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The results of 3D modelling of the flow structure in the classical symbioticsystem Z~Andromedae are presented. Outbursts in systems of this type occur whenthe accretion rate exceeds the upper limit of the steady burning range.Therefore, in order to realize the transition from a quiescent to an activestate it is necessary to find a mechanism able to sufficiently increase theaccretion rate on a time scale typical to the duration of outburst development.  Our calculations have confirmed the transition mechanism from quiescence tooutburst in classic symbiotic systems suggested earlier on the basis of 2Dcalculations (Bisikalo et al, 2002). The analysis of our results have shownthat for wind velocity of 20 km/s an accretion disc forms in the system. Theaccretion rate for the solution with the disc is ~22.5-25% of the mass lossrate of the donor, that is, ~4.5-5*10^(-8)Msun/yr for Z And. This value is inagreement with the steady burning range for white dwarf masses typicallyaccepted for this system. When the wind velocity increases from 20 to 30 km/sthe accretion disc is destroyed and the matter of the disc falls onto theaccretor's surface. This process is followed by an approximately twofoldaccretion rate jump. The resulting accretion rate growth is sufficient forpassing the upper limit of the steady burning range, thereby bringing thesystem into an active state. The time during which the accretion rate is abovethe steady burning value is in a very good agreement with observations.  The analysis of the results presented here allows us to conclude that smallvariations in the donor's wind velocity can lead to the transition from thedisc accretion to the wind accretion and, as a consequence, to the transitionfrom quiescent to active state in classic symbiotic stars.Comment: 21 pages, 7 figure

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Three-dimensional gas-dynamical modeling of changes in the flow structure during the transition from the quiescent to the active state in symbiotic stars