Flying a twin-engine aircraft with two independently governed propellers is not as acoustically smooth as it might seem. Even when both tachometers read the same RPM, the slightest difference between the two propellers creates a pulsing, low-frequency beat that passengers and crew quickly find fatiguing. Propeller synchronization and synchrophasing are systems designed to eliminate that phenomenon. Understanding how they work, why they matter, and when they are — and are not — appropriate to use is an important part of multi-engine systems knowledge for any pilot seeking an airplane rating or operating a light or turboprop twin.
These two systems are related but distinct. Synchronization matches RPM; synchrophasing goes one step further by also matching the angular position of each propeller's blades relative to the others. Both are found on modern twins and pressurized turboprops, and both are covered in the FAA Airplane Flying Handbook (FAA-H-8083-3C), Chapter 13.
The Beat-Frequency Problem
When two propellers turn at slightly different speeds, their pressure pulses reach the airframe at offset intervals. The result is a periodic reinforcement and cancellation of sound waves — the classic wah-wah-wah beat tone. The frequency of that beat equals the difference in RPM divided by 60. For example, if the left propeller turns at 2,400 RPM and the right turns at 2,402 RPM, the beat frequency is 2 cycles per second (2 Hz), which falls squarely in the range humans find most annoying and fatiguing. Even a 1 RPM difference produces a noticeable, slow pulse. Because propeller governors are mechanical devices with small tolerances, they cannot perfectly match speed under all conditions without electronic assistance.
Beyond passenger comfort, sustained vibration at beat frequencies can accelerate fatigue in airframe structures, loosen avionics connectors, and make precise instrument readings more difficult. Eliminating the beat is therefore not merely a luxury — it is an operational and maintenance concern.
How Propeller Synchronization Works
A propeller synchronization system (commonly called a prop sync) uses electronic governor control to slave one propeller's speed to the other. The pilot selects one engine as the master and the other becomes the slave. A sensor on each propeller shaft — typically a magnetic or optical pulse generator — feeds RPM data to a synchronizer control unit. The unit continuously compares the two signals and sends small electrical trim commands to the slave governor, nudging its blade pitch up or down by tiny increments until the slave RPM exactly matches the master. Once locked, both propellers turn at precisely the same speed, eliminating the beat frequency entirely.
Most synchronization systems engage automatically once the RPM difference falls within a capture range — often within about 100 to 150 RPM of each other. Outside that band, the system cannot lock in and the pilot must manually adjust the propeller controls closer before engagement. This is a common test point: the prop sync system is not a substitute for manually setting approximate RPM first.
How Synchrophasing Works
Synchrophasing extends the synchronization concept by controlling not just speed but also the phase angle — the rotational position of one propeller's blades relative to the other's at any given instant. Even with identical RPM, two propellers whose blades happen to be perfectly in phase (both passing the 12-o'clock position at exactly the same time) produce reinforced pressure pulses that are louder than if the blades were offset. By electronically adjusting the phase relationship, the synchrophaser can place the pressure pulses so they partially cancel each other inside the cabin.
The system again uses shaft-position sensors. The control unit monitors the timing of each blade-pass event and adjusts the slave governor with precise, small corrections to maintain a target phase offset — often between 15° and 30°, though the optimal angle varies with aircraft type and is typically determined during certification testing. The pilot usually has a phase control knob or switch that allows adjustment of the offset angle to find the quietest setting for a given flight condition. Some aircraft automate this entirely, while others give the pilot manual phase control.
Synchrophasing systems are found on many turboprop twins and on higher-end piston twins. They are particularly effective in aircraft with propellers close to the fuselage, where blade-pass pressure waves couple strongly into the cabin structure.
Why It Matters Operationally
The practical benefits of synchronization and synchrophasing are significant. Reduced vibration lowers crew fatigue on long flights, making for better decision-making and situational awareness. Passengers experience markedly less low-frequency noise, a major factor in charter and corporate aviation. From a maintenance perspective, lower sustained vibration reduces wear on engine mounts, avionics racks, and structural joints.
There is also an instrument-accuracy consideration. Vibration can cause instrument pointers to oscillate, making precise readings harder. On glass cockpit aircraft the effect is less pronounced, but on round-dial aircraft in older twins, sustained beat vibration can noticeably blur needle position.
Key Limitations and Appropriate Use
Synchronization and synchrophasing systems are cruise and cruise-climb tools only. They must be disengaged — and propeller controls returned to individual manual control — for:
- Takeoff and initial climb: Full, unrestricted governor authority is required; slaving one governor to the other during takeoff reduces flexibility to respond to engine anomalies and may mask developing problems.
- Single-engine operations: If an engine fails, the sync system should be turned off immediately. With one engine inoperative, the slave-master relationship is meaningless, and the system may interfere with proper propeller feathering or governor response on the operating engine.
- Approach and landing: Again, full individual governor authority is needed.
- Any abnormal or emergency procedure: Restore manual individual propeller control so the pilot has maximum authority over each propeller independently.
Most manufacturer checklists specify engaging prop sync after level-off at cruise altitude, after power is set, and disengaging it before beginning descent or any non-normal procedure. Pilots should follow the Pilot's Operating Handbook (POH) for their specific aircraft, as activation procedures and capture ranges vary.
Key Numbers and Rules
- Beat frequency: Equals the RPM difference divided by 60 (e.g., 3 RPM difference = 0.05 Hz beat — extremely slow and perceptible as a long, slow pulse).
- Capture range: Typically within 50–150 RPM of each other, depending on aircraft type, before the sync system can lock in.
- Phase offset: Synchrophasing systems typically allow adjustment from approximately 0° to 30° or more; the quietest angle is found by experiment in cruise.
- System status: Engage in cruise after power stabilized; disengage before descent, approach, single-engine operations, or any abnormal/emergency checklist.
Relationship to Other Multi-Engine Systems Knowledge
Propeller synchronization is one piece of a broader understanding of multi-engine propeller systems that includes propeller feathering, autofeather systems, and the aerodynamic effects of windmilling versus feathered propellers. A windmilling propeller on a failed engine produces significant drag — far more than a feathered propeller — which is why feathering is critical to OEI climb performance. Synchronization and synchrophasing, by contrast, deal with the operating propellers and are purely about efficiency and comfort, not emergency response. Keeping these systems conceptually separate — normal-operation tool vs. emergency procedure — is essential for both the written test and practical examination.
It is also worth noting that counter-rotating propellers, fitted to some twins, eliminate the P-factor asymmetry that defines the critical engine problem, but they do not inherently eliminate the beat-frequency issue. Those aircraft still benefit from synchronization and synchrophasing systems.
Common Test Traps
- Confusing sync with synchrophasing: Synchronization matches RPM only; synchrophasing additionally matches blade phase angle. They are related but not the same system — some aircraft have sync without synchrophasing.
- Forgetting to pre-set RPM manually: The sync system cannot lock onto a large RPM difference. Students incorrectly assume engaging the switch does all the work; manual coarse adjustment comes first.
- Using prop sync during takeoff: This is specifically inappropriate. Full, independent governor authority is required for all takeoff and critical flight phases.
- Leaving sync engaged after engine failure: The system must be turned off immediately during single-engine operations. Leaving it on can interfere with emergency procedures and is incorrect airmanship.
- Assuming synchrophasing eliminates all vibration: It reduces cabin noise and beat vibration significantly but does not eliminate all propeller-induced vibration. Engine mounts, imbalanced propellers, and airframe resonance are separate issues.