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I've never understood this, why can't you just idle the engine and let it slow down?

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    $\begingroup$ In the case of the one propeller overspeed I experienced, idling the engine reduced the overspeed, but it was still well above red line. The only way I knew that closing the throttle reduced the overspeed was that the noise, which was considerable, lessened somewhat. The cause of the overspeed was found to be congealed oil in the propeller hub. The airplane had set outside for three nights in sub-zero temperatures. The aircraft was a Cessna 310. The overspeed occurred right after raising the gear. I was alone in the airplane. $\endgroup$ – Terry Mar 7 '15 at 17:44
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Often the overspeed is caused by the airflow forcing the propeller to move faster. It's like braking on the engine while going down a steep hill, even without touching the gas pedal you may speed up and redline the engine. Very problematic if you already overheated your brakes in a 18 wheeler.

If you have a variable pitch prop you can feather it to reduce the effect of the airflow. Or just follow the procedures in the overspeed checklist for the aircraft.

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    $\begingroup$ Of course usually the reason for the overspeed is that the pitch control failed, so you may not be able to feather it. Otherwise variable pitch propellers are generally constant speed, so they automatically adjust pitch to avoid overspeeding when the pitch control works. $\endgroup$ – Jan Hudec Mar 9 '15 at 10:20
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    $\begingroup$ But wouldn't you be able to pull up and slow the airflow? $\endgroup$ – ptgflyer Mar 9 '15 at 11:44
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    $\begingroup$ @ptgflyer slowing down could cause the plane to stall and would not necessarily be enough to slow down the prop. $\endgroup$ – fooot Mar 9 '15 at 14:41
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    $\begingroup$ That's a good point. $\endgroup$ – ptgflyer Mar 9 '15 at 17:01
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Overspeed propeller is caused by a governor malfunction: instead of aligning itself so that it experiences a decent amount of torque to overcome, the blade pitch springs back to minimum pitch. A good pitch for take-off, but not at speed since there is no resistance anymore, in fact the prop can be back driven by the airflow in this scenario.

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