there a change to the general configuration?
14 CFR §
21.101(b)(1)(i) Is there a change to the principles of construction?
14 CFR §
21.101(b)(1)(i) Have the assumptions used for certification been invalidated?
14 CFR §
21.101(b)(1)(ii) Notes Short take off and landing (STOL) kit.
No No Yes Certification assumptions invalidated.
Requires a new aircraft flight manual (AFM) to address performance and flight characteristics.
A change in the rated power or thrust is likely to be regarded as significant if the design speeds are thereby changed so that compliance needs to be rejustified with a majority of requirements.
No No Yes Certification assumptions invalidated.
Requires a new aircraft flight manual (AFM) to address performance and flight characteristics.
Fuel state: such as compressed gaseous fuels, or fuel cells.
This could completely alter the fuel storage and handling systems and possibly affect the airplane structure.
No No Yes Changes in design/certification assumptions. Extensive alteration of fuel storage and handling systems.
Appendix A A-7 The following examples are for SIGNIFICANT changes for Small Airplanes (Part 23):
Description of change Is there a change to the general configuration?
14 CFR §
21.101(b)(1)(i) Is there a change to the principles of construction?
14 CFR §
21.101(b)(1)(i) Have the assumptions used for certification been invalidated?
14 CFR §
21.101(b)(1)(ii) Notes A design change that alters the aircraft flight characteristics or performance from the type design would normally be significant if it appreciably changes the kinematics or dynamics of the airplane.
No No Yes Certification assumptions invalidated.
Requires a new aircraft flight manual (AFM) to address performance and flight characteristics.
A change in the flight control concept for an aircraft, for example, to fly by wire (FBW) and sidestick control, or a change from hydraulic to electronically actuated flight controls, would in isolation normally be regarded as a significant change.
No No Yes Changes in design and certification assumptions. Requires extensive systems architecture and integration reinvestigation.
Requires a new aircraft flight manual (AFM).
Change to airplane’s cabin operating altitude, or operating pressure
No
No Yes An increase greater than 10% in maximum cabin pressure differential invalidates certification assumptions and the fundamental approach used in decompression, structural strength, and fatigue.
Appendix A A-8 The following examples are for SIGNIFICANT changes for Small Airplanes (Part 23):
Description of change Is there a change to the general configuration?
14 CFR §
21.101(b)(1)(i) Is there a change to the principles of construction?
14 CFR §
21.101(b)(1)(i) Have the assumptions used for certification been invalidated?
14 CFR §
21.101(b)(1)(ii) Notes Increase in cabin pressurization.
No No Yes Typically, a change greater than 10% in operational cabin pressure differential.
May require extensive airframe changes affecting load paths, fatigue evaluation, aeroelastic characteristics, etc. Invalidates design assumptions.
Addition of cabin pressurization system.
No Yes Yes Extensive airframe changes affecting load paths, fatigue evaluation, aeroelastic characteristics, etc. Invalidates design assumptions.
Changes in types and number of emergency exits or an increase in maximum certificated passenger capacity.
Yes No Yes Emergency egress requirements exceed those previously substantiated.
Invalidates assumptions of certification.