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since heat is applied just long enough to melt the ice face in contact with the blade. If heat supplied to an icing surface is more than that required for melting just the inner ice face, but insufficient to evaporate all the water formed, water will run back over the unheated surface and freeze. Runback of this nature causes ice formation on uncontrolled icing areas of the blade or surface. 

Cycling timers are used to energize the heating element circuits for periods of 15 to 30 seconds, with a complete cycle time of 2 minutes. A cycling timer is an electric motor driven contactor that controls power contactors in separate sections of the circuit. Controls for propeller electrical deicing systems include on-off switches, ammeters or loadmeters to indicate current in the circuits, and protective devices, such as current limiters or circuit breakers. The ammeters or loadmeters permit monitoring of individual circuit currents and reflect operation of the timer. To prevent element overheating, the propeller deicing system is used only when the propellers are rotating and for short test periods of time during the takeoff check list or system inspection. 

## Propeller Synchronization and Synchrophasing 

Most multi-engine aircraft are equipped with propeller synchronization systems. Synchronization systems provide a means of controlling and synchronizing engine rpm. Synchronization reduces vibration and eliminates the unpleasant beat produced by unsynchronized propeller operation. 

A typical synchrophasing system is an electronic system. [Figure 7-31]  It functions to match the rpm of both engines and establish a blade phase relationship between the left and right propellers to reduce cabin noise. The system is controlled by a two-position switch located forward of the throttle quadrant. Turning the control switch on supplies 

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direct current (DC) power to the electronic control box. Input signals representing propeller rpm are received from magnetic pickup on each propeller. The computed input signals are corrected to a command signal and sent to a rpm trimming coil located on the propeller governor of the slow engine. Its rpm is adjusted to that of the other propeller. 

## Autofeathering System 

An autofeather system is used normally only during takeoff, approach, and landing. It is used to feather the propeller automatically if power is lost from either engine. The system uses a solenoid valve to dump oil pressure from the propeller cylinder (this allows the prop to feather) if two torque switches sense low torque from the engine. This system has a test-off-arm switch that is used to arm the system. 

## Propeller Inspection and Maintenance 

Propellers must be inspected regularly. The exact time interval for particular propeller inspections is usually specified by the propeller manufacturer. The regular daily inspection of propellers varies little from one type to another. 

Typically, it is a visual inspection of propeller blades, hubs, controls, and accessories for security, safety, and general condition. Visual inspection of the blades does not mean a careless or casual observation. The inspection should be meticulous enough to detect any flaw or defect that may exist. Inspections performed at greater intervals of time (e.g., 25, 50, or 100 hours) usually include a visual check of: 

1. Blades, spinners, and other external surfaces for excessive oil or grease deposits. 

2. Weld and braze sections of blades and hubs for evidence of failure. 

