CS 25.335 Design airspeeds

ED Decision 2016/010/R

(See AMC 25.335)

The selected design airspeeds are equivalent airspeeds (EAS). Estimated values of V_S0 and V_S1 must be conservative.

(a)     Design cruising speed, V_C. For V_C, the following apply:

(1)     The minimum value of V_C must be sufficiently greater than V_B to provide for inadvertent speed increases likely to occur as a result of severe atmospheric turbulence.

(2)     Except as provided in sub-paragraph 25.335(d)(2), V_C may not be less than V_B + 1·32 U_ref (with U_ref as specified in sub-paragraph 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 V_D is limited by Mach number, V_C may be limited to a selected Mach number. (See CS 25.1505.)

(b)     Design dive speed, V_D. V_D must be selected so that V_C/M_C is not greater than 0·8 V_D/M_D, or so that the minimum speed margin between V_C/M_C and V_D/M_D is the greater of the following values:

(1)     (i)      For aeroplanes not equipped with a high speed protection function: From an initial condition of stabilised flight at V_C/M_C, the aeroplane 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 manoeuvre may be calculated if reliable or conservative aerodynamic data issued. Power as specified in CS 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;

(ii)     For aeroplanes equipped with a high speed protection function: In lieu of subparagraph (b)(1)(i), the speed increase above V_C/M_C resulting from the greater of the following manoeuvres must be established:

(A)     From an initial condition of stabilised flight at V_C/M_C, the aeroplane is upset so as to take up a new flight path 7.5° below the initial path. Control application, up to full authority, is made to try and maintain this new flight path. Twenty seconds after achieving the new flight path, manual recovery is made at a load factor of 1.5 g (0.5 g acceleration increment), or such greater load factor that is automatically applied by the system with the pilot’s pitch control neutral. The speed increase occurring in this manoeuvre may be calculated if reliable or conservative aerodynamic data is used. Power as specified in CS 25.175(b)(1)(iv) is assumed until recovery is made, at which time power reduction and the use of pilot controlled drag devices may be assumed.

(B)     From a speed below V_C/M_C, with power to maintain stabilised level flight at this speed, the aeroplane is upset so as to accelerate through V_C/M_C at a flight path 15° below the initial path (or at the steepest nose down attitude that the system will permit with full control authority if less than 15°). Pilot controls may be in neutral position after reaching V_C/M_C and before recovery is initiated. Recovery may be initiated 3 seconds after operation of high speed, attitude, or other alerting system by application of a load factor of 1.5 g (0.5 g acceleration increment), or such greater load factor that is automatically applied by the system with the pilot’s pitch control neutral. Power may be reduced simultaneously. All other means of decelerating the aeroplane, the use of which is authorised up to the highest speed reached in the manoeuvre, may be used. The interval between successive pilot actions must not be less than 1 second (See AMC 25.335(b)(1)(ii)).

(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 be 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. (See AMC 25.335(b)(2))

(c)      Design manoeuvring speed, V_A. For V_A, the following apply:

(1)     V_A may not be less than VS1 √n where –

(i)      n is the limit positive manoeuvring load factor at V_C; and

(ii)     V_S1 is the stalling speed with wing-flaps retracted.

(2)     V_A and V_S must be evaluated at the design weight and altitude under consideration.

(3)     V_A need not be more than V_C or the speed at which the positive C_Nmax curve intersects the positive manoeuvre load factor line, whichever is less.

(d)     Design speed for maximum gust intensity, V_B.

(1)     V_B may not be less than

where –

V_sl   =  the 1-g stalling speed based on C_NAmax with the flaps retracted at the particular weight under consideration;

C_NAmax = the maximum aeroplane normal force coefficient;

V_c   =  design cruise speed (knots equivalent airspeed);

U_ref =  the reference gust velocity (feet per second equivalent airspeed) from CS 25.341(a)(5)(i);

w   =  average wing loading (pounds per square foot) at the particular weight under consideration.

K_g   =   

µ    = 

ρ    =  density of air (slugs/ft³);

c    =   mean geometric chord of the wing (feet);

g    =   acceleration due to gravity (ft/sec²);

a    =   slope of the aeroplane 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 wing-flap speeds, V_F. For V_F, the following apply:

(1)     The design wing-flap speed for each wing-flap position (established in accordance with CS 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 wing-flap position to another.

(2)     If an automatic wing-flap positioning or load limiting device is used, the speeds and corresponding wing-flap positions programmed or allowed by the device may be used.

(3)     V_F may not be less than –

(i)      1·6 V_S1 with the wing-flaps in take-off position at maximum take-off weight;

(ii)     1·8 V_S1 with the wing-flaps in approach position at maximum landing weight; and

(iii)     1·8 V_S0 with the wing-flaps in landing position at maximum landing weight.

(f)      Design drag device speeds, V_DD. 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, V_DD may not be less than V_D. 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.

[Amdt 25/13]

[Amdt 25/18]