  Simple derivation of the condition for the transition point from absolute
instability of plane dark solitons to their convective instability is
suggested. It is shown that unstable wave packet expands with velocity equal to
the minimal group velocity of the disturbance waves propagating along a dark
soliton. The growth rate of the length of dark solitons generated by the flow
of Bose-Einstein condensate past an obstacle is estimated. Analytical theory is
confirmed by the results of numerical simulations.
