Deice boot DC bus Syncrophasing controller DC power input to controller Output to left governor Left engine input to controller Right engine input to controller Output to right governor Propeller spinner Propeller governor Propeller synchronization OFF RPM sensor RPM sensor Synchroscope Figure 7-31.  Synchrophasing system.  Figure 7-30.  Electric deice boot.  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. 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 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. 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.  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. Propeller Synchronization and Synchrophasing Propeller Inspection and Maintenance Autofeathering System 7-20