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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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225 

Federal Aviation Administration, DOT 

§ 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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