It seems now important to consider bulk velocity gradients inside jets (Blandford, 1993; Biretta et al, 1995) and recent results from tomography technique suggest the presence of different components in radio sources (Rudnick, this symposium). A few jet models take explicitly into account two components with different bulk velocities (Smith, Raine, 1985; Baker et al, 1988; Sol et al, 1989; Achatz et al, 1990). A fast beam comes from the vicinity of the black hole while a slower collimated wind is emitted by the accretion disk. When stable, the two components can survive along the jet. If a fast instability develops at a distance Dc
from core, the beam is destroyed over some relaxation length. Its energy and momentum are transmitted to the wind. Apparent change in the flow regime such as slowing down, decollimation, local bending or discontinuity in surface brightness is the expected observational signature of such critical zones on radio maps. The exact appearance of the zone depends on the parameters of the two flows and on their interaction regime.