Sec. 25.341  Gust and turbulence loads.
(a) Discrete Gust Design Criteria. The airplane is assumed to be subjected to symmetrical vertical and lateral gusts in level flight. Limit gust loads must be determined in accordance with the provisions: (1) Loads on each part of the structure must be determined by dynamic analysis. The analysis must take into account unsteady aerodynamic characteristics and all significant structural degrees of freedom including rigid body motions. (2) The shape of the gust must be: $$
for 0 ≤ s ≤ 2H where  s=distance penetrated into the gust (feet); U H=the gust gradient which is the distance (feet)
parallel to the airplane's flight path for the gust to reach its peak
velocity. (3) A sufficient number of gust gradient distances in the range 30 feet to 350 feet must be investigated to find the critical response for each load quantity. (4) The design gust velocity must be: $$
where  U F (5) The following reference gust velocities apply: (i) At the airplane design speed V (ii) At the airplane design speed V (6) The flight profile alleviation
factor, F
Z (7) When a stability augmentation system is included in the analysis, the effect of any significant system nonlinearities should be accounted for when deriving limit loads from limit gust conditions. (b) Continuous Gust Design Criteria. The dynamic response of the airplane to vertical and lateral continuous turbulence must be taken into account. The continuous gust design criteria of appendix G of this part must be used to establish the dynamic response unless more rational criteria are shown. [Doc. No. 27902, 61 FR 5221, Feb. 9, 1996; 61 FR
9533, Mar. 8, 1996]
