Sec. 25.335 - Design 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
(2) Except as provided in §25.335(d)(2),
VC may not be less than VB + 1.32 UREF
(with UREF as specified in §25.341(a)(5)(i)). However VC
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
(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 1.5g (0.5g
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 MC 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
(c) Design maneuvering speed VA.
For VA, the following apply:
(1) VA may not be less than
VS1 √n where --
(i) n is the limit positive
maneuvering load factor at VC; and
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
(d) Design speed for maximum gust
(1) VB may not be less than
VS1=the 1-g stalling speed based on CNAmax with
the flaps retracted at the particular weight under consideration;
Vc=design cruise speed (knots equivalent airspeed);
Uref=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.
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,
CNA per radian;
(2) At altitudes where VC is
limited by Mach number --
(i) VB may be chosen to
provide an optimum margin between low and high speed buffet boundaries;
(ii) VB need not be greater
(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
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as
amended by Amdt. 25-23, 35 FR 5672, Apr. 8, 1970; Amdt. 25-86, 61 FR 5220,
Feb. 9, 1996; Amdt. 25-91, 62 FR 40704, July 29, 1997]