224
14 CFR Ch. I (1–1–24 Edition)
§ 25.171
angle of bank may not exceed five de-
grees) at 1.3
V
SR1
during climbing flight
with—
(1) The critical engine inoperative;
(2) The remaining engines at max-
imum continuous power; and
(3) The landing gear and flaps re-
tracted.
(e) Airplanes with four or more en-
gines. Each airplane with four or more
engines must also maintain trim in
rectilinear flight with the most unfa-
vorable center of gravity and at the
climb speed, configuration, and power
required by § 25.123(a) for the purpose of
establishing the en route flight paths
with two engines inoperative.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as
amended by Amdt. 25–23, 35 FR 5671, Apr. 8,
1970; Amdt. 25–38, 41 FR 55466, Dec. 20, 1976;
Amdt. 25–108, 67 FR 70827, Nov. 26, 2002;
Amdt. 25–115, 69 FR 40527, July 2, 2004]
S
TABILITY
§ 25.171
General.
The airplane must be longitudinally,
directionally, and laterally stable in
accordance with the provisions of
§§ 25.173 through 25.177. In addition,
suitable stability and control feel
(static stability) is required in any con-
dition normally encountered in service,
if flight tests show it is necessary for
safe operation.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as
amended by Amdt. 25–7, 30 FR 13117, Oct. 15,
1965]
§ 25.173
Static longitudinal stability.
Under the conditions specified in
§ 25.175, the characteristics of the eleva-
tor control forces (including friction)
must be as follows:
(a) A pull must be required to obtain
and maintain speeds below the speci-
fied trim speed, and a push must be re-
quired to obtain and maintain speeds
above the specified trim speed. This
must be shown at any speed that can be
obtained except speeds higher than the
landing gear or wing flap operating
limit speeds or
V
FC
/M
FC,
whichever is
appropriate, or lower than the min-
imum speed for steady unstalled flight.
(b) The airspeed must return to with-
in 10 percent of the original trim speed
for the climb, approach, and landing
conditions specified in § 25.175 (a), (c),
and (d), and must return to within 7.5
percent of the original trim speed for
the cruising condition specified in
§ 25.175(b), when the control force is
slowly released from any speed within
the range specified in paragraph (a) of
this section.
(c) The average gradient of the stable
slope of the stick force versus speed
curve may not be less than 1 pound for
each 6 knots.
(d) Within the free return speed range
specified in paragraph (b) of this sec-
tion, it is permissible for the airplane,
without control forces, to stabilize on
speeds above or below the desired trim
speeds if exceptional attention on the
part of the pilot is not required to re-
turn to and maintain the desired trim
speed and altitude.
[Amdt. 25–7, 30 FR 13117, Oct. 15, 1965]
§ 25.175
Demonstration of static longi-
tudinal stability.
Static longitudinal stability must be
shown as follows:
(a)
Climb. The stick force curve must
have a stable slope at speeds between
85 and 115 percent of the speed at which
the airplane—
(1) Is trimmed, with—
(i) Wing flaps retracted;
(ii) Landing gear retracted;
(iii) Maximum takeoff weight; and
(iv) 75 percent of maximum contin-
uous power for reciprocating engines or
the maximum power or thrust selected
by the applicant as an operating limi-
tation for use during climb for turbine
engines; and
(2) Is trimmed at the speed for best
rate-of-climb except that the speed
need not be less than 1.3
V
SR1
.
(b)
Cruise. Static longitudinal sta-
bility must be shown in the cruise con-
dition as follows:
(1) With the landing gear retracted at
high speed, the stick force curve must
have a stable slope at all speeds within
a range which is the greater of 15 per-
cent of the trim speed plus the result-
ing free return speed range, or 50 knots
plus the resulting free return speed
range, above and below the trim speed
(except that the speed range need not
include speeds less than 1.3
V
SR1
, nor
speeds greater than
V
FC
/M
FC,
nor speeds
that require a stick force of more than
50 pounds), with—
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§ 25.177
(i) The wing flaps retracted;
(ii) The center of gravity in the most
adverse position (see § 25.27);
(iii) The most critical weight be-
tween the maximum takeoff and max-
imum landing weights;
(iv) 75 percent of maximum contin-
uous power for reciprocating engines or
for turbine engines, the maximum
cruising power selected by the appli-
cant as an operating limitation (see
§ 25.1521), except that the power need
not exceed that required at
V
MO
/
M
MO
;
and
(v) The airplane trimmed for level
flight with the power required in para-
graph (b)(1)(iv) of this section.
