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Prop Sync Types (which do you have?) - Tom Clements, King Air Academy


Prop Sync Types (which do you have?) - Tom Clements, King Air Academy

How can you tell if your King Air has the older Type I or the newer Type II Propeller Synchrophaser system? There are two ways to tell from the cockpit.

First, does the label by the Prop Sync switch merely say “On-Off” or does it also have the statement “Must Be Off for Takeoff and Landing?” If you have the former, you have Type II. If you have the extra comment of the latter, it’s Type I.

The second way to tell involves turning the battery switch on. When you turn on the Prop Sync switch here, on the ground with the gear handle down – I sure hope so! – do you get a yellow “Prop Sync ON” annunciator? Yes? Then you have Type I. No? Then it’s Type II. Type I uses an electromechanical device on the right engine that can make minor adjustments to the governor attachment end of the propeller cable. It can make the cable longer to slightly increase the governor’s speed setting or shorter to decrease the speed setting in an attempt to make the right propeller speed match the left. The range is quite small, about 30 RPM total, or 15 RPM from neutral. Although limit switches inside the assembly should stop the motor and prevent any harm when and if the device drives the adjustment mechanism to an extreme, it is not uncommon to find that in fact, the mechanism binds up or jams when an extreme travel limit is reached.

The pilot has two ways of telling that his sync motor has jammed. First, the dang system is inop. Unless the propeller levers are carefully adjusted by the pilot, the disconcerting whaa-whaa sound of out-of-sync propellers is heard. Second, there now appears a significant misalignment, or stagger, between the left and right propeller knobs in the cockpit.

If the right knob is well behind the left, then the sync motor has probably jammed in the “Increase RPM” position, making the cable longer and requiring the knob in the cockpit to be further back to compensate for the longer cable. Vice versa, if the right knob is forward, the likelihood is that the mechanism is jammed in the “Decrease RPM,” or short, position.

Luckily, it is usually fairly easy for maintenance personnel to access the assembly, get it back to neutral, and adjust the limit switches as needed.

This tendency to jam when reaching a limit is why the extra panel statement – Must be Off for Takeoff and Landing – is a required placard. Whenever the combination of power and airspeed are both sufficiently low such that the propellers are not yet on their governors, there is no way that they will always have their speeds perfectly matched. This will certainly occur during the early stages of the takeoff roll and during the flare and rollout stages of the landing. If sync is on now, the poor mechanism will assuredly be driven to a limit. In fact, as the pilot or the wind gusts cause some speed fluctuations, there is an excellent chance that the mechanism will be driven back and forth from one extreme to the other…just asking for a problem.

Although the placard only speaks of takeoffs and landings, the switch should always be off whenever there is a reasonable chance that the propellers will not be at or near the identical speed. Doing slow flight and stall practice, during any simulated or actual single-engine maneuvering…make sure the switch is off. When you fly a King Air for the first time, you should have the switch off when moving the propeller levers from takeoff to climb speed and from climb, speed to cruise speed. This allows you to find and know what, if any, propeller lever stagger is required in this particular airplane to set the governors at the same speed. Since the adjustment range of the sync system is small, you need to make sure the propeller speeds are very close together before using sync. However, once you know, for example, that the left knob needs to trail the right knob by 1/8 of an inch, then it is perfectly acceptable to leave sync on when changing RPM.

Type II, on the other hand, does not have a slave (right) unit attempting to follow a master (left) unit. Instead, the slower always tries to rise to the faster speed, but again in a very limited range. Furthermore, and more important, there is no electromechanical mechanism to bind up or jam! Instead, speed adjustments are made by varying the strength of a magnetic field inside the governor itself. Cool! Since there is no worry about the mechanism binding, we can leave it on all the time even when the propellers are not on the governors, not in sync.

However, even Type II has its potential problems, although minor.

First, the darn thing seems to go nuts every so often. There you are in steady-state cruise and suddenly the props go well out-of-sync. What the heck?! Turn off the sync switch, sync the propellers manually, and then turn the switch back on. You will find in almost every case that the system goes back to being its usually obedient self. If this happens once or twice every few flights, you will just learn to live with it because no problem will be found when troubleshooting takes place. Only if it becomes a continuous bother on nearly every flight should you spend money having the mechanic find and fix the problem.

The second problem with Type II appears rarely and is more common on the 300-series than the 90- and 200-series. When nearing the runway for landing – or when slowing toward a stall during flight training – once in a great while you will feel and hear the airplane start doing a rhythmic left-and-right yawing “dance.” When you examine the engine instruments, you will probably notice that torque and propeller speed is fluctuating.

The cause of this is a battle between the two governors as the propellers just start slowing off of the selected speed. One side may drop a little speed first and so the sync system speeds it up. Now the other side is slow, so it gets boosted up. A little battle ensues, with each side increasing then easing its governor’s speed setting…leading to the yawing dance you are feeling. Solution? Turn the sync switch off and – Voila! – things immediately smooth out as they should.

I hope this provides some interesting and important details about this little system that contributes its share to passenger comfort.

Author: Tom Clements, King Air Academy.

Image: Skylar Manning (@skylarmanning IG)