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14 CFR Ch. I (1–1–19 Edition) 

§ 29.571 

each float. If the floats are deployed in 
flight, appropriate air loads derived 
from the flight limitations with the 
floats deployed shall be used in sub-
stantiation of the floats and their at-
tachment to the rotorcraft. For this 
purpose, the design airspeed for limit 
load is the float deployed airspeed op-
erating limit multiplied by 1.11. 


Floats deployed after initial water 


Each float must be designed for 

full or partial immersion prescribed in 
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 

[Amdt. 27–26, 55 FR 8003, Mar. 6, 1990] 






§ 29.571

Fatigue Tolerance Evaluation 

of Metallic Structure. 

(a) A fatigue tolerance evaluation of 

each principal structural element 
(PSE) must be performed, and appro-
priate inspections and retirement time 
or approved equivalent means must be 
established to avoid catastrophic fail-
ure during the operational life of the 
rotorcraft. The fatigue tolerance eval-
uation must consider the effects of 
both fatigue and the damage deter-
mined under paragraph (e)(4) of this 
section. Parts to be evaluated include 
PSEs of the rotors, rotor drive systems 
between the engines and rotor hubs, 
controls, fuselage, fixed and movable 
control surfaces, engine and trans-
mission mountings, landing gear, and 
their related primary attachments. 

(b) For the purposes of this section, 

the term— 


Catastrophic failure 

means an 

event that could prevent continued 
safe flight and landing. 


Principal structural element (PSE) 

means a structural element that con-
tributes significantly to the carriage of 
flight or ground loads, and the fatigue 
failure of that structural element could 
result in catastrophic failure of the air-

(c) The methodology used to estab-

lish compliance with this section must 

be submitted to and approved by the 

(d) Considering all rotorcraft struc-

ture, structural elements, and assem-
blies, each PSE must be identified. 

(e) Each fatigue tolerance evaluation 

required by this section must include: 

(1) In-flight measurements to deter-

mine the fatigue loads or stresses for 
the PSEs identified in paragraph (d) of 
this section in all critical conditions 
throughout the range of design limita-
tions required by § 29.309 (including al-
titude effects), except that maneu-
vering load factors need not exceed the 
maximum values expected in oper-

(2) The loading spectra as severe as 

those expected in operations based on 
loads or stresses determined under 
paragraph (e)(1) of this section, includ-
ing external load operations, if applica-
ble, and other high frequency power- 
cycle operations. 

(3) Takeoff, landing, and taxi loads 

when evaluating the landing gear and 
other affected PSEs. 

(4) For each PSE identified in para-

graph (d) of this section, a threat as-
sessment which includes a determina-
tion of the probable locations, types, 
and sizes of damage, taking into ac-
count fatigue, environmental effects, 
intrinsic and discrete flaws, or acci-
dental damage that may occur during 
manufacture or operation. 

(5) A determination of the fatigue 

tolerance characteristics for the PSE 
with the damage identified in para-
graph (e)(4) of this section that sup-
ports the inspection and retirement 
times, or other approved equivalent 

(6) Analyses supported by test evi-

dence and, if available, service experi-

(f) A residual strength determination 

is required that substantiates the max-
imum damage size assumed in the fa-
tigue tolerance evaluation. In deter-
mining inspection intervals based on 
damage growth, the residual strength 
evaluation must show that the remain-
ing structure, after damage growth, is 
able to withstand design limit loads 
without failure. 

(g) The effect of damage on stiffness, 

dynamic behavior, loads, and func-
tional performance must be considered. 

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