501
Federal Aviation Administration, DOT
§ 27.427
the system allows the pilot to apply,
but not less than 0.60 times the forces
specified in § 27.397.
(b) Each primary control system, in-
cluding its supporting structure, must
be designed as follows:
(1) The system must withstand loads
resulting from the limit pilot forces
prescribed in § 27.397.
(2) Notwithstanding paragraph (b)(3)
of this section, when power-operated
actuator controls or power boost con-
trols are used, the system must also
withstand the loads resulting from the
force output of each normally ener-
gized power device, including any sin-
gle power boost or actuator system
failure.
(3) If the system design or the normal
operating loads are such that a part of
the system cannot react to the limit
pilot forces prescribed in § 27.397, that
part of the system must be designed to
withstand the maximum loads that can
be obtained in normal operation. The
minimum design loads must, in any
case, provide a rugged system for serv-
ice use, including consideration of fa-
tigue, jamming, ground gusts, control
inertia, and friction loads. In the ab-
sence of rational analysis, the design
loads resulting from 0.60 of the speci-
fied limit pilot forces are acceptable
minimum design loads.
(4) If operational loads may be ex-
ceeded through jamming, ground gusts,
control inertia, or friction, the system
must withstand the limit pilot forces
specified in § 27.397, without yielding.
[Doc. No. 5074, 29 FR 15695, Nov. 24, 1964, as
amended by Amdt. 27–26, 55 FR 7999, Mar. 6,
1990]
§ 27.397
Limit pilot forces and torques.
(a) Except as provided in paragraph
(b) of this section, the limit pilot
forces are as follows:
(1) For foot controls, 130 pounds.
(2) For stick controls, 100 pounds fore
and aft, and 67 pounds laterally.
(b) For flap, tab, stabilizer, rotor
brake, and landing gear operating con-
trols, the follows apply (R = radius in
inches):
(1) Crank, wheel, and lever controls,
[1 + R]/3
×
50 pounds, but not less than
50 pounds nor more than 100 pounds for
hand operated controls or 130 pounds
for foot operated controls, applied at
any angle within 20 degrees of the
plane of motion of the control.
(2) Twist controls, 80R inch-pounds.
[Amdt. 27–11, 41 FR 55469, Dec. 20, 1976, as
amended by Amdt. 27–40, 66 FR 23538, May 9,
2001]
§ 27.399
Dual control system.
Each dual primary flight control sys-
tem must be designed to withstand the
loads that result when pilot forces of
0.75 times those obtained under § 27.395
are applied—
(a) In opposition; and
(b) In the same direction.
§ 27.411
Ground clearance: tail rotor
guard.
(a) It must be impossible for the tail
rotor to contact the landing surface
during a normal landing.
(b) If a tail rotor guard is required to
show compliance with paragraph (a) of
this section—
(1) Suitable design loads must be es-
tablished for the guard; and
(2) The guard and its supporting
structure must be designed to with-
stand those loads.
§ 27.427
Unsymmetrical loads.
(a) Horizontal tail surfaces and their
supporting structure must be designed
for unsymmetrical loads arising from
yawing and rotor wake effects in com-
bination with the prescribed flight con-
ditions.
(b) To meet the design criteria of
paragraph (a) of this section, in the ab-
sence of more rational data, both of the
following must be met:
(1) One hundred percent of the max-
imum loading from the symmetrical
flight conditions acts on the surface on
one side of the plane of symmetry, and
no loading acts on the other side.
(2) Fifty percent of the maximum
loading from the symmetrical flight
conditions acts on the surface on each
side of the plane of symmetry but in
opposite directions.
(c) For empennage arrangements
where the horizontal tail surfaces are
supported by the vertical tail surfaces,
the vertical tail surfaces and sup-
porting structure must be designed for
the combined vertical and horizontal
surface loads resulting from each pre-
scribed flight condition, considered
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14 CFR Ch. I (1–1–24 Edition)
§ 27.471
separately. The flight conditions must
be selected so the maximum design
loads are obtained on each surface. In
the absence of more rational data, the
unsymmetrical horizontal tail surface
loading distributions described in this
section must be assumed.
[Amdt. 27–26, 55 FR 7999, Mar. 6, 1990, as
amended by Amdt. 27–27, 55 FR 38966, Sept.
21, 1990]
G
ROUND
L
OADS
§ 27.471
General.
(a)
Loads and equilibrium. For limit
ground loads—
(1) The limit ground loads obtained
in the landing conditions in this part
must be considered to be external loads
that would occur in the rotorcraft
structure if it were acting as a rigid
body; and
(2) In each specified landing condi-
tion, the external loads must be placed
in equilibrium with linear and angular
inertia loads in a rational or conserv-
ative manner.
(b)
Critical centers of gravity. The crit-
ical centers of gravity within the range
for which certification is requested
must be selected so that the maximum
design loads are obtained in each land-
ing gear element.
§ 27.473
Ground loading conditions
and assumptions.
(a) For specified landing conditions,
a design maximum weight must be
used that is not less than the max-
imum weight. A rotor lift may be as-
sumed to act through the center of
gravity throughout the landing impact.
This lift may not exceed two-thirds of
the design maximum weight.
(b) Unless otherwise prescribed, for
each specified landing condition, the
rotorcraft must be designed for a limit
load factor of not less than the limit
inertia load factor substantiated under
§ 27.725.
[Amdt. 27–2, 33 FR 963, Jan. 26, 1968]
§ 27.475
Tires and shock absorbers.
Unless otherwise prescribed, for each
specified landing condition, the tires
must be assumed to be in their static
position and the shock absorbers to be
in their most critical position.
§ 27.477
Landing gear arrangement.
Sections 27.235, 27.479 through 27.485,
and 27.493 apply to landing gear with
two wheels aft, and one or more wheels
forward, of the center of gravity.
§ 27.479
Level landing conditions.
(a)
Attitudes. Under each of the load-
ing conditions prescribed in paragraph
(b) of this section, the rotorcraft is as-
sumed to be in each of the following
level landing attitudes:
(1) An attitude in which all wheels
contact the ground simultaneously.
(2) An attitude in which the aft
wheels contact the ground with the for-
ward wheels just clear of the ground.
(b)
Loading conditions. The rotorcraft
must be designed for the following
landing loading conditions:
(1) Vertical loads applied under
§ 27.471.
(2) The loads resulting from a com-
bination of the loads applied under
paragraph (b)(1) of this section with
drag loads at each wheel of not less
than 25 percent of the vertical load at
that wheel.
(3) If there are two wheels forward, a
distribution of the loads applied to
those wheels under paragraphs (b)(1)
and (2) of this section in a ratio of
40:60.
(c)
Pitching moments. Pitching mo-
ments are assumed to be resisted by—
(1) In the case of the attitude in para-
graph (a)(1) of this section, the forward
landing gear; and
(2) In the case of the attitude in para-
graph (a)(2) of this section, the angular
inertia forces.
[Doc. No. 5074, 29 FR 15695, Nov. 24, 1964; 29
FR 17885, Dec. 17, 1964]
§ 27.481
Tail-down landing conditions.
(a) The rotorcraft is assumed to be in
the maximum nose-up attitude allow-
ing ground clearance by each part of
the rotorcraft.
(b) In this attitude, ground loads are
assumed to act perpendicular to the
ground.
§ 27.483
One-wheel landing conditions.
For the one-wheel landing condition,
the rotorcraft is assumed to be in the
level attitude and to contact the
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