V_RA

A rough air speed, V_RA, for use as the recommended turbulence penetration airspeed in §25.1585(a)(8), must be established, which—

(1) Is not greater than the design airspeed for maximum gust intensity, selected for V_B; and

(2) Is not less than the minimum value of V_B specified in §25.335(d); and

(3) Is sufficiently less than V_MO to ensure that likely speed variation during rough air encounters will not cause the overspeed warning to operate too frequently. In the absence of a rational investigation substantiating the use of other values, V_RA must be less than V_MO—35 knots (TAS).

Source: 14 CFR 25, §25.1517

Operating Procedures

(a) Information and instructions regarding the peculiarities of normal operations (including starting and warming the engines, taxiing, operation of wing flaps, landing gear, and the automatic pilot) must be furnished, together with recommended procedures for—

(8) Operation in turbulence for turbine powered airplanes (including recommended turbulence penetration airspeeds, flight peculiarities, and special control instructions);

[Final Rule. Docket No. FAA-2000-8511; Issued 06/15/01] The Federal Aviation Administration amends the airworthiness standards for transport category airplanes concerning airplane operating limitations and the content of airplane flight manuals. Issuing this amendment eliminates regulatory differences between the airworthiness standards of the U.S. and the Joint Aviation Requirements of Europe, without affecting current industry design practices.

Source: 14 CFR 25, §25.1585(a)(8), Amdt. 25-46, Eff. 12/1/78

Design speed for maximum gust intensity, V_B.

(1) V_B may not be less than

$${V_{\mathrm{S1}}(1+\frac{K_g{U}_{\mathrm{REF}}{V}_{C}a}{498w})}^{½}$$

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.

$$K_g=\frac{88\mu }{5.3+\mu }$$ $$\mu =\frac{2w}{\rho \mathrm{cag}}$$

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.

Source: 14 CFR 25, §25.335(d)

• Gusts are associated with the vertical and horizontal velocity gradients in the atmosphere. A horizontal gust produces a change in dynamic pressure on the airplane but causes relatively small and unimportant changes in flight load factor. The more important gusts are the vertical gusts which cause changes in angle of attack.
• The operation of any aircraft is subject to specific operating strength limitations. A single large overstress may cause structural failure or damage severe enough to require costly overhaul. Less severe overstress repeated for sufficient time will cause fatigue cracking and require replacement of parts to prevent subsequent failure.

Source: ATCM 51-3, Aerodynamics for Pilots, pg. 332

The operating flight strength limitations of an airplane are presented in the form of a V-n or V-g diagram. This chart usually is included in the aircraft flight handbook in the section dealing with operating limitations.

Source: ATCM 51-3, Aerodynamics for Pilots, pg. 334

The flight operating strength of an airplane is presented on a graph whose horizontal scale is airspeed (V) and vertical scale is load factor (n). The presentation of airplane strength is contingent on four factors being known: (1) the aircraft gross weight, (2) the configuration of the aircraft (clean, external stores, flaps and landing gear position, etc.), (3) symmetry of loading (since a rolling pullout at high speed can reduce structural limits to approximately two-thirds of the symmetrical load limits) and (4) the applicable altitude. A change in any one of these four factors can cause important changes in operating limits.

Source: ATCM 51-3, Aerodynamics for Pilots, pg. 334

• The lines of maximum lift capability are the first points of importance on the V-n diagram. The subject aircraft is capable of developing no more than one positive "g" at 100 knots, the wing stall speed of the airplane. Since maximum load factor varies with the square of the airspeed, the maximum positive lift capability of this airplane is 4 "g" at 200 knots, 9 "g" at 300 knots, 16 "g" at 400 knots, etc. Any load factor above this line is unavailable aerodynamically, i.e., the subject airplane cannot fly above the line of maximum lift capability. Essentially the same situation exists for negative lift flight with the exception that the speed necessary to produce a given negative load factor is higher than that to produce the same positive load factor.
• Thus, the airplane in flight is limited to a regime of airspeeds and g's which do not exceed the limit (or red line) speed, do not exceed the limit load factor, and cannot exceed the maximum lift capability. The airplane must be operated within this "envelope" to prevent structural damage and ensure that the anticipated service life of the airplane is obtained. The pilot must appreciate the V-n diagram as describing the allowable combination of airspeeds and load factors for safe operation. Any maneuver, gust, or gust plus maneuver outside the structural envelope can cause structural damage and effectively shorten the service life of the airplane.
• There are two points of great importance on the V-n diagram of [the figure]. Point B is the intersection of the negative limit load factor and line of maximum negative lift capability. Any airspeed greater than point B provides a negative lift capability sufficient to damage the airplane; any airspeed less than Point B does not provide negative lift capability sufficient to damage the airplane from excessive flight loads. Point A is the intersection of the positive limit load factor and the line of maximum positive lift capability. The airspeed at this point is the minimum airspeed at which the limit load can be developed aerodynamically. Any airspeed greater than Point A provides a positive lift capability sufficient to damage the airplane; any airspeed less than Point A does not provide lift capability sufficient to cause damage from excessive flight loads. The usual term given to the speed at Point A is the "maneuver speed," since consideration of subsonic aerodynamics would predict minimum usable turn radius to occur at this condition.
• The maneuver speed can be computed from the following equation:
$$V_P=V_s\sqrt{\mathrm{n\; limit}}$$

where

V_p = maneuver speed

V_s = stall speed

N limit = limit load factor

Source: ATCM 51-3, Aerodynamics for Pilots, pg. 334

• Recommended turbulence penetration airspeed is 270 KCAS / 0.75 M_T, whichever is lower.
• A minimum speed of 1.5 V_S is recommended to maintain adequate buffet margins.
• Severe turbulence will cause large and often rapid variations in indicated airspeed.

Source: G450 Aircraft Operating Manual, §07-02-30-03-40; ¶ 3.A.