# Synchroscope Operation: Multi-Engine Propeller RPM Synchronization

> Learn synchroscope principles for multi-engine propeller aircraft, including AC frequency comparison, master-slave adjustment, and vibration reduction.

- **Subject:** 022 — Instrumentation
- **Section:** 022-01 — Sensors and Instruments
- **Topic code:** 022-01-08
- **Last reviewed:** 2026-03-27
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/instrumentation/sensors-and-instruments/synchroscope/

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In multi-engine propeller aircraft, unsynchronized engine speeds (RPM) generate uncomfortable "beat" noises and vibrations, leading to structural fatigue and passenger discomfort.

## Purpose and Function
A **Synchroscope** allows pilots to manually synchronize the RPM of a "slave" engine to a "master" engine with high precision.
*   **Mechanism:** The instrument receives AC signals from the **tacho-generators** (alternators) of both engines. It compares the **frequencies** of these AC signals.
*   **Indication:**
    *   If frequencies match, the engines are synchronized, and the indicator (a small rotating propeller or pointer) stops.
    *   If the slave is faster, the pointer rotates in one direction.
    *   If the slave is slower, it rotates in the opposite direction.
*   **Operation:** The pilot adjusts the throttle of the slave engine until rotation stops. Modern aircraft perform this synchronization automatically.

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Free study notes from pilotsyllabus.com. Not a substitute for training at an approved ATO.