264
14 CFR Ch. I (1–1–24 Edition)
§ 25.619
structural failures due to material var-
iability. Except as provided in para-
graphs (e) and (f) of this section, com-
pliance must be shown by selecting ma-
terial design values which assure mate-
rial strength with the following prob-
ability:
(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.
(2) For redundant structure, in which
the failure of individual elements
would result in applied loads being
safely distributed to other load car-
rying members, 90 percent probability
with 95 percent confidence.
(c) The effects of environmental con-
ditions, such as temperature and mois-
ture, on material design values used in
an essential component or structure
must be considered where these effects
are significant within the airplane op-
erating envelope.
(d) [Reserved]
(e) Greater material design values
may be used if a ‘‘premium selection’’
of the material is made in which a
specimen of each individual item is
tested before use to determine that the
actual strength properties of that par-
ticular item will equal or exceed those
used in design.
(f) Other material design values may
be used if approved by the Adminis-
trator.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as
amended by Amdt. 25–46, 43 FR 50595, Oct. 30,
1978; Amdt. 25–72, 55 FR 29776, July 20, 1990;
Amdt. 25–112, 68 FR 46431, Aug. 5, 2003]
§ 25.619
Special factors.
The factor of safety prescribed in
§ 25.303 must be multiplied by the high-
est pertinent special factor of safety
prescribed in §§ 25.621 through 25.625 for
each part of the structure whose
strength is—
(a) Uncertain;
(b) Likely to deteriorate in service
before normal replacement; or
(c) Subject to appreciable variability
because of uncertainties in manufac-
turing processes or inspection methods.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as
amended by Amdt. 25–23, 35 FR 5674, Apr. 8,
1970]
§ 25.621
Casting factors.
(a)
General. For castings used in
structural applications, the factors,
tests, and inspections specified in para-
graphs (b) through (d) of this section
must be applied in addition to those
necessary to establish foundry quality
control. The inspections must meet ap-
proved specifications. Paragraphs (c)
and (d) of this section apply to any
structural castings, except castings
that are pressure tested as parts of hy-
draulic or other fluid systems 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. Each casting
whose failure could preclude continued
safe flight and landing of the airplane
or could result in serious injury to oc-
cupants is a critical casting. Each crit-
ical casting must have a factor associ-
ated with it for showing compliance
with strength and deformation require-
ments of § 25.305, and must comply with
the following criteria associated with
that factor:
(1) A casting factor of 1.0 or greater
may be used, provided that—
(i) It is demonstrated, in the form of
process qualification, proof of product,
and process monitoring that, for each
casting design and part number, the
castings produced by each foundry and
process combination have coefficients
of variation of the material properties
that are equivalent to those of wrought
alloy products of similar composition.
Process monitoring must include test-
ing of coupons cut from the prolonga-
tions of each casting (or each set of
castings, if produced from a single pour
into a single mold in a runner system)
and, on a sampling basis, coupons cut
from critical areas of production cast-
ings. The acceptance criteria for the
process monitoring inspections and
tests must be established and included
in the process specifications to ensure
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