The calculations of this investigation showed that critical flutter speed goes down as altitude increases. In case of control surface flutter the decrease may exceed Skm/h for each thousand meters of altitude, and in exceptional cases may be similar to the decrease designated by TAS= const. There are also some kinds of flutter which are possible at high altitude only. As the result of this situation if the critical flutter mode is not known, there is no basis for altering the existing reduction is Vne over 3000m (or over the test flight altitude) according to the law TAS= const.