On any multi-engine aircraft, each engine-propeller combination produces its own pattern of vibration and noise. When two or more propellers turn at slightly different speeds, their pressure pulses interact in the cabin as an uncomfortable, pulsating beat frequency — the audible "wah-wah" drone familiar to passengers on twin-engine transports and general aviation twins alike. Two related but distinct systems were developed to combat this problem: propeller synchronizing and propeller synchrophasing. Both systems are found on a wide range of piston and turboprop multi-engine aircraft, and understanding how each works, why it matters, and how technicians maintain these systems is essential knowledge for the Aviation Mechanic Technician (AMT) Powerplant certificate.
While the two terms are sometimes used loosely, they address different aspects of the same root problem. A synchronizer matches rotational speed; a synchrophasor goes further and also controls the precise angular position of corresponding blades on each propeller. The result of full synchrophasing is not only identical RPM across all propellers but also a fixed, optimized relationship between blade positions — dramatically cutting the constructive interference of pressure pulses that causes cabin noise and airframe fatigue.
Propeller Synchronizing: Matching RPM
A basic propeller synchronizing system has one simple goal: keep all propellers turning at exactly the same rotational speed. Without synchronization, even a small RPM difference between two propellers — sometimes only a few RPM — is enough to create a noticeable beat frequency inside the cabin. The beat frequency equals the difference in RPM divided by 60, expressed in Hertz. For example, propellers differing by 12 RPM produce a beat of 0.2 Hz, perceived as a slow, nauseating throb.
In a typical light twin installation, the pilot first manually adjusts propeller controls until the RPM gauges read as close to identical as practical. The synchronizer then takes over as a fine-tuning device. The system designates one propeller as the master and one or more as slaves. A magnetic or optical pulse generator mounted on each propeller governor or engine accessory case generates electrical pulses at a rate proportional to propeller RPM. The control unit continuously compares the pulse rate of each slave to the master. Any detected difference produces a correction signal that adjusts the slave governor's speeder spring tension — in effect, slightly raising or lowering the governed RPM set point — until pulse rates match. The correction is continuous and automatic, keeping all propellers precisely synchronized without further pilot input.
It is important to understand that the synchronizer only trims the governor reference; it does not replace the governor. The governor remains the primary speed-controlling device. Synchronizers typically have a limited authority range that varies by manufacturer and installation, so the pilot must first set approximately equal RPM before the synchronizer can do its job. If the manual settings differ by more than the authority range, the synchronizer cannot bridge the gap and will simply drive the slave governor to its correction limit without achieving true synchronization.
Propeller Synchrophasing: Adding Phase Control
Synchrophasing builds on synchronization by adding precise control of the phase angle — the angular relationship between the blade positions of one propeller relative to another at any given instant. Even two propellers turning at exactly identical RPM will produce maximum noise if their blades happen to pass through the same angular position at the same time, because their pressure pulses add together (constructive interference). By rotating one propeller's blades to a specific offset angle relative to the other, the system can arrange for the pressure pulses to partially cancel each other (destructive interference), significantly reducing the overall cabin noise level.
The synchrophasor uses more sophisticated sensors than a basic synchronizer. Typically a phase reference sensor — often a magnetic pickup triggered by a single reference tooth or magnet on the propeller shaft or spinner — generates one pulse per revolution, establishing an absolute angular reference for each propeller. The electronic control unit compares not just the frequency of these once-per-revolution pulses (which reveals RPM difference) but also their timing offset (which reveals the phase angle between propellers). By commanding small, continuous corrections to the slave governor, the controller drives both RPM and blade phase angle to the desired values simultaneously.
The optimum phase angle for minimum noise is not always the same for every aircraft type and is typically determined by the manufacturer through flight testing and acoustic measurement. On many aircraft the phase angle setting is a fixed value built into the control unit. Some more sophisticated systems allow the pilot or crew to select from several preset phase angles using a cockpit switch, letting occupants find the quietest combination for a given cruise condition. The noise reduction achieved by synchrophasing can be substantial, and the human ear can perceive a meaningful decrease in loudness when phase angle is properly optimized.
System Components and Operation
A complete synchrophasing system typically includes the following major components:
- Phase/speed sensors: Magnetic pickups or Hall-effect sensors mounted close to a toothed wheel or trigger magnet on each propeller shaft. These generate the raw electrical pulses the control unit uses to determine both speed and phase.
