this line from its usual return pressure 17 value to supply pressure 16 the signal of the state line 29 may be used as a criteria to initiate the control transfer sequence from this main servo actuation system to a back up servo actuation system this transfer must be accomplished by elements of the global servo actuation system which are not the subject of this invention the transfer may be 1 partial keeping piston 3 and position transducer 4 as part of the backup servo actuation system i e disconnecting control lines 18 19 from the piston 3 in points 31 32 and connecting those points to the control lines of the back up servo actuation system 2 total where the back up servo actuation system has its own piston and position transducer should this be the case the control transfer should be made between the outlet functions of both pistons the type of transfer made will be greatly dependent upon the reliability of the piston used if the potential of this invention needs to be used to override e g possible piston seizures the use of the type of transfer indicated in point 2 is recommended b system with four way servovalves 1 and 2 fig3 and 4 this type of system is designed for the actuation of a piston subjected to any loading and will be able to detect and self compensate for any single failure in the feedback position control loop or any servovalve the principle of operation of this system is very similar to that of three way servovalves 1 2 in fig1 and 2 and as such the description made in section a is most applicable the description.