14 CFR § 25.1517 - Rough air speed, VRA.
(a) A rough air speed, VRA, for use as the recommended turbulence penetration airspeed, and a rough air Mach number, MRA, for use as the recommended turbulence penetration Mach number, must be established. VRA/MRA must be sufficiently less than VMO/MMO to ensure that likely speed variation during rough air encounters will not cause the overspeed warning to operate too frequently.
(b) At altitudes where VMO is not limited by Mach number, in the absence of a rational investigation substantiating the use of other values, VRA must be less than VMO minus 35 KTAS.
(c) At altitudes where VMO is limited by Mach number, MRA may be chosen to provide an optimum margin between low and high speed buffet boundaries.