- Electronic control unit (ECU) or synchronizer box: The brain of the system. It receives sensor signals, computes speed and phase errors, and outputs correction signals to the slave governor(s). Modern units are solid-state and highly reliable.
- Governor trim motor or solenoid: A small electric actuator — sometimes called a trimmer motor — integrated into or attached to the propeller governor. It adjusts the speeder spring preload in response to ECU commands, altering the governed RPM by a small amount. This is the only moving part the ECU directly controls.
- Cockpit controls: Typically a simple ON/OFF switch, and on some aircraft a phase angle selector. A synchronizer light or annunciator may indicate system status.
When the system is engaged, the ECU operates continuously. The pilot sets the desired RPM with the propeller control levers, engages the synchronizer/synchrophasor switch, and the system handles the rest. Most installations require that the manual RPM settings be within a certain tolerance before the system is effective, reinforcing the need for proper manual technique before relying on automation.
Why It Matters: Safety and Airframe Health
Propeller synchrophasing and synchronizing systems are not merely comfort features. Uncontrolled vibration from unsynchronized propellers accelerates fatigue in airframe structures, engine mounts, and avionics. Over time, beat-frequency vibration can loosen fasteners, crack brackets, and cause premature failure of components whose design life assumed controlled vibration environments. By eliminating the beat frequency entirely, these systems protect the airframe investment and extend component service life.
For flight crew, prolonged exposure to low-frequency beat vibration causes fatigue, impairs concentration, and in extreme cases contributes to spatial disorientation. Reduced cabin noise also improves intercom clarity and passenger communication, which has practical safety value during IFR operations and crew coordination tasks.
Key Numbers and Rules
- Beat frequency: Equals the RPM difference between two propellers divided by 60 (in Hz). A 12 RPM difference produces a 0.2 Hz beat.
- Synchronizer authority range: Limited and varies by manufacturer and system design; consult the aircraft/system maintenance manual for the specific range.
- Manual pre-set requirement: Pilot must manually match RPM within the system's authority range before engaging the synchronizer.
- Phase angle: The angular offset between blade positions of master and slave propellers; optimum angle is aircraft-specific and determined by the manufacturer.
- Noise reduction: Properly phased systems can achieve a meaningful, perceptible reduction in dominant propeller harmonic noise levels, with the specific amount depending on the aircraft and system.
- Master vs. slave: One propeller is designated master and all others are slaves that follow it; which engine/propeller is designated master is aircraft- and system-specific, as determined by the manufacturer's design — not a universal rule.
- System does NOT replace the governor: The governor remains primary; the trim actuator only nudges the governed speed reference.
AMT Maintenance Considerations
From a maintenance standpoint, AMTs must understand that synchrophasing system faults often show up as persistent cabin vibration complaints even when individual engine run-ups appear normal. Troubleshooting begins with verifying proper sensor gap — the clearance between the magnetic pickup and the trigger tooth or magnet must fall within the manufacturer's specified range, which varies by system and must be confirmed against the applicable maintenance manual. An incorrect gap produces weak or erratic pulses that confuse the ECU.
Wiring integrity is critical; shielded cables must be properly grounded to prevent electromagnetic interference from ignition systems from corrupting sensor signals. The ECU itself is generally a line-replaceable unit (LRU); if sensor checks and wiring are satisfactory, replacement of the ECU is the standard next step. After any maintenance affecting the system, a functional check must confirm that the synchronizer captures and holds both speed and, for synchrophasing systems, phase angle within published tolerances. This check is typically performed during a ground run-up and confirmed in flight per the aircraft maintenance manual.
Common Test Traps
- Confusing synchronizing with synchrophasing: Synchronizing matches RPM only. Synchrophasing matches both RPM and blade phase angle. Know the distinction.
- Forgetting the manual pre-set step: The synchronizer cannot function if RPM settings differ by more than its authority range. The pilot must manually set approximately equal RPM first.
- Thinking the ECU controls blade pitch: The system adjusts governor speed reference via a trim actuator — it does not directly command blade pitch angle changes.
- Assuming the system replaces the governor: The constant-speed governor remains the primary propeller speed control device; the synchrophasor merely trims its reference point.
- Overlooking sensor gap as a fault source: When a synchrophasing system malfunctions, incorrect magnetic pickup gap is one of the first things to check — it is a common, easily overlooked maintenance item.