(2) With the landing gear retracted at
low speed, the stick force curve must
have a stable slope at all speeds within
a range which is the greater of 15 per-
cent of the trim speed plus the result-
ing free return speed range, or 50 knots
plus the resulting free return speed
range, above and below the trim speed
(except that the speed range need not
include speeds less than 1.3
V
SR1
, nor
speeds greater than the minimum
speed of the applicable speed range pre-
scribed in paragraph (b)(1), nor speeds
that require a stick force of more than
50 pounds), with—
(i) Wing flaps, center of gravity posi-
tion, and weight as specified in para-
graph (b)(1) of this section;
(ii) Power required for level flight at
a speed equal to (
V
MO
+ 1.3
V
SR1
)/2; and
(iii) The airplane trimmed for level
flight with the power required in para-
graph (b)(2)(ii) of this section.
(3) With the landing gear extended,
the stick force curve must have a sta-
ble slope at all speeds within a range
which is the greater of 15 percent of the
trim speed plus the resulting free re-
turn speed range, or 50 knots plus the
resulting free return speed range,
above and below the trim speed (except
that the speed range need not include
speeds less than 1.3
V
SR1
, nor speeds
greater than
V
LE,
nor speeds that re-
quire a stick force of more than 50
pounds), with—
(i) Wing flap, center of gravity posi-
tion, and weight as specified in para-
graph (b)(1) of this section;
(ii) 75 percent of maximum contin-
uous power for reciprocating engines
or, for turbine engines, the maximum
cruising power selected by the appli-
cant as an operating limitation, except
that the power need not exceed that re-
quired for level flight at
V
LE
; and
(iii) The aircraft trimmed for level
flight with the power required in para-
graph (b)(3)(ii) of this section.
(c)
Approach. The stick force curve
must have a stable slope at speeds be-
tween
V
SW
and 1.7
V
SR1
, with—
(1) Wing flaps in the approach posi-
tion;
(2) Landing gear retracted;
(3) Maximum landing weight; and
(4) The airplane trimmed at 1.3
V
SR1
with enough power to maintain level
flight at this speed.
(d)
Landing. The stick force curve
must have a stable slope, and the stick
force may not exceed 80 pounds, at
speeds between
V
SW
and 1.7
V
SR0
with—
(1) Wing flaps in the landing position;
(2) Landing gear extended;
(3) Maximum landing weight;
(4) The airplane trimmed at 1.3 V
SR0
with—
(i) Power or thrust off, and
(ii) Power or thrust for level flight.
(5) The airplane trimmed at 1.3
V
SR0
with power or thrust off.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as
amended by Amdt. 25–7, 30 FR 13117, Oct. 15,
1965; Amdt. 25–108, 67 FR 70827, Nov. 26, 2002;
Amdt. 25–115, 69 FR 40527, July 2, 2004]
§ 25.177
Static lateral-directional sta-
bility.
(a) The static directional stability
(as shown by the tendency to recover
from a skid with the rudder free) must
be positive for any landing gear and
flap position and symmetric power con-
dition, at speeds from 1.13 V
SR1
, up to
V
FE
, V
LE
, or V
FC
/M
FC
(as appropriate for
the airplane configuration).
(b) The static lateral stability (as
shown by the tendency to raise the low
wing in a sideslip with the aileron con-
trols free) for any landing gear and flap
position and symmetric power condi-
tion, may not be negative at any air-
speed (except that speeds higher than
V
FE
need not be considered for flaps ex-
tended configurations nor speeds high-
er than V
LE
for landing gear extended
configurations) in the following air-
speed ranges:
(1) From 1.13 V
SR1
to V
MO
/M
MO
.
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