A controllability analysis of a paper machine Forming zone in a newsprint mill was performed to determine the maximum allowable variability of the pulp furnished to the process. The goal was to keep the output variables, basis weight and first-pass retention between certain limits in spite of disturbances and paper grade changes. The results indicate that the process is more disturbed at high frequencies, and that the two output variables may be maintained within the interval +/-1% of their nominal values if the maximum variability in the consistency and fines content of the thick-stock is kept lower than +/-1 and +/-2%, respectively.