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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