Francis turbines operate in many cases out of its best effici ency point, in order to regulate their output power according to the instantaneous ener gy demand of the grid. Therefore, it is of paramount importance to analyse and determine the unst able operating points for these kind of units. In the framework of the HYPERBOLE project (FP7-E NERGY-2013-1; Project number 608532) a large Francis unit was investigated numericall y, experimentally in a reduced scale model and also experimen tally and numerically in the real prototype. This paper shows the unstable operating points identified during the experimental te sts on the real Francis unit and the analysis of the main characteristics of these instabilities. Fi nally, it is shown that similar phenomena have been identified on previous research in the LMH (Laboratory for Hydraulic Machines, Lausanne) with t he reduced scale model.