Sec. 23.425 - Gust loads. (a) Each horizontal surface, other than a main wing, must be designed for loads resulting from -- (1) Gust velocities specified in §23.333(c) with flaps retracted; and (2) Positive and negative gusts of 25 f.p.s. nominal intensity at VF corresponding to the flight conditions specified in §23.345(a)(2). (b) [Reserved] (c) When determining the total load on the horizontal surfaces for the conditions specified in paragraph (a) of this section, the initial balancing loads for steady unaccelerated flight at the pertinent design speeds VF, VC, and VD must first be determined. The incremental load resulting from the gusts must be added to the initial balancing load to obtain the total load. (d) In the absence of a more rational analysis, the incremental load due to the gust must be computed as follows only on airplane configurations with aft-mounted, horizontal surfaces, unless its use elsewhere is shown to be conservative:  where -- ΔLht=Incremental horizontal tailload (lbs.); Kg=Gust alleviation factor defined in §23.341; Ude=Derived gust velocity (f.p.s.); V=Airplane equivalent speed (knots); aht=Slope of aft horizontal lift curve (per radian) Sht=Area of aft horizontal lift surface (ft 2); and [Doc. No. 4080, 29 FR 17955, Dec. 18, 1964, as amended by Amdt. 23-7, 34 FR 13089 Aug. 13, 1969; Amdt. 23-42, 56 FR 353, Jan. 3, 1991]