658 

14 CFR Ch. I (1–1–14 Edition) 

§ 27.573 

time furnished under section A27.4 of 
appendix A. 

(d) 

Fail-safe evaluation. The following 

apply to fail-safe evaluation: 

(1) It must be shown that all partial 

failures will become readily detectable 
under inspection procedures furnished 
under section A27.4 of appendix A. 

(2) The interval between the time 

when any partial failure becomes read-
ily detectable under paragraph (d)(1) of 
this section, and the time when any 
such failure is expected to reduce the 
remaining strength of the structure to 
limit or maximum attainable loads 
(whichever is less), must be deter-
mined. 

(3) It must be shown that the interval 

determined under paragraph (d)(2) of 
this section is long enough, in relation 
to the inspection intervals and related 
procedures furnished under section 
A27.4 of appendix A, to provide a prob-
ability of detection great enough to en-
sure that the probability of cata-
strophic failure is extremely remote. 

(e) 

Combination of replacement time 

and failsafe evaluations. A component 
may be evaluated under a combination 
of paragraphs (c) and (d) of this sec-
tion. For such component it must be 
shown that the probability of cata-
strophic failure is extremely remote 
with an approved combination of re-
placement time, inspection intervals, 
and related procedures furnished under 
section A27.4 of appendix A. 

(Secs. 313(a), 601, 603, 604, and 605, 72 Stat. 752, 
775, and 778, (49 U.S.C. 1354(a), 1421, 1423, 1424, 
and 1425; sec. 6(c), 49 U.S.C. 1655(c))) 

[Amdt. 27–3, 33 FR 14106, Sept. 18, 1968, as 
amended by Amdt. 27–12, 42 FR 15044, Mar. 17, 
1977; Amdt. 27–18, 45 FR 60177, Sept. 11 1980; 
Amdt. 27–26, 55 FR 8000, Mar. 6, 1990] 

§ 27.573

Damage Tolerance and Fa-

tigue Evaluation of Composite 
Rotorcraft Structures. 

(a) Each applicant must evaluate the 

composite rotorcraft structure under 
the damage tolerance standards of 
paragraph (d) of this section unless the 
applicant establishes that a damage 
tolerance evaluation is impractical 
within the limits of geometry, 
inspectability, and good design prac-
tice. If an applicant establishes that it 
is impractical within the limits of ge-
ometry, inspectability, and good design 

practice, the applicant must do a fa-
tigue evaluation in accordance with 
paragraph (e) of this section. 

(b) The methodology used to estab-

lish compliance with this section must 
be submitted to and approved by the 
Administrator. 

(c) Definitions: 
(1) 

Catastrophic failure is an event 

that could prevent continued safe 
flight and landing. 

(2) 

Principal Structural Elements (PSEs) 

are structural elements that con-
tribute significantly to the carrying of 
flight or ground loads, the failure of 
which could result in catastrophic fail-
ure of the rotorcraft. 

(3) 

Threat Assessment is an assessment 

that specifies the locations, types, and 
sizes of damage, considering fatigue, 
environmental effects, intrinsic and 
discrete flaws, and impact or other ac-
cidental damage (including the discrete 
source of the accidental damage) that 
may occur during manufacture or oper-
ation. 

(d) Damage Tolerance Evaluation: 
(1) Each applicant must show that 

catastrophic failure due to static and 
fatigue loads, considering the intrinsic 
or discrete manufacturing defects or 
accidental damage, is avoided through-
out the operational life or prescribed 
inspection intervals of the rotorcraft 
by performing damage tolerance eval-
uations of the strength of composite 
PSEs and other parts, detail design 
points, and fabrication techniques. 
Each applicant must account for the 
effects of material and process varia-
bility along with environmental condi-
tions in the strength and fatigue eval-
uations. Each applicant must evaluate 
parts that include PSEs of the air-
frame, main and tail rotor drive sys-
tems, main and tail rotor blades and 
hubs, rotor controls, fixed and movable 
control surfaces, engine and trans-
mission mountings, landing gear, other 
parts, detail design points, and fabrica-
tion techniques deemed critical by the 
FAA. Each damage tolerance evalua-
tion must include: 

(i) The identification of all PSEs; 
(ii) In-flight and ground measure-

ments for determining the loads or 
stresses for all PSEs for all critical 
conditions throughout the range of 

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