On propeller-driven multiengine aircraft, even a slight difference in RPM can create uncomfortable noise and vibrations. You can realize that engine RPMs are not synchronized during the flight if you hear a strange intermittent beat. The RPM indicator only allows the pilot to set the RPM roughly, but discrepancies in engines’ speed can be hard to adjust. To overcome this problem, some aircraft are fitted with an instrument called a synchroscope.

In a synchroscope, one engine is identified as the master, and the others are the slaves. The instrument indicates on a dial by means of a pointer, the variation in RPM relative to the master engine. Generally, the synchroscopes use each engine's tachometers to convert the rotational speed into an electrical current. In this way, if the slave engine is turning faster, it produces a bigger current, and the pointer will turn in a given direction. If the slave engine is rotating slower, the current provided to the synchroscope will be lower, and the pointer will turn in the opposite direction.

Synchroscopes may have different operating principles; some may work based on a supplied voltage; others may compare each engine's alternators' frequencies. Modern propeller-driven multiengine airplanes have their engine RPMs automatically synchronized through various control units.