Low-energy phonon spectra of lead zirconate being classic antiferroelectric have been simulated using the Vaks model with 5 modes. This model takes into account a flexoelectric coupling between 5 low-energy modes (3 acoustical and 2 optical ones). The Vaks model was shown to allow not only to achieve good agreement of calculated dispersions with experimental data, but to reproduce the temperature behaviour of TA phonon frequencies as well. As our calculations revealed, the system with 5 low-energy modes was close to structural instability stage. Even a weak change in coupling parameters could result in lattice instability which was characterized by qcrit wave vector. The qcriе-vector took any value including irrational one. In latter case the system would be close to go over into an incommensurate phase. As our calculations revealed, the system with 5 low-energy modes was close to structural instability stage. Even a weak change in coupling parameters could result in lattice instability which was characterized by qcrit wave vector. The qcri-vector took any value including irrational one. In latter case the system would be close to go over into an incommensurate phase.