The paper presents an investigation into the design and operation of an intelligent controller for a multimicroprocessor-based fault-tolerant aircraft control system. This unit, called an asynchronous consistency unit (ACU), uses an interactive procedure and a mutual feedback mechanism (called synchronisation voting) for the timing control of the exchange of data among processors. Fault tolerance is achieved by majority voting of the exchanges between various processors. This is implemented by using both hardware and software techniques (hence the name hybrid). The ACUs govern the reliable operation of the system so that no module failure (processor, ACU or bus) has hard core characteristics; i.e. it cannot affect the reliable operation of the system, causing a catastrophic failure. The design provides full fault assertion for the ACUs and notification of failures to the processors. An ACU requires no master clock operation and it operates independently of its host processor in an asynchronous mode. A four-module system is presented. A disadvantage of this design is that the total number of data and control lines of the bus is substantial. If N data bits are exchanged in a four-module system, the total number of lines in the ACU is at least 4N+32. This disadvantage is, however, offset by the stringent requirement of full system fault tolerance in both the data and control signals of the aircraft control system.