Engine control system

A power turbine speed control system (10) for a helicopter is disclosed which includes a) means for generating a power turbine speed signal (NF) and a power turbine shaft torque signal (QS) from a power turbine and rotor drive train; b) filtering means (40) for filtering the power turbine speed signal (NF) by effectuating a rapid attenuation of main and tail rotor torsional frequencies in the power turbine speed signal (NF) without compromising phase at low frequencies; and c) damping means (14) for actively damping main and tail rotor torsional frequencies in the power turbine shaft torque signal (QS).