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512 

14 CFR Ch. I (1–1–24 Edition) 

§ 27.605 

§ 27.605

Fabrication methods. 

(a) The methods of fabrication used 

must produce consistently sound struc-
tures. If a fabrication process (such as 
gluing, spot welding, or heat-treating) 
requires close control to reach this ob-
jective, the process must be performed 
according to an approved process speci-
fication. 

(b) Each new aircraft fabrication 

method must be substantiated by a 
test program. 

(Secs. 313(a), 601, 603, 604, and 605 of the Fed-
eral Aviation Act of 1958 (49 U.S.C. 1354(a), 
1421, 1423, 1424 and 1425); sec. 6(c) of the Dept. 
of Transportation Act (49 U.S.C. 1655(c))) 

[Doc. No. 5074, 29 FR 15695, Nov. 24, 1964, as 
amended by Amdt. 27–16, 43 FR 50599, Oct. 30, 
1978] 

§ 27.607

Fasteners. 

(a) Each removable bolt, screw, nut, 

pin, or other fastener whose loss could 
jeopardize the safe operation of the 
rotorcraft must incorporate two sepa-
rate locking devices. The fastener and 
its locking devices may not be ad-
versely affected by the environmental 
conditions associated with the par-
ticular installation. 

(b) No self-locking nut may be used 

on any bolt subject to rotation in oper-
ation unless a nonfriction locking de-
vice is used in addition to the self-lock-
ing device. 

[Amdt. 27–4, 33 FR 14533, Sept. 27, 1968] 

§ 27.609

Protection of structure. 

Each part of the structure must— 
(a) Be suitably protected against de-

terioration or loss of strength in serv-
ice due to any cause, including— 

(1) Weathering; 
(2) Corrosion; and 
(3) Abrasion; and 
(b) Have provisions for ventilation 

and drainage where necessary to pre-
vent the accumulation of corrosive, 
flammable, or noxious fluids. 

§ 27.610

Lightning and static elec-

tricity protection. 

(a) The rotorcraft must be protected 

against catastrophic effects from light-
ning. 

(b) For metallic components, compli-

ance with paragraph (a) of this section 
may be shown by— 

(1) Electrically bonding the compo-

nents properly to the airframe; or 

(2) Designing the components so that 

a strike will not endanger the rotor-
craft. 

(c) For nonmetallic components, 

compliance with paragraph (a) of this 
section may be shown by— 

(1) Designing the components to min-

imize the effect of a strike; or 

(2) Incorporating acceptable means of 

diverting the resulting electrical cur-
rent so as not to endanger the rotor-
craft. 

(d) The electrical bonding and protec-

tion against lightning and static elec-
tricity must— 

(1) Minimize the accumulation of 

electrostatic charge; 

(2) Minimize the risk of electric 

shock to crew, passengers, and service 
and maintenance personnel using nor-
mal precautions; 

(3) Provide an electrical return path, 

under both normal and fault condi-
tions, on rotorcraft having grounded 
electrical systems; and 

(4) Reduce to an acceptable level the 

effects of static electricity on the func-
tioning of essential electrical and elec-
tronic equipment. 

[Amdt. 27–21, 49 FR 44433, Nov. 6, 1984, as 
amended by Amdt. 27–37, 64 FR 45094, Aug. 18, 
1999; Amdt. 27–46, 76 FR 33135, June 8, 2011] 

§ 27.611

Inspection provisions. 

There must be means to allow the 

close examination of each part that re-
quires— 

(a) Recurring inspection; 
(b) Adjustment for proper alignment 

and functioning; or 

(c) Lubrication. 

§ 27.613

Material strength properties 

and design values. 

(a) Material strength properties must 

be based on enough tests of material 
meeting specifications to establish de-
sign values on a statistical basis. 

(b) Design values must be chosen to 

minimize the probability of structural 
failure due to material variability. Ex-
cept as provided in paragraphs (d) and 
(e) of this section, compliance with 
this paragraph must be shown by se-
lecting design values that assure mate-
rial strength with the following prob-
ability— 

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513 

Federal Aviation Administration, DOT 

§ 27.621 

(1) Where applied loads are eventu-

ally distributed through a single mem-
ber within an assembly, the failure of 
which would result in loss of structural 
integrity of the component, 99 percent 
probability with 95 percent confidence; 
and 

(2) For redundant structure, those in 

which the failure of individual ele-
ments would result in applied loads 
being safely distributed to other load- 
carrying members, 90 percent prob-
ability with 95 percent confidence. 

