Volume: 1Date: 2024-01-01Original Date: 2024-01-01Title: Section 25.335 - Design airspeeds.Context:
Title 14 - Aeronautics and Space. CHAPTER I - FEDERAL AVIATION ADMINISTRATION, DEPARTMENT OF TRANSPORTATION. SUBCHAPTER C - AIRCRAFT. PART 25 - AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY AIRPLANES. Subpart C - Structure. - Flight Maneuver and Gust Conditions.
§ 25.335Design airspeeds.
The selected design airspeeds are equivalent airspeeds (EAS). Estimated values of
VS0
and
VS1
must be conservative.
(a)
Design cruising speed, VC
. For
VC,
the following apply:
(1) The minimum value of
VC
must be sufficiently greater than
VB
to provide for inadvertent speed increases likely to occur as a result of severe atmospheric turbulence.
(2) Except as provided in § 25.335(d)(2), V
C
may not be less than V
B
+ 1.32 U
REF
(with U
REF
as specified in § 25.341(a)(5)(i)). However V
C
need not exceed the maximum speed in level flight at maximum continuous power for the corresponding altitude.
(3) At altitudes where
VD
is limited by Mach number,
VC
may be limited to a selected Mach number.
(b)
Design dive speed, VD
.
VD
must be selected so that
VC
/
MC
is not greater than 0.8
VD
/
MD,
or so that the minimum speed margin between
VC
/
MC
and
VD
/
MD
is the greater of the following values:
(1) From an initial condition of stabilized flight at
VC
/
MC,
the airplane is upset, flown for 20 seconds along a flight path 7.5° below the initial path, and then pulled up at a load factor of 1.5
g
(0.5
g
acceleration increment). The speed increase occurring in this maneuver may be calculated if reliable or conservative aerodynamic data is used. Power as specified in § 25.175(b)(1)(iv) is assumed until the pullup is initiated, at which time power reduction and the use of pilot controlled drag devices may be assumed;
(2) The minimum speed margin must be enough to provide for atmospheric variations (such as horizontal gusts, and penetration of jet streams and cold fronts) and for instrument errors and airframe production variations. These factors may be considered on a probability basis. The margin at altitude where M
C
is limited by compressibility effects must not less than 0.07M unless a lower margin is determined using a rational analysis that includes the effects of any automatic systems. In any
case, the margin may not be reduced to less than 0.05M.
Code of Federal Regulations /
Title 14 - Aeronautics and Space /
Vol. 1 / 2024-01-01237
(c)
Design maneuvering speed VA
. For
VA
, the following apply:
(1) V
A
may not be less than V
S1
√n where—
(i)
n
is the limit positive maneuvering load factor at
VC
; and
(ii)
VS1
is the stalling speed with flaps retracted.
(2)
VA
and
VS
must be evaluated at the design weight and altitude under consideration.
(3)
VA
need not be more than
VC
or the speed at which the positive
CN max
curve intersects the positive maneuver load factor line, whichever is less.
(d)
Design speed for maximum gust intensity, VB
.
(1) V
B
may not be less than
[Please see PDF for Formula: ER09fe96.016]
where—
V
S1
= the 1-g stalling speed based on C
NAmax
with the flaps retracted at the particular weight under consideration;
V
c
= design cruise speed (knots equivalent airspeed);
U
ref
= the reference gust velocity (feet per second equivalent airspeed) from § 25.341(a)(5)(i);
w = average wing loading (pounds per square foot) at the particular weight under consideration.
[Please see PDF for Formula: ER09FE96.017]
ρ = density of air (slugs/ft
3
);
c = mean geometric chord of the wing (feet);
g = acceleration due to gravity (ft/sec
2
);
a = slope of the airplane normal force coefficient curve, C
NA
per radian;
(2) At altitudes where V
C
is limited by Mach number—
(i) V
B
may be chosen to provide an optimum margin between low and high speed buffet boundaries; and,
(ii) V
B
need not be greater than V
C
.
(e)
Design flap speeds, VF
. For
VF
, the following apply:
(1) The design flap speed for each flap position (established in accordance with § 25.697(a)) must be sufficiently greater than the operating speed recommended for the corresponding stage of flight (including balked landings) to allow for probable variations in control of airspeed and for transition from one flap position to another.(2) If an automatic flap positioning or load limiting device is used, the speeds and corresponding flap positions programmed or allowed by the device may be used.
(3)
VF
may not be less than—
(i) 1.6
VS1
with the flaps in takeoff position at maximum takeoff weight;
(ii) 1.8
VS1
with the flaps in approach position at maximum landing weight, and
(iii) 1.8
VS0
with the flaps in landing position at maximum landing weight.
(f)
Design drag device speeds, VDD
. The selected design speed for each drag device must be sufficiently greater than the speed recommended for the operation of the device to allow for probable variations in speed control. For drag devices intended for use in high speed descents,
VDD
may not be less than
VD
. When an automatic drag device positioning or load limiting means is used, the speeds and corresponding drag device positions programmed or allowed by the automatic means must be used for design.