Patent US 4,130,241

A quadruplex, double-failure survival, fly-by-wire aircraft control system includes two monitor lanes whose outputs are consolidated with those of the four main control lanes to drive the aircraft control-surfaces by hydraulic rams via individual spool-valve actuators. Nominally-identical sets of pilot's-demand, instrument, and position-feedback signals are derived independently for the four lanes respectively, and digital representations of all these are exchanged between the control lanes for the selection of their mean or median values by digital processors that also serve to compute the control outputs of the lanes from the selected values. The sets of signals supplied to one pair of the four lanes are reversed in sense compared with the sets supplied to the other pair so as to ensure that the same digitally-encoded data is represented differently in the different pairs and the possibility of a context-dependent failure occurring simultaneously in both pairs is thereby avoided. The outputs of the control lanes are amalgamated to provide intermediate-value inputs to the monitor lanes. An alternative monitored-duplex single-failure survival fly-by-wire system has two control lanes with reversal of sense of input and feedback signals as between one lane and the other. Difference of digital representation in a third, monitor lane is provided by halving the amplitude of the input and feedback signals supplied to this lane.