509 

Federal Aviation Administration, DOT 

§ 27.573 

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 
limits in § 27.309 (including altitude ef-
fects), except that maneuvering load 
factors need not exceed the maximum 
values expected in service; 

(iii) The loading spectra as severe as 

those expected in service based on 
loads or stresses determined under 
paragraph (d)(1)(ii) of this section, in-
cluding external load operations, if ap-
plicable, and other operations includ-
ing high-torque events; 

(iv) A threat assessment for all PSEs 

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; and 

(v) An assessment of the residual 

strength and fatigue characteristics of 
all PSEs that supports the replacement 
times and inspection intervals estab-
lished under paragraph (d)(2) of this 
section. 

(2) Each applicant must establish re-

placement times, inspections, or other 
procedures for all PSEs to require the 
repair or replacement of damaged parts 
before a catastrophic failure. These re-
placement times, inspections, or other 
procedures must be included in the Air-
worthiness Limitations Section of the 
Instructions for Continued Airworthi-
ness required by § 27.1529. 

(i) Replacement times for PSEs must 

be determined by tests, or by analysis 
supported by tests, and must show that 
the structure is able to withstand the 
repeated loads of variable magnitude 
expected in-service. In establishing 
these replacement times, the following 
items must be considered: 

(A) Damage identified in the threat 

assessment required by paragraph 
(d)(1)(iv) of this section; 

(B) Maximum acceptable manufac-

turing defects and in-service damage 
(

i.e.

, those that do not lower the resid-

ual strength below ultimate design 
loads and those that can be repaired to 
restore ultimate strength); and 

(C) Ultimate load strength capability 

after applying repeated loads. 

(ii) Inspection intervals for PSEs 

must be established to reveal any dam-
age identified in the threat assessment 

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