14 CFR §25.393

1. (a)Control surfaces and supporting hinge brackets must be designed for inertia loads acting parallel to the hinge line.
2. (b)In the absence of more rational data, the inertia loads may be assumed to be equal to KW, where—
   1. (1)K = 24 for vertical surfaces;
   2. (2)K = 12 for horizontal surfaces; and
   3. (3)W = weight of the movable surfaces.