(c) The strength, detail design, and 

fabrication of the structure must mini-
mize the probability of disastrous fa-
tigue failure, particularly at points of 
stress concentration. 

(d) Design values may be those con-

tained in the following publications 
(available from the Naval Publications 
and Forms Center, 5801 Tabor Avenue, 
Philadelphia, Pennsylvania 19120) or 
other values approved by the Adminis-
trator: 

(1) MIL-HDBK-5, ‘‘Metallic Materials 

and Elements for Flight Vehicle Struc-
ture’’. 

(2) MIL-HDBK-17, ‘‘Plastics for 

Flight Vehicles’’. 

(3) ANC-18, ‘‘Design of Wood Aircraft 

Structures’’. 

(4) MIL-HDBK-23, ‘‘Composite Con-

struction for Flight Vehicles’’. 

(e) Other design values may be used if 

a selection of the material is made in 
which a specimen of each individual 
item is tested before use and it is de-
termined that the actual strength 
properties of that particular item will 
equal or exceed those used in design. 

(Secs. 313(a), 601, 603, 604, Federal Aviation 
Act of 1958 (49 U.S.C. 1354(a), 1421, 1423, 1424), 
sec. 6(c), Dept. of Transportation Act (49 
U.S.C. 1655(c))) 

[Doc. No. 5074, 29 FR 15695, Nov. 24, 1964, as 
amended by Amdt. 27–16, 43 FR 50599, Oct. 30, 
1978; Amdt. 27–26, 55 FR 8000, Mar. 6, 1990] 

§ 27.619

Special factors. 

(a) The special factors prescribed in 

§§ 27.621 through 27.625 apply to each 
part of the structure whose strength 
is— 

(1) Uncertain; 
(2) Likely to deteriorate in service 

before normal replacement; or 

(3) Subject to appreciable variability 

due to— 

(i) Uncertainties in manufacturing 

processes; or 

(ii) Uncertainties in inspection meth-

ods. 

(b) For each part to which §§ 27.621 

through 27.625 apply, the factor of safe-
ty prescribed in § 27.303 must be multi-
plied by a special factor equal to— 

(1) The applicable special factors pre-

scribed in §§ 27.621 through 27.625; or 

(2) Any other factor great enough to 

ensure that the probability of the part 
being understrength because of the un-
certainties specified in paragraph (a) of 
this section is extremely remote. 

§ 27.621

Casting factors. 

(a) 

General. The factors, tests, and in-

spections specified in paragraphs (b) 
and (c) of this section must be applied 
in addition to those necessary to estab-
lish foundry quality control. The in-
spections must meet approved speci-
fications. Paragraphs (c) and (d) of this 
section apply to structural castings ex-
cept castings that are pressure tested 
as parts of hydraulic or other fluid sys-
tems and do not support structural 
loads. 

(b) 

Bearing stresses and surfaces. The 

casting factors specified in paragraphs 
(c) and (d) of this section— 

(1) Need not exceed 1.25 with respect 

to bearing stresses regardless of the 
method of inspection used; and 

(2) Need not be used with respect to 

the bearing surfaces of a part whose 
bearing factor is larger than the appli-
cable casting factor. 

(c) 

Critical castings. For each casting 

whose failure would preclude continued 
safe flight and landing of the rotorcraft 
or result in serious injury to any occu-
pant, the following apply: 

(1) Each critical casting must— 
(i) Have a casting factor of not less 

than 1.25; and 

(ii) Receive 100 percent inspection by 

visual, radiographic, and magnetic par-
ticle (for ferromagnetic materials) or 
penetrant (for nonferromagnetic mate-
rials) inspection methods or approved 
equivalent inspection methods. 

(2) For each critical casting with a 

casting factor less than 1.50, three sam-
ple castings must be static tested and 
shown to meet— 

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