Comparing a Flight In Turbulent Conditions At Va And Vb

Question

First, let's cover definitions for both airspeeds:

1. Turbulence penetration airspeed (Vb) is the design speed for maximum gust intensity (i.e. the intersection between the 66 fps gust line and the maximum certified g load of the aircraft).
2. Maneuvering speed (Va) is defined as the speed at which the aircraft reaches its maximum positive load factor limit at the same time it experiences an aerodynamic stall when flight controls are fully deflected in flight (or when encountering a vertical gust).

After both definitions, shouldn't be more logical to fly in turbulent conditions at Va, since any encountered gust would give you maximum and equal margin from overstressing the airframe and stalling the aircraft?

For example, if turbulence penetration speed (Vb) is greater than Va for your specific aircraft, and you encounter severe turbulence while flying at Vb, that would mean that a potentially hazardous gust of wind would overstress the airframe first and therefore, not provide maximum protection from damaging the aircraft? On the other hand, if Vb is smaller than Va and assuming the same scenario as described above, you would first stall the aircraft (which again does not provide maximum protection in turbulence)?

Additional: What is the exact difference between Vb and Vra (Rough Air Speed), if any?

Answer 1

The requirements for V_A and V_B can be found in Code of Federal Regulations, more precicely in Title 14, Chapter I, Subchapter C, Part 25, Subpart C, Flight Maneuver and Gust Conditions § 25.335

(c) Design maneuvering speed V_A. For V_A, the following apply: [sic]

(1) V_A may not be less than V_S1 √n where—

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

(ii) V_s1 is the stalling speed with > 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_{N max} curve intersects the positive maneuver 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_S1 = the 1-g stalling speed based on C_{NA max} 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.

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 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.

Now, as you can see the formula for V_A is pretty straightforward, whereas the one for V_B is more complicated. While I have not tested these formulas, I'm quite certain the simple reason for the seemingly illogical V_A and V_B values simply stem from the way they are "regulated".

_{sorry for using pictures for formulas, proved too difficult for me to get them here in anynother way}