Synchronization of Turbofan Engines

It is widely accepted that aircraft with multi-engine configurations are likely to experience physical phenomena mainly caused by variations in the rotational speed of moving assemblies of installed turbofan engines including the fan, low, and high-pressure compressors. These physical phenomena can take the form of beat frequencies, annoying noises, and sometimes severe vibrations. An initiative to keep these occurrences and further consequences to minimum is to implement a system that constantly monitors and controls the operating speed of the fan of the engines. This method, usually titled engine synchronization, provides the aircraft and engine manufacturers to reduce the consequences of engines running at different speeds. Aircraft with 4 engines, such as BAe 146 are more prone to experience these phenomena.

The synchronization system architecture generally incorporates the Thrust Modulation Computer (TMS) of engines as the main processing unit of the system operation. TMS receives various signals necessary for its internal calculations processes from a wide variety of computers and sensors. Each engine features its independent processing unit which is linked to other engines’ processors. This interconnection enables the modulating computers the constantly compare their commands with other counterparts and achieve the best performance requirements. TMS computer mechanically (on BAe 146) moves an electromechanical actuator on engine Hydromechanical Unit (HMU) which regulates the amount and flow of fuel into the fuel nozzles, controlling the RPM of the engine.

The synchronization system selects on the engine (automatically or manually by the pilot) as the master and the other engines as slaves following the RPM of the master. This ensures the uniformity and accuracy of the system and reduction in the crew workload since they will no longer have to constantly monitor and control the fan speed of 4 engines simultaneously. 

Like many other systems installed on the aircraft, synchronization and Thrust Modulation Systems are designed to enhance the safety and comfort of the crew and on-board passengers and reduce the crew workload as well, but as it affects the operation of all the four engines of the aircraft by commanding the slave engines to follow the master, flight crew are recommended to constantly monitor the operation of the system through observing N1 and N2 parameters of the engine to take appropriate actions if needed.

Here are some images of the actual system installed on BAe 146. The system enables the pilot to select either engine No.1 or No.2 as master and choose between N1 (fan speed) and N2 (high-pressure compressor speed) as the parameter to be synchronized. However, it should be mentioned that TMS and synchronization system on BAe 146 provide a limited amount of correction in throttle movement and thrust generation in comparison with Full Authority Digital Engine Control (FADEC) on latest aircraft generations.