Sec. 25.171 General.
The airplane must be longitudinally, directionally, and laterally stable in
accordance with the provisions of Secs. 25.173 through 25.177. In addition,
suitable stability and control feel (static stability) is required in any
condition 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]
Sec. 25.173 Static longitudinal stability.
Under the conditions specified in Sec. 25.175, the characteristics of the
elevator control forces (including friction) must be as follows:
(a) A pull must be required to obtain and maintain speeds below the
specified trim speed, and a push must be required 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 VFC/MFC, whichever is appropriate, or lower than
the minimum speed for steady unstalled flight.
(b) The airspeed must return to within 10 percent of the original trim
speed for the climb, approach, and landing conditions specified in Sec.
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 Sec. 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
section, 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 return to and maintain
the desired trim speed and altitude.
[Amdt. 25-7, 30 FR 13117, Oct. 15, 1965]
Sec. 25.175 Demonstration of static longitudinal 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 continuous power for reciprocating engines or
the maximum power or thrust selected by the applicant as an operating
limitation 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 1.4 VS1.
(b) Cruise. Static longitudinal stability must be shown in the cruise
condition 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 percent of the trim speed plus the resulting 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 1.4 VS1,
nor speeds greater than VFC/MFC, nor speeds that require a stick force of
more than 50 pounds), with--
(i) The wing flaps retracted;
(ii) The center of gravity in the most adverse position (see Sec. 25.27);
(iii) The most critical weight between the maximum takeoff and maximum
landing weights;
(iv) 75 percent of maximum continuous power for reciprocating engines or
for turbine engines, the maximum cruising power selected by the applicant as
an operating limitation (see Sec. 25.1521), except that the power need not
exceed that required at VMO/MMO; and
(v) The airplane trimmed for level flight with the power required in
paragraph (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 percent of the trim speed plus the resulting 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 1.4 VS1,
nor speeds greater than the minimum speed of the applicable speed range
prescribed in paragraph (b)(1), nor speeds that require a stick force of more
than 50 pounds), with--
(i) Wing flaps, center of gravity position, and weight as specified in
paragraph (b)(1) of this section;
(ii) Power required for level flight at a speed equal to VMO + 1.4 VS1/2;
and
(iii) The airplane trimmed for level flight with the power required in
paragraph (b)(2)(ii) of this section.
(3) With the landing gear extended, the stick force curve must have a
stable slope at all speeds within a range which is the greater of 15 percent
of the trim speed plus the resulting 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 1.4 VS1, nor
speeds greater than VLE, nor speeds that require a stick force of more than
50 pounds), with--
(i) Wing flap, center of gravity position, and weight as specified in
paragraph (b)(1) of this section;
(ii) 75 percent of maximum continuous power for reciprocating engines or,
for turbine engines, the maximum cruising power selected by the applicant as
an operating limitation, except that the power need not exceed that required
for level flight at VLE; and
(iii) The aircraft trimmed for level flight with the power required in
paragraph (b)(3)(ii) of this section.
(c) Approach. The stick force curve must have a stable slope at speeds
between 1.1 VS1 and 1.8 VS1, with--
(1) Wing flaps in the approach position;
(2) Landing gear retracted;
(3) Maximum landing weight; and
(4) The airplane trimmed at 1.4 VS1 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 1.1 VS0 and 1.3 VS0 with--
(1) Wing flaps in the landing position;
(2) Landing gear extended;
(3) Maximum landing weight;
(4) Power or thrust off on the engines; and
(5) The airplane trimmed at 1.4 VS0 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]
Sec. 25.177 Static lateral-directional stability.
(a) [Reserved]
(b) [Reserved]
(c) In straight, steady sideslips, the aileron and rudder control movements
and forces must be substantially proportional to the angle of sideslip in a
stable sense; and the factor of proportionality must lie between limits found
necessary for safe operation throughout the range of sideslip angles
appropriate to the operation of the airplane. At greater angles, up to the
angle at which full rudder is used or a rudder force of 180 pounds is
obtained, the rudder pedal forces may not reverse; and increased rudder
deflection must be needed for increased angles of sideslip. Compliance with
this paragraph must be demonstrated for all landing gear and flap positions
and symmetrical power conditions at speeds from 1.2 VS1 to VFE, VLE, or VFC/
MFC, as appropriate.
(d) The rudder gradi@ts must meet the requirements of paragraph (c) at
speeds between VMO/MMO and VFC/MFC except that the dihedral effect (aileron
deflection opposite the corresponding rudder input) may be negative provided
the divergence is gradual, easily recognized, and easily controlled by the
pilot.
[Doc. No. 24344, Amdt. 25-72, 55 FR 29774, July 20, 1990; 55 FR 37607,
Sept. 12, 1990]
*****************************************************************************
55 FR 29756, No. 140, July 20, 1990
SUMMARY: These amendments to the Federal Aviation Regulations (FAR) update
the standards for type certification of transport category airplanes for
clarity and accuracy, and ensure that the standards are appropriate and
practicable for the smaller transport category airplanes common to regional
air carrier operation.
EFFECTIVE DATE: August 20, 1990.
*****************************************************************************
Sec. 25.181 Dynamic stability.
(a) Any short period oscillation, not including combined lateral-
directional oscillations, occurring between 1.2 VS and maximum allowable
speed appropriate to the configuration of the airplane must be heavily
damped with the primary controls--
(1) Free; and
(2) In a fixed position.
(b) Any combined lateral-directional oscillations ("Dutch roll") occurring
between 1.2 VS and maximum allowable speed appropriate to the configuration
of the airplane must be positively damped with controls free, and must be
controllable with normal use of the primary controls without requiring
exceptional pilot skill.