The results of 3D modelling of the flow structure in the classical symbiotic system Z Andromedae are presented .
Outbursts in systems of this type occur when the accretion rate exceeds the upper limit of the steady burning range .
Therefore , in order to realize the transition from a quiescent to an active state it is necessary to find a mechanism able to sufficiently increase the accretion rate on a time scale typical to the duration of outburst development .

Our calculations have confirmed the transition mechanism from quiescence to outburst in classic symbiotic systems suggested earlier on the basis of 2D calculations ( Bisikalo et al , 2002 ) .
The analysis of our results have shown that for wind velocity of 20 km/s an accretion disc forms in the system .
The accretion rate for the solution with the disc is \sim 22.5 - 25 \% of the mass loss rate of the donor , that is , \sim 4.5 - 5 \cdot 10 ^ { -8 } M _ { \odot } /yr for Z And .
This value is in agreement with the steady burning range for white dwarf masses typically accepted for this system .
When the wind velocity increases from 20 to 30 km/s the accretion disc is destroyed and the matter of the disc falls onto the accretor ’ s surface .
This process is followed by an approximately twofold accretion rate jump .
The resulting accretion rate growth is sufficient for passing the upper limit of the steady burning range , thereby bringing the system into an active state .
The time during which the accretion rate is above the steady burning value is in a very good agreement with observations .

The analysis of the results presented here allows us to conclude that small variations in the donor ’ s wind velocity can lead to the transition from the disc accretion to the wind accretion and , as a consequence , to the transition from quiescent to active state in classic symbiotic stars .