14 CFR § 21.93

In addition to changes in type design specified in paragraph (b) of this section, changes in type design are classified as minor and major. A “minor change” is one that has no appreciable effect on the weight, balance, structural strength, reliability, operational characteristics, or other characteristics affecting the airworthiness of the product. All other changes are “major changes” (except as provided in paragraph (b) of this section).

For the purpose of complying with Part 36 of this chapter, and except as provided in paragraphs (b)(2), (b)(3), and (b)(4) of this section, any voluntary change in the type design of an aircraft that may increase the noise levels of that aircraft is an “acoustical change” (in addition to being a minor or major change as classified in paragraph (a) of this section) for the following aircraft:

Transport category large airplanes.

Jet (Turbojet powered) airplanes (regardless of category). For airplanes to which this paragraph applies, “acoustical changes” do not include changes in type design that are limited to one of the following—

Gear down flight with one or more retractable landing gear down during the entire flight, or

Spare engine and nacelle carriage external to the skin of the airplane (and return of the pylon or other external mount), or

Time-limited engine and/or nacelle changes, where the change in type design specifies that the airplane may not be operated for a period of more than 90 days unless compliance with the applicable acoustical change provisions of Part 36 of this chapter is shown for that change in type design.

Propeller driven commuter category and small airplanes in the primary, normal, utility, acrobatic, transport, and restricted categories, except for airplanes that are:

Designated for “agricultural aircraft operations” (as defined in § 137.3 of this chapter, effective January 1, 1966) to which § 36.1583 of this chapter does not apply, or

Designated for dispensing fire fighting materials to which § 36.1583 of this chapter does not apply, or

U.S. registered, and that had flight time prior to January 1, 1955 or

Land configured aircraft reconfigured with floats or skis. This reconfiguration does not permit further exception from the requirements of this section upon any acoustical change not enumerated in § 21.93(b).

Helicopters except:

Those helicopters that are designated exclusively:

For “agricultural aircraft operations”, as defined in § 137.3 of this chapter, as effective on January 1, 1966;

For dispensing fire fighting materials; or

For carrying external loads, as defined in § 133.1(b) of this chapter, as effective on December 20, 1976.

Those helicopters modified by installation or removal of external equipment. For purposes of this paragraph, “external equipment” means any instrument, mechanism, part, apparatus, appurtenance, or accessory that is attached to, or extends from, the helicopter exterior but is not used nor is intended to be used in operating or controlling a helicopter in flight and is not part of an airframe or engine. An “acoustical change” does not include:

Addition or removal of external equipment;

Changes in the airframe made to accommodate the addition or removal of external equipment, to provide for an external load attaching means, to facilitate the use of external equipment or external loads, or to facilitate the safe operation of the helicopter with external equipment mounted to, or external loads carried by, the helicopter;

Reconfiguration of the helicopter by the addition or removal of floats and skis;

Flight with one or more doors and/or windows removed or in an open position; or

Any changes in the operational limitations placed on the helicopter as a consequence of the addition or removal of external equipment, floats, and skis, or flight operations with doors and/or windows removed or in an open position.

Tiltrotors.

For purposes of complying with part 34 of this chapter, any voluntary change in the type design of the airplane or engine which may increase fuel venting or exhaust emissions is an “emissions change.”

For the purpose of maintaining compliance with part 38 of this chapter, any voluntary change in the type design of an airplane that may increase the fuel efficiency metric value or the MTOM of that airplane is a “fuel efficiency change”, in addition to being a minor or major change as classified in paragraph (a) of this section.