There are two types of prop sync systems used in the Twin Commander—prop sync and syncrophase. In this article I am referring to the prop sync system, as the syncrophase uses a different method of adjusting RPM. Syncrophase also adds the functionality of controlling prop phase angles between the two sides, instead of just matching RPM as the prop sync system does. The three main components of the prop sync system are the controller (mounted on the forward side of the rear pressure bulkhead), actuator (mounted on the righthand engine firewall), and RPM pickups (mounted to a bracket on the engine output gearbox with targets on the prop slip-ring platter, or the pick-ups in the prop governor are utilized in parallel with the three-speed switch).
The actuator is a simple ratchet mechanism with solenoids, one for each direction. The output of the actuator connects to a flex shaft which turns a jack screw in the linkage actuator attached to the FCU speed shaft and prop governor. The actuator receives a short pulse of 28 volts to either the increase or decrease solenoids, which drives the ratchet mechanism to trim prop governor speed setting. The range of motion in the mechanism can trim prop governed RPM only approximately 1.5 to 3 percent. The prop sync controller receives input pulses from each of the pick-ups and sends trim pulses out to the actuator to drive the right engine RPM up or down until they match. Any time the prop sync switch is on, the controller will send output pulses to drive the actuator.
The most common cause of failure I see is the actuator sticking against one of the extreme limits, a result of either dirt build-up causing the jack screw to stick or the ratchet mechanism being driven against its internal stop. Flushing out the actuator jack screw and re-lubricating with light oil (LPS1 or ACF-50) usually resolves the sticking actuator. If the servo is against its stop, applying voltage pulses to re-center the actuator will unstick the mechanism.
The other common cause of failure is RPM signal not getting to the controller. Typically, there is some kind of disconnect between the pick-up and the firewall connector. A knife- or handshake-style splice tend to be common culprits here. If during troubleshooting you’d like to check for good signals, a portable oscilloscope connected to the monopole terminal block at the controller is all that is needed. Simply turning the prop by hand while observing for a clean square wave will confirm you have signal. You can also measure the resistance through the two monopoles from this point for comparison. Pay particular attention to the diodes here as they can cause signal issues if not working properly.
For the pilots, some recommended best practices will greatly help the prop sync system perform. Only utilize prop sync when RPM is reduced below 100 percent and large power changes aren’t expected. Manually match engine RPM to within 0.5 percent before turning the system on. And don’t forget to turn it off when landing or going back to 100 percent RPM. Avoiding situations that would drive the actuator against its limits always helps, such as shutting down with prop sync still on or adjusting engine RPMs also with the sync on.
Andre Pridgen is the service center manager at Aero Air, and an experienced Twin Commander technician.
