  We follow the approach used by Davies and Pringle (1981) and discuss the
trans-sonic substage of the propeller regime. This substage is intermediate
between the supersonic and subsonic propeller substages. In the trans-sonic
regime an envelope around a magnetosphere of a neutron star passes through a
kind of a reorganization process. The envelope in this regime consists of two
parts. In the bottom one turbulent motions are subsonic. Then at some distance
$r_\mathrm{s}$ the turbulent velocity becomes equal to the sound velocity.
During this substage the boundary $r_\mathrm{s}$ propagates outwards till it
reaches the outer boundary, and so the subsonic regime starts.
  We found that the trans-sonic substage is unstable, so the transition between
supersonic and subsonic substages proceeds on the dynamical time scale. For
realistic parameters this time is in the range from weeks to years.
