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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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514 

14 CFR Ch. I (1–1–24 Edition) 

§ 27.623 

(i) The strength requirements of 

§ 27.305 at an ultimate load cor-
responding to a casting factor of 1.25; 
and 

(ii) The deformation requirements of 

§ 27.305 at a load of 1.15 times the limit 
load. 

(d) 

Noncritical castings. For each cast-

ing other than those specified in para-
graph (c) of this section, the following 
apply: 

(1) Except as provided in paragraphs 

(d)(2) and (3) of this section, the casting 
factors and corresponding inspections 
must meet the following table: 

Casting factor 

Inspection 

2.0 or greater ...............

100 percent visual. 

Less than 2.0, greater 

than 1.5.

100 percent visual, and magnetic 

particle (ferromagnetic materials), 
penetrant (nonferromagnetic ma-
terials), or approved equivalent 
inspection methods. 

1.25 through 1.50 ........

100 percent visual, and magnetic 

particle (ferromagnetic materials). 
penetrant (nonferromagnetic ma-
terials), and radiographic or ap-
proved equivalent inspection 
methods. 

(2) The percentage of castings in-

spected by nonvisual methods may be 
reduced below that specified in para-
graph (d)(1) of this section when an ap-
proved quality control procedure is es-
tablished. 

(3) For castings procured to a speci-

fication that guarantees the mechan-
ical properties of the material in the 
casting and provides for demonstration 
of these properties by test of coupons 
cut from the castings on a sampling 
basis— 

(i) A casting factor of 1.0 may be 

used; and 

(ii) The castings must be inspected as 

provided in paragraph (d)(1) of this sec-
tion for casting factors of ‘‘1.25 through 
1.50’’ and tested under paragraph (c)(2) 
of this section. 

[Doc. No. 5074, 29 FR 15695, Nov. 24, 1964, as 
amended by Amdt. 27–34, 62 FR 46173, Aug. 29, 
1997] 

§ 27.623

Bearing factors. 

(a) Except as provided in paragraph 

(b) of this section, each part that has 
clearance (free fit), and that is subject 
to pounding or vibration, must have a 
bearing factor large enough to provide 

for the effects of normal relative mo-
tion. 

(b) No bearing factor need be used on 

a part for which any larger special fac-
tor is prescribed. 

§ 27.625

Fitting factors. 

For each fitting (part or terminal 

used to join one structural member to 
another) the following apply: 

(a) For each fitting whose strength is 

not proven by limit and ultimate load 
tests in which actual stress conditions 
are simulated in the fitting and sur-
rounding structures, a fitting factor of 
at least 1.15 must be applied to each 
part of— 

(1) The fitting; 
(2) The means of attachment; and 
(3) The bearing on the joined mem-

bers. 

(b) No fitting factor need be used— 
(1) For joints made under approved 

practices and based on comprehensive 
test data (such as continuous joints in 
metal plating, welded joints, and scarf 
joints in wood); and 

(2) With respect to any bearing sur-

face for which a larger special factor is 
used. 

(c) For each integral fitting, the part 

must be treated as a fitting up to the 
point at which the section properties 
become typical of the member. 

(d) Each seat, berth, litter, safety 

belt, and harness attachment to the 
structure must be shown by analysis, 
tests, or both, to be able to withstand 
the inertia forces prescribed in 
§ 27.561(b)(3) multiplied by a fitting fac-
tor of 1.33. 

[Doc. No. 5074, 29 FR 15695, Nov. 24, 1964, as 
amended by Amdt. 27–35, 63 FR 43285, Aug. 12, 
1998] 

§ 27.629

Flutter. 

Each aerodynamic surface of the 

rotorcraft must be free from flutter 
under each appropriate speed and 
power condition. 

[Doc. No. 5074, 29 FR 15695, Nov. 24, 1964, as 
amended by Amdt. 27–26, 55 FR 8000, Mar. 6, 
1990] 

R

OTORS

 

§ 27.653

Pressure venting and drain-

age of rotor blades. 

(a) For each rotor blade— 

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