Text in force at CS25-28: from CS25-1, effective 12 Dec 2005.
(a) A rough air speed VRA for use as the recommended turbulence penetration air speed, and a rough air Mach number MRA, for use as the recommended turbulence penetration Mach number, must be established 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 - 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.
ED Decision 2005/006/R · Source