an apparatus , for example a gas turbine engine is normally controlled by a digital first computer which is responsive to engine operating conditions . a second computer is responsive to selected ones of said conditions . control signals from the digital computer are monitored by a circuit which generates analog signals which lie within acceptable ranges and which correspond to the values generated by the digital computer . the monitor circuit supplies the engine with signals selected from whichever of the analog value and corresponding digital value lies farthest from a predetermined limit . predetermined levels of discrepancy between any of the analog values and its corresponding digital value cause control of the engine to be transferred to the second computer , which may also be a digital computer .