- Abstract The evolution of weak large-scale disturbances in the helical turbulence is considered. High-order statistical moments are taken into account in the frames of two-scale analog of Orszag diffusion approximation. The appearance of the instability of second moments is demonstrated for k< α/ν (k is wave vector, α and ν are proportional to the mean helicity and mean energy viscosity, respectively). Turbulent viscosity diminishing in comparison with the non-helical case is also demonstrated. These phenomenona qualitatively agree with previous results of other authors on the slow-down of energy transfer along the spectrum, from large to small scales at non-zero helicity.