146 

14 CFR Ch. I (1–1–14 Edition) 

§ 21.91 

the type certificate for the number of 
hours found necessary by the FAA. 

[Amdt. 21–12, 31 FR 13388, Oct. 15, 1966, as 
amended by Amdt. 21–66, 54 FR 34329, Aug. 18, 
1989] 

Subpart D—Changes to Type 

Certificates 

S

OURCE

: Docket No. 5085, 29 FR 14567, Oct. 

24, 1964, unless otherwise noted. 

§ 21.91

Applicability. 

This subpart prescribes procedural 

requirements for the approval of 
changes to type certificates. 

§ 21.93

Classification of changes in 

type design. 

(a) In addition to changes in type de-

sign 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 ap-
preciable effect on the weight, balance, 
structural strength, reliability, oper-
ational characteristics, or other char-
acteristics affecting the airworthiness 
of the product. All other changes are 
‘‘major changes’’ (except as provided in 
paragraph (b) of this section). 

(b) 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: 

(1) Transport category large air-

planes. 

(2) 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— 

(i) Gear down flight with one or more 

retractable landing gear down during 
the entire flight, or 

(ii) Spare engine and nacelle carriage 

external to the skin of the airplane 
(and return of the pylon or other exter-
nal mount), or 

(iii) Time-limited engine and/or na-

celle 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 provi-
sions of Part 36 of this chapter is shown 
for that change in type design. 

(3) Propeller driven commuter cat-

egory and small airplanes in the pri-
mary, normal, utility, acrobatic, trans-
port, and restricted categories, except 
for airplanes that are: 

(i) Designated for ‘‘agricultural air-

craft operations’’ (as defined in § 137.3 
of this chapter, effective January 1, 
1966) to which § 36.1583 of this chapter 
does not apply, or 

(ii) Designated for dispensing fire 

fighting materials to which § 36.1583 of 
this chapter does not apply, or 

(iii) U.S. registered, and that had 

flight time prior to January 1, 1955 or 

(iv) Land configured aircraft recon-

figured with floats or skis. This recon-
figuration does not permit further ex-
ception from the requirements of this 
section upon any acoustical change not 
enumerated in § 21.93(b). 

(4) Helicopters except: 
(i) Those helicopters that are des-

ignated exclusively: 

(A) For ‘‘agricultural aircraft oper-

ations’’, as defined in § 137.3 of this 
chapter, as effective on January 1, 1966; 

(B) For dispensing fire fighting mate-

rials; or 

(C) For carrying external loads, as 

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

(ii) Those helicopters modified by in-

stallation or removal of external equip-
ment. For purposes of this paragraph, 
‘‘external equipment’’ means any in-
strument, mechanism, part, apparatus, 
appurtenance, or accessory that is at-
tached to, or extends from, the heli-
copter 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: 

(A) Addition or removal of external 

equipment; 

(B) 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 equip-
ment or external loads, or to facilitate 
the safe operation of the helicopter 

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