P-120-A stands for automatic pitch control. A selector switch activates the desired mode of operation. For normal flight the switch has to be set to "AUTO" = automatic mode.
On P-120-A control units with serial no. WA01-xxx both switches must be set to "AUTO" mode. In "AUTO" mode the desired RPM is set by a RPM dial selector, installed in the center of the control unit's panel. For take off, the RPM is set to the max. allowed RPM. A sensor in the brush block assembly picks up six impulses per revolution from bolt heads installed in the spinner back plate and generates the RPM information for the control unit. In the control unit these impulses are compared to the set value and the electric current is automatically activated in the needed direction of the pitch change. The continuous RPM comparison results in a constant speed operation with the blade pitch being automatically changed according to changes in power and airspeed. If the power of the engine is reduced and the RPM falls below the preset value, the blades will change pitch until low pitch stop is reached. When reaching low pitch stop, the control unit switches the current off and the green light indicating take off position goes on. The set RPM is kept constant by approx. ± 30 RPM.

In "START" mode , the pitch is changed until the low pitch stop (take-off position) has been reached and blocked in this position. In "SEGEL" mode, the pitch is changed until high pitch stop (feathering position) has been reached. Once the propeller is at either one of the pitch stops, the control unit switches the current off automatically.
For P-120-A controls with serial no. WA01-xxx the left switch must be set to "MANU" mode and the right switch to "START" mode. If "START" mode is selected, the blades are blocked in low pitch position. If "SEGEL" mode is selected, the pitch is changed until the high pitch stop (feathering position) has been reached. Once the propeller is at either one of the pitch stops, the control unit switches the current off automatically.

These two modes of operation (START and SEGEL) are only used for changing pitch into feathering or for returning from feathering position back into low pitch position. In case of a malfunction in the automatic control, the RPM can still be changed by putting the selector switch into one of the two positions (START for changing pitch towards low pitch or SEGEL for changing pitch towards feathering) for a short moment and shutting the current off with the circuit breaker, which stops the blade pitch at the present position.

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