[en] A new method of studying tokamak helical instabilities is developed. An equation of small MHD oscillations written in covariant terms is derived. An ''ideal'' limit of the equation is considered, and its connection with the potential energy functional present in the energy method of MHD-instability investigation has been established. It is considered how with the help of covarient equation of small oscillations it is possible to obtain the known results concerning small-scale MHD perturbations. Derived are initial equations of the theory of the lowest helical modes of the tokamak with an arbitrary form of the cross section. It is shown how plasma inertia is taken into account in ideal MHD modes in a covariant way. The general theory developed is applied to the problem of ideal internal helical modes