509 

Federal Aviation Administration, DOT 

§ 27.573 

paragraph (b)(1) of this section. In addi-
tion, each float must be designed for 
combined vertical and drag loads using 
a relative limit speed of 20 knots be-
tween the rotorcraft and the water. 
The vertical load may not be less than 
the highest likely buoyancy load deter-
mined under paragraph (b)(1) of this 
section. 

[Amdt. 27–26, 55 FR 8000, Mar. 6, 1990] 

F

ATIGUE

E

VALUATION

 

§ 27.571

Fatigue evaluation of flight 

structure. 

(a) 

General. Each portion of the flight 

structure (the flight structure includes 
rotors, rotor drive systems between the 
engines and the rotor hubs, controls, 
fuselage, landing gear, and their re-
lated primary attachments), the failure 
of which could be catastrophic, must be 
identified and must be evaluated under 
paragraph (b), (c), (d), or (e) of this sec-
tion. The following apply to each fa-
tigue evaluation: 

(1) The procedure for the evaluation 

must be approved. 

(2) The locations of probable failure 

must be determined. 

(3) Inflight measurement must be in-

cluded in determining the following: 

(i) Loads or stresses in all critical 

conditions throughout the range of 
limitations in § 27.309, except that ma-
neuvering load factors need not exceed 
the maximum values expected in oper-
ation. 

(ii) The effect of altitude upon these 

loads or stresses. 

(4) The loading spectra must be as se-

vere as those expected in operation in-
cluding, but not limited to, external 
cargo operations, if applicable, and 
ground-air-ground cycles. The loading 
spectra must be based on loads or 
stresses determined under paragraph 
(a)(3) of this section. 

(b) 

Fatigue tolerance evaluation. It 

must be shown that the fatigue toler-
ance of the structure ensures that the 
probability of catastrophic fatigue fail-
ure is extremely remote without estab-
lishing replacement times, inspection 
intervals or other procedures under 
section A27.4 of appendix A. 

(c) 

Replacement time evaluation. it 

must be shown that the probability of 
catastrophic fatigue failure is ex-

tremely remote within a replacement 
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 

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