With the introduction of large multi-engine aircraft the problem arose of how to measure various quantities such as pressure, temperature, engine speed and fuel tank contents at points located at greater distances from the cockpit. Many of the instruments then available operated on mechanical principles which could be adapted to suitably transmit the required information. For example, on one very early twin-engine aircraft, mechanically-operated engine speed indicators were designed with large-diameter dials so that by mounting the indicators in the engine nacelles they could be read from the cockpit.
Synchronous systems fall into two classes: direct-current and alternating-current. The principles of some of those commonly used form the subject of this chapter. Although varying in the method of data transmission, all the systems have one common feature: they consist of a transmitter located at the source of measurement and a receiver which is used to position the indicating element.
