Manual override relays:
When the manual-override relays are not energised, this current flows through brushes riding on slip rings mounted on the propeller spinner bulkhead and into the heating elements bonded to the propeller blades. The slip rings are connected to the heater elements through flexible conductors that allow the blades to change their pitch angle.
The timer sends current through the right propeller for about 90 seconds, then switches over and sends current through the left propeller for 90 seconds.
Some propeller de-icing systems have two separate heating elements on each blade. Current flows through the right propeller outboard element for about 30 seconds, then through the right propeller inboard element for the same length of time. After the right propeller is de-iced, the timer shifts over and sends current through the left propeller outboard elements and then the left propeller inboard elements.
Current cycles of the two propellers are controlled by the timer as long as the propeller Auto Prop De-ice switch is on. When the Manual Prop De-icer switch is held in its momentary on position, the two manual-override relays are energised, and current flows directly from the bus to the blades without going through the timer.
The pilot can easily tell whether or not the de-icing system is operating correctly in the Automatic mode by watching the propeller ammeter. It will indicate a flow of current each time one of the heater elements draws current.